Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Geoffrey Garen]

> On Feb 22, 2017, at 12:16 PM, Filip Pizlo  wrote:
> 
> 
>> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen  wrote:
>> 
>> I’ve lost countless hours to investigating CrashTracers that would have been 
>> easy to solve if I had access to register state.
> 
> The current RELEASE_ASSERT means that every assertion in what the compiler 
> thinks is a function (i.e. some function and everything inlined into it) is 
> coalesced into a single trap site.  I’d like to understand how you use the 
> register state if you don’t even know which assertion you are at.

Not all functions suffer from this problem. Few enough functions suffer from 
this problem that I haven’t felt an urgent need to address it.

> I believe that if you do want to analyze register state, then switching back 
> to calling some function that prints out diagnostic information is strictly 
> better.  Sure, you get less register state, but at least you know where you 
> crashed.  Knowing where you crashed is much more important than knowing the 
> register state, since the register state is not useful if you don’t know 
> where you crashed.

See above.

Also, printed diagnostic information is not available in CrashTracer reports, 
while register state is.

> 
>> 
>> I also want the freedom to add RELEASE_ASSERT without ruining performance 
>> due to bad register allocation or making the code too large to inline. For 
>> example, hot paths in WTF::Vector use RELEASE_ASSERT.
> 
> Do we have data about the performance benefits of the current RELEASE_ASSERT 
> implementation?

Oliver needed it in order to turn on assertions in side Vector.h without 
regression. You can follow the svn / bugzilla history to find out more.

> 
>> 
>> Is some compromise solution possible?
>> 
>> Some options:
>> 
>> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
> 
> The point of C++ assert macros is that I don’t have to add a custom string.  
> I want a RELEASE_ASSERT macro that automatically stringifies the expression 
> and uses that as the string.

FWIW, all of the existing cases of dataLog followed by RELEASE_ASSERT that I 
could find wanted to log something other than just the expression — for 
example, a pointer value.

> If I had a choice between a RELEASE_ASSERT that can accurate report where it 
> crashed but sometimes trashes the register state, and a RELEASE_ASSERT that 
> always gives me the register state but cannot tell me which assert in the 
> function it’s coming from, then I would always choose the one that can tell 
> me where it crashed.  That’s much more important, and the register state is 
> not useful without that information.

I don’t think it’s helpful to conflate (a) I want a string message with (b) I 
want to solve a coalescing problem.

> 
>> 
>> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug 
>> builds. (There’s not much need to preserve register state in debug builds.)
> 
> That would be nice, but doesn’t make RELEASE_ASSERT useful for debugging 
> issues where timing is important.  I no longer use RELEASE_ASSERTS for those 
> kinds of assertions, because if I do it then I will never know where I 
> crashed.  So, I use the explicit:
> 
> if (!thing) {
>   dataLog(“…”);
>   RELEASE_ASSERT_NOT_REACHED();
> }

If asserts are generally set up to provide the extra data we want, and we just 
need a temporary change in assertion policy to satisfy a use case like this, 
that’s easy enough to achieve with a local configuration change to Assertions.h.

Geoff

> 
> -Filip
> 
> 
>> 
>> Geoff
>> 
>>> On Feb 22, 2017, at 11:09 AM, Filip Pizlo  wrote:
>>> 
>>> I disagree actually.  I've lost countless hours to converting this:
>>> 
>>> RELEASE_ASSERT(blah)
>>> 
>>> into this:
>>> 
>>> if (!blah) {
>>> dataLog("Reason why I crashed");
>>> RELEASE_ASSERT_NOT_REACHED();
>>> }
>>> 
>>> Look in the code - you'll find lots of stuff like this.
>>> 
>>> I don't think analyzing register state at crashes is more important than 
>>> keeping our code sane.
>>> 
>>> -Filip
>>> 
>>> 
 On Feb 21, 2017, at 5:56 PM, Mark Lam  wrote:
 
 Oh yeah, I forgot about that.  I think the register state is more 
 important for crash analysis, especially if we can make sure that the 
 compiler does not aggregate the int3s.  I’ll explore alternatives.
 
> On Feb 21, 2017, at 5:54 PM, Saam barati  wrote:
> 
> I thought the main point of moving to SIGTRAP was to preserve register 
> state?
> 
> That said, there are probably places where we care more about the message 
> than the registers.
> 
> - Saam
> 
>> On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
>> 
>> Is there a reason why RELEASE_ASSERT (and friends) does not call 
>> WTFReportAssertionFailure() to report where the assertion occur?  Is 
>> this purely to save memory?  svn blame tells me that 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Mark Lam]

To give an update: I plan to experiment and get some size and perf numbers and 
report back later.  But since this is not a high priority task for me, it may 
be a while before I get back to this.

Mark

> On Feb 22, 2017, at 1:25 PM, Ryosuke Niwa  wrote:
> 
> On Wed, Feb 22, 2017 at 12:41 PM, Mark Lam  wrote:
>> 
>> I would like to point out that we might be able to get the best of both 
>> worlds.  Here’s how we can do it:
>> 
>> define RELEASE_ASSERT(assertion) do { \
>>if (UNLIKELY(!(assertion))) { \
>>preserveRegisterState(); \
>>WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
>> #assertion); \
>>restoreRegisterState(); \
>>CRASH(); \
>>} \
>> 
>> preserveRegisterState() and restoreRegisterState() will carefully push and 
>> pop registers onto / off the stack (like how the JIT probe works).
>> 
> 
> I'm afraid this would bloat the binary size too much. You can test but
> I'd be surprised if this didn't negatively impact perf especially in
> WebCore code.
> 
> - R. Niwa

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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Ryosuke Niwa]

On Wed, Feb 22, 2017 at 12:41 PM, Mark Lam  wrote:
>
> I would like to point out that we might be able to get the best of both 
> worlds.  Here’s how we can do it:
>
> define RELEASE_ASSERT(assertion) do { \
> if (UNLIKELY(!(assertion))) { \
> preserveRegisterState(); \
> WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
> #assertion); \
> restoreRegisterState(); \
> CRASH(); \
> } \
>
> preserveRegisterState() and restoreRegisterState() will carefully push and 
> pop registers onto / off the stack (like how the JIT probe works).
>

I'm afraid this would bloat the binary size too much. You can test but
I'd be surprised if this didn't negatively impact perf especially in
WebCore code.

- R. Niwa
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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Mark Lam]

> On Feb 22, 2017, at 12:46 PM, Filip Pizlo  wrote:
> 
>> 
>> On Feb 22, 2017, at 12:41 PM, Mark Lam > > wrote:
>> 
>>> 
>>> On Feb 22, 2017, at 12:35 PM, Filip Pizlo >> > wrote:
>>> 
 
 On Feb 22, 2017, at 12:33 PM, Mark Lam > wrote:
 
 
> On Feb 22, 2017, at 12:16 PM, Filip Pizlo  > wrote:
> 
> 
>> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen > > wrote:
>> 
>> I’ve lost countless hours to investigating CrashTracers that would have 
>> been easy to solve if I had access to register state.
> 
> The current RELEASE_ASSERT means that every assertion in what the 
> compiler thinks is a function (i.e. some function and everything inlined 
> into it) is coalesced into a single trap site.  I’d like to understand 
> how you use the register state if you don’t even know which assertion you 
> are at.
 
 Correction: they are not coalesced.  I was mistaken about that.  The fact 
 that we turn them into inline asm (for emitting the int3) means the 
 compiler cannot optimize it away or coalesce it.  The compiler does move 
 it to the end of the emitted code for the function though because we end 
 the CRASH() macro with __builtin_unreachable().
 
 Hence, each int3 can be correlated back to the RELEASE_ASSERT that 
 triggered it (with some extended disassembly work).
>>> 
>>> This never works for me.  I tested it locally.  LLVM will even coalesce 
>>> similar inline assembly.
>> 
>> With my proposal, I’m emitting different inline asm now after the int3 trap 
>> because I’m embedding line number and file strings.  Hence, even if the 
>> compiler is smart enough to compare inline asm code blobs, it will find them 
>> to be different, and hence, it doesn’t make sense to coalesce.
> 
> Are you claiming that LLVM does not currently now coalesce RELEASE_ASSERTS, 
> or that it will not coalesce them anymore after you make some change?

Here, I’m claiming that it will not coalesce after I make some changes.

> 
>> 
>>> 
 
> I believe that if you do want to analyze register state, then switching 
> back to calling some function that prints out diagnostic information is 
> strictly better.  Sure, you get less register state, but at least you 
> know where you crashed.  Knowing where you crashed is much more important 
> than knowing the register state, since the register state is not useful 
> if you don’t know where you crashed.
> 
 
 I would like to point out that we might be able to get the best of both 
 worlds.  Here’s how we can do it:
 
 define RELEASE_ASSERT(assertion) do { \
 if (UNLIKELY(!(assertion))) { \
 preserveRegisterState(); \
 WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
 #assertion); \
 restoreRegisterState(); \
 CRASH(); \
 } \
 
 preserveRegisterState() and restoreRegisterState() will carefully push and 
 pop registers onto / off the stack (like how the JIT probe works).
>>> 
>>> Why not do the preserve/restore inside the WTFReport call?
>> 
>> Because I would like to preserve the register values that were used in the 
>> comparison that failed the assertion.
> 
> That doesn't change anything.  You can create a WTFFail that is written in 
> assembly and first saves all registers, and restores them prior to trapping.

A meaningful call here requires passing __FILE__, __LINE__, 
WTF_PRETTY_FUNCTION, and #assertion as arguments.  Hence, this will necessarily 
perturb register state at the call site.  The compiler is also free to load the 
reporting function into a register to make the call.  My approach of preserving 
regs before any code the compiler emits to make the call guarantees that we 
have the register immediately after the assertion compare.

Mark

> 
> -Filip
> 
> 
>> 
>> Mark
>> 
>>> 
 This allows us to get a log message on the terminal when we’re running 
 manually.
 
 In addition, we can capture some additional information about the 
 assertion site by forcing the compiler to emit code to capture the code 
 location info after the trapping instruction.  This is redundant but 
 provides an easy place to find this info (i.e. after the int3 instruction).
 
 #define WTFBreakpointTrap() do { \
 __asm__ volatile ("int3"); \
 __asm__ volatile( "" :  : "r"(__FILE__), "r"(__LINE__), 
 "r"(WTF_PRETTY_FUNCTION)); \
 } while (false)
 
 We can easily get the line number this way.  However, the line number is 
 not very useful by itself when we have inlining.  Hence, I also capture 
 the __FILE__ and 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Filip Pizlo]

> On Feb 22, 2017, at 12:41 PM, Mark Lam  wrote:
> 
>> 
>> On Feb 22, 2017, at 12:35 PM, Filip Pizlo > > wrote:
>> 
>>> 
>>> On Feb 22, 2017, at 12:33 PM, Mark Lam >> > wrote:
>>> 
>>> 
 On Feb 22, 2017, at 12:16 PM, Filip Pizlo > wrote:
 
 
> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen  > wrote:
> 
> I’ve lost countless hours to investigating CrashTracers that would have 
> been easy to solve if I had access to register state.
 
 The current RELEASE_ASSERT means that every assertion in what the compiler 
 thinks is a function (i.e. some function and everything inlined into it) 
 is coalesced into a single trap site.  I’d like to understand how you use 
 the register state if you don’t even know which assertion you are at.
>>> 
>>> Correction: they are not coalesced.  I was mistaken about that.  The fact 
>>> that we turn them into inline asm (for emitting the int3) means the 
>>> compiler cannot optimize it away or coalesce it.  The compiler does move it 
>>> to the end of the emitted code for the function though because we end the 
>>> CRASH() macro with __builtin_unreachable().
>>> 
>>> Hence, each int3 can be correlated back to the RELEASE_ASSERT that 
>>> triggered it (with some extended disassembly work).
>> 
>> This never works for me.  I tested it locally.  LLVM will even coalesce 
>> similar inline assembly.
> 
> With my proposal, I’m emitting different inline asm now after the int3 trap 
> because I’m embedding line number and file strings.  Hence, even if the 
> compiler is smart enough to compare inline asm code blobs, it will find them 
> to be different, and hence, it doesn’t make sense to coalesce.

Are you claiming that LLVM does not currently now coalesce RELEASE_ASSERTS, or 
that it will not coalesce them anymore after you make some change?

> 
>> 
>>> 
 I believe that if you do want to analyze register state, then switching 
 back to calling some function that prints out diagnostic information is 
 strictly better.  Sure, you get less register state, but at least you know 
 where you crashed.  Knowing where you crashed is much more important than 
 knowing the register state, since the register state is not useful if you 
 don’t know where you crashed.
 
>>> 
>>> I would like to point out that we might be able to get the best of both 
>>> worlds.  Here’s how we can do it:
>>> 
>>> define RELEASE_ASSERT(assertion) do { \
>>> if (UNLIKELY(!(assertion))) { \
>>> preserveRegisterState(); \
>>> WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
>>> #assertion); \
>>> restoreRegisterState(); \
>>> CRASH(); \
>>> } \
>>> 
>>> preserveRegisterState() and restoreRegisterState() will carefully push and 
>>> pop registers onto / off the stack (like how the JIT probe works).
>> 
>> Why not do the preserve/restore inside the WTFReport call?
> 
> Because I would like to preserve the register values that were used in the 
> comparison that failed the assertion.

That doesn't change anything.  You can create a WTFFail that is written in 
assembly and first saves all registers, and restores them prior to trapping.

-Filip


> 
> Mark
> 
>> 
>>> This allows us to get a log message on the terminal when we’re running 
>>> manually.
>>> 
>>> In addition, we can capture some additional information about the assertion 
>>> site by forcing the compiler to emit code to capture the code location info 
>>> after the trapping instruction.  This is redundant but provides an easy 
>>> place to find this info (i.e. after the int3 instruction).
>>> 
>>> #define WTFBreakpointTrap() do { \
>>> __asm__ volatile ("int3"); \
>>> __asm__ volatile( "" :  : "r"(__FILE__), "r"(__LINE__), 
>>> "r"(WTF_PRETTY_FUNCTION)); \
>>> } while (false)
>>> 
>>> We can easily get the line number this way.  However, the line number is 
>>> not very useful by itself when we have inlining.  Hence, I also capture the 
>>> __FILE__ and WTF_PRETTY_FUNCTION.  However, I haven’t been able to figure 
>>> out how to decode those from the otool disassembler yet.
>>> 
>>> The only downside of doing this extra work is that it increases the code 
>>> size for each RELEASE_ASSERT site.  This is probably insignificant in total.
>>> 
>>> Performance-wise, it should be neutral-ish because the 
>>> __builtin_unreachable() in the CRASH() macro + the UNLIKELY() macro would 
>>> tell the compiler to put this in a slow path away from the main code path.
>>> 
>>> Any thoughts on this alternative?
>>> 
>>> Mark
>>> 
>>> 
> 
> I also want the freedom to add RELEASE_ASSERT without ruining performance 
> due to bad register allocation or making the code too large to inline. 
> For example, 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Filip Pizlo]

Mark,

I know that you keep saying this.  I remember you told me this even when I was 
sitting on a RELEASE_ASSERT that had gotten rage-coalesced.

Your reasoning sounds great, but this just isn't what happens in clang.  
__builtin_trap gets coalesced, as does inline asm.

-Filip


> On Feb 22, 2017, at 12:38 PM, Mark Lam  wrote:
> 
> For some context, we used to see aggregation of the CRASH() for 
> RELEASE_ASSERT() in the old days before we switched to using the int3 trap.  
> Back then we called a crash() function that never returns.  As a result, the 
> C++ compiler was able to coalesce all the calls.  With the int3 trap emitted 
> by inline asm, the C++ compiler has less ability to determine that the crash 
> sites have the same code (probably because it doesn’t bother comparing what’s 
> in the inline asm blobs).
> 
> Mark
> 
> 
>> On Feb 22, 2017, at 12:33 PM, Mark Lam > > wrote:
>> 
>>> 
>>> On Feb 22, 2017, at 12:16 PM, Filip Pizlo >> > wrote:
>>> 
>>> 
 On Feb 22, 2017, at 11:58 AM, Geoffrey Garen > wrote:
 
 I’ve lost countless hours to investigating CrashTracers that would have 
 been easy to solve if I had access to register state.
>>> 
>>> The current RELEASE_ASSERT means that every assertion in what the compiler 
>>> thinks is a function (i.e. some function and everything inlined into it) is 
>>> coalesced into a single trap site.  I’d like to understand how you use the 
>>> register state if you don’t even know which assertion you are at.
>> 
>> Correction: they are not coalesced.  I was mistaken about that.  The fact 
>> that we turn them into inline asm (for emitting the int3) means the compiler 
>> cannot optimize it away or coalesce it.  The compiler does move it to the 
>> end of the emitted code for the function though because we end the CRASH() 
>> macro with __builtin_unreachable().
>> 
>> Hence, each int3 can be correlated back to the RELEASE_ASSERT that triggered 
>> it (with some extended disassembly work).
>> 
>>> I believe that if you do want to analyze register state, then switching 
>>> back to calling some function that prints out diagnostic information is 
>>> strictly better.  Sure, you get less register state, but at least you know 
>>> where you crashed.  Knowing where you crashed is much more important than 
>>> knowing the register state, since the register state is not useful if you 
>>> don’t know where you crashed.
>>> 
>> 
>> I would like to point out that we might be able to get the best of both 
>> worlds.  Here’s how we can do it:
>> 
>> define RELEASE_ASSERT(assertion) do { \
>> if (UNLIKELY(!(assertion))) { \
>> preserveRegisterState(); \
>> WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
>> #assertion); \
>> restoreRegisterState(); \
>> CRASH(); \
>> } \
>> 
>> preserveRegisterState() and restoreRegisterState() will carefully push and 
>> pop registers onto / off the stack (like how the JIT probe works).
>> This allows us to get a log message on the terminal when we’re running 
>> manually.
>> 
>> In addition, we can capture some additional information about the assertion 
>> site by forcing the compiler to emit code to capture the code location info 
>> after the trapping instruction.  This is redundant but provides an easy 
>> place to find this info (i.e. after the int3 instruction).
>> 
>> #define WTFBreakpointTrap() do { \
>> __asm__ volatile ("int3"); \
>> __asm__ volatile( "" :  : "r"(__FILE__), "r"(__LINE__), 
>> "r"(WTF_PRETTY_FUNCTION)); \
>> } while (false)
>> 
>> We can easily get the line number this way.  However, the line number is not 
>> very useful by itself when we have inlining.  Hence, I also capture the 
>> __FILE__ and WTF_PRETTY_FUNCTION.  However, I haven’t been able to figure 
>> out how to decode those from the otool disassembler yet.
>> 
>> The only downside of doing this extra work is that it increases the code 
>> size for each RELEASE_ASSERT site.  This is probably insignificant in total.
>> 
>> Performance-wise, it should be neutral-ish because the 
>> __builtin_unreachable() in the CRASH() macro + the UNLIKELY() macro would 
>> tell the compiler to put this in a slow path away from the main code path.
>> 
>> Any thoughts on this alternative?
>> 
>> Mark
>> 
>> 
 
 I also want the freedom to add RELEASE_ASSERT without ruining performance 
 due to bad register allocation or making the code too large to inline. For 
 example, hot paths in WTF::Vector use RELEASE_ASSERT.
>>> 
>>> Do we have data about the performance benefits of the current 
>>> RELEASE_ASSERT implementation?
>>> 
 
 Is some compromise solution possible?
 
 Some options:
 
 (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
>>> 
>>> The point of C++ 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Mark Lam]

> On Feb 22, 2017, at 12:35 PM, Filip Pizlo  wrote:
> 
>> 
>> On Feb 22, 2017, at 12:33 PM, Mark Lam > > wrote:
>> 
>> 
>>> On Feb 22, 2017, at 12:16 PM, Filip Pizlo >> > wrote:
>>> 
>>> 
 On Feb 22, 2017, at 11:58 AM, Geoffrey Garen > wrote:
 
 I’ve lost countless hours to investigating CrashTracers that would have 
 been easy to solve if I had access to register state.
>>> 
>>> The current RELEASE_ASSERT means that every assertion in what the compiler 
>>> thinks is a function (i.e. some function and everything inlined into it) is 
>>> coalesced into a single trap site.  I’d like to understand how you use the 
>>> register state if you don’t even know which assertion you are at.
>> 
>> Correction: they are not coalesced.  I was mistaken about that.  The fact 
>> that we turn them into inline asm (for emitting the int3) means the compiler 
>> cannot optimize it away or coalesce it.  The compiler does move it to the 
>> end of the emitted code for the function though because we end the CRASH() 
>> macro with __builtin_unreachable().
>> 
>> Hence, each int3 can be correlated back to the RELEASE_ASSERT that triggered 
>> it (with some extended disassembly work).
> 
> This never works for me.  I tested it locally.  LLVM will even coalesce 
> similar inline assembly.

With my proposal, I’m emitting different inline asm now after the int3 trap 
because I’m embedding line number and file strings.  Hence, even if the 
compiler is smart enough to compare inline asm code blobs, it will find them to 
be different, and hence, it doesn’t make sense to coalesce.

> 
>> 
>>> I believe that if you do want to analyze register state, then switching 
>>> back to calling some function that prints out diagnostic information is 
>>> strictly better.  Sure, you get less register state, but at least you know 
>>> where you crashed.  Knowing where you crashed is much more important than 
>>> knowing the register state, since the register state is not useful if you 
>>> don’t know where you crashed.
>>> 
>> 
>> I would like to point out that we might be able to get the best of both 
>> worlds.  Here’s how we can do it:
>> 
>> define RELEASE_ASSERT(assertion) do { \
>> if (UNLIKELY(!(assertion))) { \
>> preserveRegisterState(); \
>> WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
>> #assertion); \
>> restoreRegisterState(); \
>> CRASH(); \
>> } \
>> 
>> preserveRegisterState() and restoreRegisterState() will carefully push and 
>> pop registers onto / off the stack (like how the JIT probe works).
> 
> Why not do the preserve/restore inside the WTFReport call?

Because I would like to preserve the register values that were used in the 
comparison that failed the assertion.

Mark

> 
>> This allows us to get a log message on the terminal when we’re running 
>> manually.
>> 
>> In addition, we can capture some additional information about the assertion 
>> site by forcing the compiler to emit code to capture the code location info 
>> after the trapping instruction.  This is redundant but provides an easy 
>> place to find this info (i.e. after the int3 instruction).
>> 
>> #define WTFBreakpointTrap() do { \
>> __asm__ volatile ("int3"); \
>> __asm__ volatile( "" :  : "r"(__FILE__), "r"(__LINE__), 
>> "r"(WTF_PRETTY_FUNCTION)); \
>> } while (false)
>> 
>> We can easily get the line number this way.  However, the line number is not 
>> very useful by itself when we have inlining.  Hence, I also capture the 
>> __FILE__ and WTF_PRETTY_FUNCTION.  However, I haven’t been able to figure 
>> out how to decode those from the otool disassembler yet.
>> 
>> The only downside of doing this extra work is that it increases the code 
>> size for each RELEASE_ASSERT site.  This is probably insignificant in total.
>> 
>> Performance-wise, it should be neutral-ish because the 
>> __builtin_unreachable() in the CRASH() macro + the UNLIKELY() macro would 
>> tell the compiler to put this in a slow path away from the main code path.
>> 
>> Any thoughts on this alternative?
>> 
>> Mark
>> 
>> 
 
 I also want the freedom to add RELEASE_ASSERT without ruining performance 
 due to bad register allocation or making the code too large to inline. For 
 example, hot paths in WTF::Vector use RELEASE_ASSERT.
>>> 
>>> Do we have data about the performance benefits of the current 
>>> RELEASE_ASSERT implementation?
>>> 
 
 Is some compromise solution possible?
 
 Some options:
 
 (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
>>> 
>>> The point of C++ assert macros is that I don’t have to add a custom string. 
>>>  I want a RELEASE_ASSERT macro that automatically stringifies the 
>>> expression and uses that as the string.
>>> 
>>> If I had a choice 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Mark Lam]

For some context, we used to see aggregation of the CRASH() for 
RELEASE_ASSERT() in the old days before we switched to using the int3 trap.  
Back then we called a crash() function that never returns.  As a result, the 
C++ compiler was able to coalesce all the calls.  With the int3 trap emitted by 
inline asm, the C++ compiler has less ability to determine that the crash sites 
have the same code (probably because it doesn’t bother comparing what’s in the 
inline asm blobs).

Mark


> On Feb 22, 2017, at 12:33 PM, Mark Lam  wrote:
> 
>> 
>> On Feb 22, 2017, at 12:16 PM, Filip Pizlo > > wrote:
>> 
>> 
>>> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen >> > wrote:
>>> 
>>> I’ve lost countless hours to investigating CrashTracers that would have 
>>> been easy to solve if I had access to register state.
>> 
>> The current RELEASE_ASSERT means that every assertion in what the compiler 
>> thinks is a function (i.e. some function and everything inlined into it) is 
>> coalesced into a single trap site.  I’d like to understand how you use the 
>> register state if you don’t even know which assertion you are at.
> 
> Correction: they are not coalesced.  I was mistaken about that.  The fact 
> that we turn them into inline asm (for emitting the int3) means the compiler 
> cannot optimize it away or coalesce it.  The compiler does move it to the end 
> of the emitted code for the function though because we end the CRASH() macro 
> with __builtin_unreachable().
> 
> Hence, each int3 can be correlated back to the RELEASE_ASSERT that triggered 
> it (with some extended disassembly work).
> 
>> I believe that if you do want to analyze register state, then switching back 
>> to calling some function that prints out diagnostic information is strictly 
>> better.  Sure, you get less register state, but at least you know where you 
>> crashed.  Knowing where you crashed is much more important than knowing the 
>> register state, since the register state is not useful if you don’t know 
>> where you crashed.
>> 
> 
> I would like to point out that we might be able to get the best of both 
> worlds.  Here’s how we can do it:
> 
> define RELEASE_ASSERT(assertion) do { \
> if (UNLIKELY(!(assertion))) { \
> preserveRegisterState(); \
> WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
> #assertion); \
> restoreRegisterState(); \
> CRASH(); \
> } \
> 
> preserveRegisterState() and restoreRegisterState() will carefully push and 
> pop registers onto / off the stack (like how the JIT probe works).
> This allows us to get a log message on the terminal when we’re running 
> manually.
> 
> In addition, we can capture some additional information about the assertion 
> site by forcing the compiler to emit code to capture the code location info 
> after the trapping instruction.  This is redundant but provides an easy place 
> to find this info (i.e. after the int3 instruction).
> 
> #define WTFBreakpointTrap() do { \
> __asm__ volatile ("int3"); \
> __asm__ volatile( "" :  : "r"(__FILE__), "r"(__LINE__), 
> "r"(WTF_PRETTY_FUNCTION)); \
> } while (false)
> 
> We can easily get the line number this way.  However, the line number is not 
> very useful by itself when we have inlining.  Hence, I also capture the 
> __FILE__ and WTF_PRETTY_FUNCTION.  However, I haven’t been able to figure out 
> how to decode those from the otool disassembler yet.
> 
> The only downside of doing this extra work is that it increases the code size 
> for each RELEASE_ASSERT site.  This is probably insignificant in total.
> 
> Performance-wise, it should be neutral-ish because the 
> __builtin_unreachable() in the CRASH() macro + the UNLIKELY() macro would 
> tell the compiler to put this in a slow path away from the main code path.
> 
> Any thoughts on this alternative?
> 
> Mark
> 
> 
>>> 
>>> I also want the freedom to add RELEASE_ASSERT without ruining performance 
>>> due to bad register allocation or making the code too large to inline. For 
>>> example, hot paths in WTF::Vector use RELEASE_ASSERT.
>> 
>> Do we have data about the performance benefits of the current RELEASE_ASSERT 
>> implementation?
>> 
>>> 
>>> Is some compromise solution possible?
>>> 
>>> Some options:
>>> 
>>> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
>> 
>> The point of C++ assert macros is that I don’t have to add a custom string.  
>> I want a RELEASE_ASSERT macro that automatically stringifies the expression 
>> and uses that as the string.
>> 
>> If I had a choice between a RELEASE_ASSERT that can accurate report where it 
>> crashed but sometimes trashes the register state, and a RELEASE_ASSERT that 
>> always gives me the register state but cannot tell me which assert in the 
>> function it’s coming from, then I would always choose the one that can tell 
>> me where it 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Filip Pizlo]

> On Feb 22, 2017, at 12:33 PM, Mark Lam  wrote:
> 
> 
>> On Feb 22, 2017, at 12:16 PM, Filip Pizlo > > wrote:
>> 
>> 
>>> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen >> > wrote:
>>> 
>>> I’ve lost countless hours to investigating CrashTracers that would have 
>>> been easy to solve if I had access to register state.
>> 
>> The current RELEASE_ASSERT means that every assertion in what the compiler 
>> thinks is a function (i.e. some function and everything inlined into it) is 
>> coalesced into a single trap site.  I’d like to understand how you use the 
>> register state if you don’t even know which assertion you are at.
> 
> Correction: they are not coalesced.  I was mistaken about that.  The fact 
> that we turn them into inline asm (for emitting the int3) means the compiler 
> cannot optimize it away or coalesce it.  The compiler does move it to the end 
> of the emitted code for the function though because we end the CRASH() macro 
> with __builtin_unreachable().
> 
> Hence, each int3 can be correlated back to the RELEASE_ASSERT that triggered 
> it (with some extended disassembly work).

This never works for me.  I tested it locally.  LLVM will even coalesce similar 
inline assembly.

> 
>> I believe that if you do want to analyze register state, then switching back 
>> to calling some function that prints out diagnostic information is strictly 
>> better.  Sure, you get less register state, but at least you know where you 
>> crashed.  Knowing where you crashed is much more important than knowing the 
>> register state, since the register state is not useful if you don’t know 
>> where you crashed.
>> 
> 
> I would like to point out that we might be able to get the best of both 
> worlds.  Here’s how we can do it:
> 
> define RELEASE_ASSERT(assertion) do { \
> if (UNLIKELY(!(assertion))) { \
> preserveRegisterState(); \
> WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
> #assertion); \
> restoreRegisterState(); \
> CRASH(); \
> } \
> 
> preserveRegisterState() and restoreRegisterState() will carefully push and 
> pop registers onto / off the stack (like how the JIT probe works).

Why not do the preserve/restore inside the WTFReport call?

> This allows us to get a log message on the terminal when we’re running 
> manually.
> 
> In addition, we can capture some additional information about the assertion 
> site by forcing the compiler to emit code to capture the code location info 
> after the trapping instruction.  This is redundant but provides an easy place 
> to find this info (i.e. after the int3 instruction).
> 
> #define WTFBreakpointTrap() do { \
> __asm__ volatile ("int3"); \
> __asm__ volatile( "" :  : "r"(__FILE__), "r"(__LINE__), 
> "r"(WTF_PRETTY_FUNCTION)); \
> } while (false)
> 
> We can easily get the line number this way.  However, the line number is not 
> very useful by itself when we have inlining.  Hence, I also capture the 
> __FILE__ and WTF_PRETTY_FUNCTION.  However, I haven’t been able to figure out 
> how to decode those from the otool disassembler yet.
> 
> The only downside of doing this extra work is that it increases the code size 
> for each RELEASE_ASSERT site.  This is probably insignificant in total.
> 
> Performance-wise, it should be neutral-ish because the 
> __builtin_unreachable() in the CRASH() macro + the UNLIKELY() macro would 
> tell the compiler to put this in a slow path away from the main code path.
> 
> Any thoughts on this alternative?
> 
> Mark
> 
> 
>>> 
>>> I also want the freedom to add RELEASE_ASSERT without ruining performance 
>>> due to bad register allocation or making the code too large to inline. For 
>>> example, hot paths in WTF::Vector use RELEASE_ASSERT.
>> 
>> Do we have data about the performance benefits of the current RELEASE_ASSERT 
>> implementation?
>> 
>>> 
>>> Is some compromise solution possible?
>>> 
>>> Some options:
>>> 
>>> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
>> 
>> The point of C++ assert macros is that I don’t have to add a custom string.  
>> I want a RELEASE_ASSERT macro that automatically stringifies the expression 
>> and uses that as the string.
>> 
>> If I had a choice between a RELEASE_ASSERT that can accurate report where it 
>> crashed but sometimes trashes the register state, and a RELEASE_ASSERT that 
>> always gives me the register state but cannot tell me which assert in the 
>> function it’s coming from, then I would always choose the one that can tell 
>> me where it crashed.  That’s much more important, and the register state is 
>> not useful without that information.
>> 
>>> 
>>> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug 
>>> builds. (There’s not much need to preserve register state in debug builds.)
>> 
>> That would be nice, but doesn’t make RELEASE_ASSERT 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Mark Lam]

> On Feb 22, 2017, at 12:16 PM, Filip Pizlo  wrote:
> 
> 
>> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen  wrote:
>> 
>> I’ve lost countless hours to investigating CrashTracers that would have been 
>> easy to solve if I had access to register state.
> 
> The current RELEASE_ASSERT means that every assertion in what the compiler 
> thinks is a function (i.e. some function and everything inlined into it) is 
> coalesced into a single trap site.  I’d like to understand how you use the 
> register state if you don’t even know which assertion you are at.

Correction: they are not coalesced.  I was mistaken about that.  The fact that 
we turn them into inline asm (for emitting the int3) means the compiler cannot 
optimize it away or coalesce it.  The compiler does move it to the end of the 
emitted code for the function though because we end the CRASH() macro with 
__builtin_unreachable().

Hence, each int3 can be correlated back to the RELEASE_ASSERT that triggered it 
(with some extended disassembly work).

> I believe that if you do want to analyze register state, then switching back 
> to calling some function that prints out diagnostic information is strictly 
> better.  Sure, you get less register state, but at least you know where you 
> crashed.  Knowing where you crashed is much more important than knowing the 
> register state, since the register state is not useful if you don’t know 
> where you crashed.
> 

I would like to point out that we might be able to get the best of both worlds. 
 Here’s how we can do it:

define RELEASE_ASSERT(assertion) do { \
if (UNLIKELY(!(assertion))) { \
preserveRegisterState(); \
WTFReportAssertionFailure(__FILE__, __LINE__, WTF_PRETTY_FUNCTION, 
#assertion); \
restoreRegisterState(); \
CRASH(); \
} \

preserveRegisterState() and restoreRegisterState() will carefully push and pop 
registers onto / off the stack (like how the JIT probe works).
This allows us to get a log message on the terminal when we’re running manually.

In addition, we can capture some additional information about the assertion 
site by forcing the compiler to emit code to capture the code location info 
after the trapping instruction.  This is redundant but provides an easy place 
to find this info (i.e. after the int3 instruction).

#define WTFBreakpointTrap() do { \
__asm__ volatile ("int3"); \
__asm__ volatile( "" :  : "r"(__FILE__), "r"(__LINE__), 
"r"(WTF_PRETTY_FUNCTION)); \
} while (false)

We can easily get the line number this way.  However, the line number is not 
very useful by itself when we have inlining.  Hence, I also capture the 
__FILE__ and WTF_PRETTY_FUNCTION.  However, I haven’t been able to figure out 
how to decode those from the otool disassembler yet.

The only downside of doing this extra work is that it increases the code size 
for each RELEASE_ASSERT site.  This is probably insignificant in total.

Performance-wise, it should be neutral-ish because the __builtin_unreachable() 
in the CRASH() macro + the UNLIKELY() macro would tell the compiler to put this 
in a slow path away from the main code path.

Any thoughts on this alternative?

Mark


>> 
>> I also want the freedom to add RELEASE_ASSERT without ruining performance 
>> due to bad register allocation or making the code too large to inline. For 
>> example, hot paths in WTF::Vector use RELEASE_ASSERT.
> 
> Do we have data about the performance benefits of the current RELEASE_ASSERT 
> implementation?
> 
>> 
>> Is some compromise solution possible?
>> 
>> Some options:
>> 
>> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
> 
> The point of C++ assert macros is that I don’t have to add a custom string.  
> I want a RELEASE_ASSERT macro that automatically stringifies the expression 
> and uses that as the string.
> 
> If I had a choice between a RELEASE_ASSERT that can accurate report where it 
> crashed but sometimes trashes the register state, and a RELEASE_ASSERT that 
> always gives me the register state but cannot tell me which assert in the 
> function it’s coming from, then I would always choose the one that can tell 
> me where it crashed.  That’s much more important, and the register state is 
> not useful without that information.
> 
>> 
>> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug 
>> builds. (There’s not much need to preserve register state in debug builds.)
> 
> That would be nice, but doesn’t make RELEASE_ASSERT useful for debugging 
> issues where timing is important.  I no longer use RELEASE_ASSERTS for those 
> kinds of assertions, because if I do it then I will never know where I 
> crashed.  So, I use the explicit:
> 
> if (!thing) {
>   dataLog(“…”);
>   RELEASE_ASSERT_NOT_REACHED();
> }
> 
> -Filip
> 
> 
>> 
>> Geoff
>> 
>>> On Feb 22, 2017, at 11:09 AM, Filip Pizlo  wrote:
>>> 
>>> I disagree actually.  I've lost countless hours to 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Saam barati]

> On Feb 22, 2017, at 12:16 PM, Filip Pizlo  wrote:
> 
> 
>> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen  wrote:
>> 
>> I’ve lost countless hours to investigating CrashTracers that would have been 
>> easy to solve if I had access to register state.
> 
> The current RELEASE_ASSERT means that every assertion in what the compiler 
> thinks is a function (i.e. some function and everything inlined into it) is 
> coalesced into a single trap site.  I’d like to understand how you use the 
> register state if you don’t even know which assertion you are at.
When I disassemble JavaScriptCore, I often find a succession of int3s at the 
bottom of a function. Does LLVM sometimes combine them and sometimes not?

For example, this is what the bottom of the 
__ZN3JSC20AbstractModuleRecord18getModuleNamespaceEPNS_9ExecStateE looks like:

5c25popq%r14
5c27popq%r15
5c29popq%rbp
5c2aretq
5c2bint3
5c2cint3
5c2dint3
5c2eint3
5c2fnop

- Saam
> 
> I believe that if you do want to analyze register state, then switching back 
> to calling some function that prints out diagnostic information is strictly 
> better.  Sure, you get less register state, but at least you know where you 
> crashed.  Knowing where you crashed is much more important than knowing the 
> register state, since the register state is not useful if you don’t know 
> where you crashed.
> 
>> 
>> I also want the freedom to add RELEASE_ASSERT without ruining performance 
>> due to bad register allocation or making the code too large to inline. For 
>> example, hot paths in WTF::Vector use RELEASE_ASSERT.
> 
> Do we have data about the performance benefits of the current RELEASE_ASSERT 
> implementation?
> 
>> 
>> Is some compromise solution possible?
>> 
>> Some options:
>> 
>> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
> 
> The point of C++ assert macros is that I don’t have to add a custom string.  
> I want a RELEASE_ASSERT macro that automatically stringifies the expression 
> and uses that as the string.
> 
> If I had a choice between a RELEASE_ASSERT that can accurate report where it 
> crashed but sometimes trashes the register state, and a RELEASE_ASSERT that 
> always gives me the register state but cannot tell me which assert in the 
> function it’s coming from, then I would always choose the one that can tell 
> me where it crashed.  That’s much more important, and the register state is 
> not useful without that information.
> 
>> 
>> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug 
>> builds. (There’s not much need to preserve register state in debug builds.)
> 
> That would be nice, but doesn’t make RELEASE_ASSERT useful for debugging 
> issues where timing is important.  I no longer use RELEASE_ASSERTS for those 
> kinds of assertions, because if I do it then I will never know where I 
> crashed.  So, I use the explicit:
> 
> if (!thing) {
>   dataLog(“…”);
>   RELEASE_ASSERT_NOT_REACHED();
> }
> 
> -Filip
> 
> 
>> 
>> Geoff
>> 
>>> On Feb 22, 2017, at 11:09 AM, Filip Pizlo  wrote:
>>> 
>>> I disagree actually.  I've lost countless hours to converting this:
>>> 
>>> RELEASE_ASSERT(blah)
>>> 
>>> into this:
>>> 
>>> if (!blah) {
>>> dataLog("Reason why I crashed");
>>> RELEASE_ASSERT_NOT_REACHED();
>>> }
>>> 
>>> Look in the code - you'll find lots of stuff like this.
>>> 
>>> I don't think analyzing register state at crashes is more important than 
>>> keeping our code sane.
>>> 
>>> -Filip
>>> 
>>> 
 On Feb 21, 2017, at 5:56 PM, Mark Lam  wrote:
 
 Oh yeah, I forgot about that.  I think the register state is more 
 important for crash analysis, especially if we can make sure that the 
 compiler does not aggregate the int3s.  I’ll explore alternatives.
 
> On Feb 21, 2017, at 5:54 PM, Saam barati  wrote:
> 
> I thought the main point of moving to SIGTRAP was to preserve register 
> state?
> 
> That said, there are probably places where we care more about the message 
> than the registers.
> 
> - Saam
> 
>> On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
>> 
>> Is there a reason why RELEASE_ASSERT (and friends) does not call 
>> WTFReportAssertionFailure() to report where the assertion occur?  Is 
>> this purely to save memory?  svn blame tells me that it has been this 
>> way since the introduction of RELEASE_ASSERT in r140577 many years ago.
>> 
>> Would anyone object to adding a call to WTFReportAssertionFailure() in 
>> RELEASE_ASSERT() like we do for ASSERT()?  One of the upside 
>> (side-effect) of adding this call is that it appears to stop the 
>> compiler from aggregating all the 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Filip Pizlo]

> On Feb 22, 2017, at 12:23 PM, Saam barati  wrote:
> 
>> 
>> On Feb 22, 2017, at 12:16 PM, Filip Pizlo > > wrote:
>> 
>> 
>>> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen >> > wrote:
>>> 
>>> I’ve lost countless hours to investigating CrashTracers that would have 
>>> been easy to solve if I had access to register state.
>> 
>> The current RELEASE_ASSERT means that every assertion in what the compiler 
>> thinks is a function (i.e. some function and everything inlined into it) is 
>> coalesced into a single trap site.  I’d like to understand how you use the 
>> register state if you don’t even know which assertion you are at.
> When I disassemble JavaScriptCore, I often find a succession of int3s at the 
> bottom of a function. Does LLVM sometimes combine them and sometimes not?

Yeah.

> 
> For example, this is what the bottom of the 
> __ZN3JSC20AbstractModuleRecord18getModuleNamespaceEPNS_9ExecStateE looks like:
> 
> 5c25  popq%r14
> 5c27  popq%r15
> 5c29  popq%rbp
> 5c2a  retq
> 5c2b  int3
> 5c2c  int3
> 5c2d  int3
> 5c2e  int3
> 5c2f  nop

I’m curious how many branches target those traps.

For example in the GC, I was often getting crashes that LLVM was convinced were 
vector overflow.  Turns out that the compiler loves to coalesce other traps 
with the one from the vector overflow assert, so if you assert for some random 
reason in a function that accesses vectors, all of our tooling will report with 
total confidence that you’re overflowing a vector.

That’s way worse than not having register state.

-Filip


> 
> - Saam
>> 
>> I believe that if you do want to analyze register state, then switching back 
>> to calling some function that prints out diagnostic information is strictly 
>> better.  Sure, you get less register state, but at least you know where you 
>> crashed.  Knowing where you crashed is much more important than knowing the 
>> register state, since the register state is not useful if you don’t know 
>> where you crashed.
>> 
>>> 
>>> I also want the freedom to add RELEASE_ASSERT without ruining performance 
>>> due to bad register allocation or making the code too large to inline. For 
>>> example, hot paths in WTF::Vector use RELEASE_ASSERT.
>> 
>> Do we have data about the performance benefits of the current RELEASE_ASSERT 
>> implementation?
>> 
>>> 
>>> Is some compromise solution possible?
>>> 
>>> Some options:
>>> 
>>> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
>> 
>> The point of C++ assert macros is that I don’t have to add a custom string.  
>> I want a RELEASE_ASSERT macro that automatically stringifies the expression 
>> and uses that as the string.
>> 
>> If I had a choice between a RELEASE_ASSERT that can accurate report where it 
>> crashed but sometimes trashes the register state, and a RELEASE_ASSERT that 
>> always gives me the register state but cannot tell me which assert in the 
>> function it’s coming from, then I would always choose the one that can tell 
>> me where it crashed.  That’s much more important, and the register state is 
>> not useful without that information.
>> 
>>> 
>>> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug 
>>> builds. (There’s not much need to preserve register state in debug builds.)
>> 
>> That would be nice, but doesn’t make RELEASE_ASSERT useful for debugging 
>> issues where timing is important.  I no longer use RELEASE_ASSERTS for those 
>> kinds of assertions, because if I do it then I will never know where I 
>> crashed.  So, I use the explicit:
>> 
>> if (!thing) {
>>   dataLog(“…”);
>>   RELEASE_ASSERT_NOT_REACHED();
>> }
>> 
>> -Filip
>> 
>> 
>>> 
>>> Geoff
>>> 
 On Feb 22, 2017, at 11:09 AM, Filip Pizlo > wrote:
 
 I disagree actually.  I've lost countless hours to converting this:
 
 RELEASE_ASSERT(blah)
 
 into this:
 
 if (!blah) {
 dataLog("Reason why I crashed");
 RELEASE_ASSERT_NOT_REACHED();
 }
 
 Look in the code - you'll find lots of stuff like this.
 
 I don't think analyzing register state at crashes is more important than 
 keeping our code sane.
 
 -Filip
 
 
> On Feb 21, 2017, at 5:56 PM, Mark Lam  > wrote:
> 
> Oh yeah, I forgot about that.  I think the register state is more 
> important for crash analysis, especially if we can make sure that the 
> compiler does not aggregate the int3s.  I’ll explore alternatives.
> 
>> On Feb 21, 2017, at 5:54 PM, Saam barati > > wrote:
>> 
>> I thought the main point of moving to SIGTRAP was to 

Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Filip Pizlo]

> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen  wrote:
> 
> I’ve lost countless hours to investigating CrashTracers that would have been 
> easy to solve if I had access to register state.

The current RELEASE_ASSERT means that every assertion in what the compiler 
thinks is a function (i.e. some function and everything inlined into it) is 
coalesced into a single trap site.  I’d like to understand how you use the 
register state if you don’t even know which assertion you are at.

I believe that if you do want to analyze register state, then switching back to 
calling some function that prints out diagnostic information is strictly 
better.  Sure, you get less register state, but at least you know where you 
crashed.  Knowing where you crashed is much more important than knowing the 
register state, since the register state is not useful if you don’t know where 
you crashed.

> 
> I also want the freedom to add RELEASE_ASSERT without ruining performance due 
> to bad register allocation or making the code too large to inline. For 
> example, hot paths in WTF::Vector use RELEASE_ASSERT.

Do we have data about the performance benefits of the current RELEASE_ASSERT 
implementation?

> 
> Is some compromise solution possible?
> 
> Some options:
> 
> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.

The point of C++ assert macros is that I don’t have to add a custom string.  I 
want a RELEASE_ASSERT macro that automatically stringifies the expression and 
uses that as the string.

If I had a choice between a RELEASE_ASSERT that can accurate report where it 
crashed but sometimes trashes the register state, and a RELEASE_ASSERT that 
always gives me the register state but cannot tell me which assert in the 
function it’s coming from, then I would always choose the one that can tell me 
where it crashed.  That’s much more important, and the register state is not 
useful without that information.

> 
> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug 
> builds. (There’s not much need to preserve register state in debug builds.)

That would be nice, but doesn’t make RELEASE_ASSERT useful for debugging issues 
where timing is important.  I no longer use RELEASE_ASSERTS for those kinds of 
assertions, because if I do it then I will never know where I crashed.  So, I 
use the explicit:

if (!thing) {
   dataLog(“…”);
   RELEASE_ASSERT_NOT_REACHED();
}

-Filip


> 
> Geoff
> 
>> On Feb 22, 2017, at 11:09 AM, Filip Pizlo  wrote:
>> 
>> I disagree actually.  I've lost countless hours to converting this:
>> 
>> RELEASE_ASSERT(blah)
>> 
>> into this:
>> 
>> if (!blah) {
>>  dataLog("Reason why I crashed");
>>  RELEASE_ASSERT_NOT_REACHED();
>> }
>> 
>> Look in the code - you'll find lots of stuff like this.
>> 
>> I don't think analyzing register state at crashes is more important than 
>> keeping our code sane.
>> 
>> -Filip
>> 
>> 
>>> On Feb 21, 2017, at 5:56 PM, Mark Lam  wrote:
>>> 
>>> Oh yeah, I forgot about that.  I think the register state is more important 
>>> for crash analysis, especially if we can make sure that the compiler does 
>>> not aggregate the int3s.  I’ll explore alternatives.
>>> 
 On Feb 21, 2017, at 5:54 PM, Saam barati  wrote:
 
 I thought the main point of moving to SIGTRAP was to preserve register 
 state?
 
 That said, there are probably places where we care more about the message 
 than the registers.
 
 - Saam
 
> On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
> 
> Is there a reason why RELEASE_ASSERT (and friends) does not call 
> WTFReportAssertionFailure() to report where the assertion occur?  Is this 
> purely to save memory?  svn blame tells me that it has been this way 
> since the introduction of RELEASE_ASSERT in r140577 many years ago.
> 
> Would anyone object to adding a call to WTFReportAssertionFailure() in 
> RELEASE_ASSERT() like we do for ASSERT()?  One of the upside 
> (side-effect) of adding this call is that it appears to stop the compiler 
> from aggregating all the RELEASE_ASSERTS into a single code location, and 
> this will help with post-mortem crash debugging.
> 
> Any thoughts?
> 
> Mark
> 
> ___
> webkit-dev mailing list
> webkit-dev@lists.webkit.org
> https://lists.webkit.org/mailman/listinfo/webkit-dev
 
>>> 
>>> ___
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>>> https://lists.webkit.org/mailman/listinfo/webkit-dev
>> 
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> 

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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Michael Catanzaro]

On Wed, 2017-02-22 at 11:58 -0800, Geoffrey Garen wrote:
> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in
> Debug builds. (There’s not much need to preserve register state in
> debug builds.)

This seems like a more desirable approach.

Michael
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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Saam barati]

I like this idea. I think even doing what JF suggested could be really nice for 
suspected crashes. That way the register state would just tell us the reason.
If we do log, I think we should use WTFLogAlways instead of dataLog, because I 
think that does show up in some crash logs (but I could be wrong about this, 
I’m not 100% sure).

- Saam

> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen  wrote:
> 
> I’ve lost countless hours to investigating CrashTracers that would have been 
> easy to solve if I had access to register state.
> 
> I also want the freedom to add RELEASE_ASSERT without ruining performance due 
> to bad register allocation or making the code too large to inline. For 
> example, hot paths in WTF::Vector use RELEASE_ASSERT.
> 
> Is some compromise solution possible?
> 
> Some options:
> 
> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
> 
> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug 
> builds. (There’s not much need to preserve register state in debug builds.)
> 
> Geoff
> 
>> On Feb 22, 2017, at 11:09 AM, Filip Pizlo  wrote:
>> 
>> I disagree actually.  I've lost countless hours to converting this:
>> 
>> RELEASE_ASSERT(blah)
>> 
>> into this:
>> 
>> if (!blah) {
>>  dataLog("Reason why I crashed");
>>  RELEASE_ASSERT_NOT_REACHED();
>> }
>> 
>> Look in the code - you'll find lots of stuff like this.
>> 
>> I don't think analyzing register state at crashes is more important than 
>> keeping our code sane.
>> 
>> -Filip
>> 
>> 
>>> On Feb 21, 2017, at 5:56 PM, Mark Lam  wrote:
>>> 
>>> Oh yeah, I forgot about that.  I think the register state is more important 
>>> for crash analysis, especially if we can make sure that the compiler does 
>>> not aggregate the int3s.  I’ll explore alternatives.
>>> 
 On Feb 21, 2017, at 5:54 PM, Saam barati  wrote:
 
 I thought the main point of moving to SIGTRAP was to preserve register 
 state?
 
 That said, there are probably places where we care more about the message 
 than the registers.
 
 - Saam
 
> On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
> 
> Is there a reason why RELEASE_ASSERT (and friends) does not call 
> WTFReportAssertionFailure() to report where the assertion occur?  Is this 
> purely to save memory?  svn blame tells me that it has been this way 
> since the introduction of RELEASE_ASSERT in r140577 many years ago.
> 
> Would anyone object to adding a call to WTFReportAssertionFailure() in 
> RELEASE_ASSERT() like we do for ASSERT()?  One of the upside 
> (side-effect) of adding this call is that it appears to stop the compiler 
> from aggregating all the RELEASE_ASSERTS into a single code location, and 
> this will help with post-mortem crash debugging.
> 
> Any thoughts?
> 
> Mark
> 
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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Geoffrey Garen]

I’ve lost countless hours to investigating CrashTracers that would have been 
easy to solve if I had access to register state.

I also want the freedom to add RELEASE_ASSERT without ruining performance due 
to bad register allocation or making the code too large to inline. For example, 
hot paths in WTF::Vector use RELEASE_ASSERT.

Is some compromise solution possible?

Some options:

(1) Add a variant of RELEASE_ASSERT that takes a string and logs.

(2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug builds. 
(There’s not much need to preserve register state in debug builds.)

Geoff

> On Feb 22, 2017, at 11:09 AM, Filip Pizlo  wrote:
> 
> I disagree actually.  I've lost countless hours to converting this:
> 
> RELEASE_ASSERT(blah)
> 
> into this:
> 
> if (!blah) {
>   dataLog("Reason why I crashed");
>   RELEASE_ASSERT_NOT_REACHED();
> }
> 
> Look in the code - you'll find lots of stuff like this.
> 
> I don't think analyzing register state at crashes is more important than 
> keeping our code sane.
> 
> -Filip
> 
> 
>> On Feb 21, 2017, at 5:56 PM, Mark Lam  wrote:
>> 
>> Oh yeah, I forgot about that.  I think the register state is more important 
>> for crash analysis, especially if we can make sure that the compiler does 
>> not aggregate the int3s.  I’ll explore alternatives.
>> 
>>> On Feb 21, 2017, at 5:54 PM, Saam barati  wrote:
>>> 
>>> I thought the main point of moving to SIGTRAP was to preserve register 
>>> state?
>>> 
>>> That said, there are probably places where we care more about the message 
>>> than the registers.
>>> 
>>> - Saam
>>> 
 On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
 
 Is there a reason why RELEASE_ASSERT (and friends) does not call 
 WTFReportAssertionFailure() to report where the assertion occur?  Is this 
 purely to save memory?  svn blame tells me that it has been this way since 
 the introduction of RELEASE_ASSERT in r140577 many years ago.
 
 Would anyone object to adding a call to WTFReportAssertionFailure() in 
 RELEASE_ASSERT() like we do for ASSERT()?  One of the upside (side-effect) 
 of adding this call is that it appears to stop the compiler from 
 aggregating all the RELEASE_ASSERTS into a single code location, and this 
 will help with post-mortem crash debugging.
 
 Any thoughts?
 
 Mark
 
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 webkit-dev@lists.webkit.org
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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[JF Bastien]

Do we get the dataLog output in a crash report? It seems useful to have at
a minimum some "reason" enum which ends up in a register, so the crash
report is somewhat helpful, like we do with JIT code.

At that point the release assert might as well capture __LINE__ and other
things in a register, so that crash reporter picks it up.

Not that the dataLog isn't useful -- it is! -- just not as useful if it
isn't in crash reports.

On Wed, Feb 22, 2017 at 11:09 AM, Filip Pizlo  wrote:

> I disagree actually.  I've lost countless hours to converting this:
>
> RELEASE_ASSERT(blah)
>
> into this:
>
> if (!blah) {
>dataLog("Reason why I crashed");
>RELEASE_ASSERT_NOT_REACHED();
> }
>
> Look in the code - you'll find lots of stuff like this.
>
> I don't think analyzing register state at crashes is more important than
> keeping our code sane.
>
> -Filip
>
>
> > On Feb 21, 2017, at 5:56 PM, Mark Lam  wrote:
> >
> > Oh yeah, I forgot about that.  I think the register state is more
> important for crash analysis, especially if we can make sure that the
> compiler does not aggregate the int3s.  I’ll explore alternatives.
> >
> >> On Feb 21, 2017, at 5:54 PM, Saam barati  wrote:
> >>
> >> I thought the main point of moving to SIGTRAP was to preserve register
> state?
> >>
> >> That said, there are probably places where we care more about the
> message than the registers.
> >>
> >> - Saam
> >>
> >>> On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
> >>>
> >>> Is there a reason why RELEASE_ASSERT (and friends) does not call
> WTFReportAssertionFailure() to report where the assertion occur?  Is this
> purely to save memory?  svn blame tells me that it has been this way since
> the introduction of RELEASE_ASSERT in r140577 many years ago.
> >>>
> >>> Would anyone object to adding a call to WTFReportAssertionFailure() in
> RELEASE_ASSERT() like we do for ASSERT()?  One of the upside (side-effect)
> of adding this call is that it appears to stop the compiler from
> aggregating all the RELEASE_ASSERTS into a single code location, and this
> will help with post-mortem crash debugging.
> >>>
> >>> Any thoughts?
> >>>
> >>> Mark
> >>>
> >>> ___
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> >>> https://lists.webkit.org/mailman/listinfo/webkit-dev
> >>
> >
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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Filip Pizlo]

I disagree actually.  I've lost countless hours to converting this:

RELEASE_ASSERT(blah)

into this:

if (!blah) {
   dataLog("Reason why I crashed");
   RELEASE_ASSERT_NOT_REACHED();
}

Look in the code - you'll find lots of stuff like this.

I don't think analyzing register state at crashes is more important than 
keeping our code sane.

-Filip


> On Feb 21, 2017, at 5:56 PM, Mark Lam  wrote:
> 
> Oh yeah, I forgot about that.  I think the register state is more important 
> for crash analysis, especially if we can make sure that the compiler does not 
> aggregate the int3s.  I’ll explore alternatives.
> 
>> On Feb 21, 2017, at 5:54 PM, Saam barati  wrote:
>> 
>> I thought the main point of moving to SIGTRAP was to preserve register state?
>> 
>> That said, there are probably places where we care more about the message 
>> than the registers.
>> 
>> - Saam
>> 
>>> On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
>>> 
>>> Is there a reason why RELEASE_ASSERT (and friends) does not call 
>>> WTFReportAssertionFailure() to report where the assertion occur?  Is this 
>>> purely to save memory?  svn blame tells me that it has been this way since 
>>> the introduction of RELEASE_ASSERT in r140577 many years ago.
>>> 
>>> Would anyone object to adding a call to WTFReportAssertionFailure() in 
>>> RELEASE_ASSERT() like we do for ASSERT()?  One of the upside (side-effect) 
>>> of adding this call is that it appears to stop the compiler from 
>>> aggregating all the RELEASE_ASSERTS into a single code location, and this 
>>> will help with post-mortem crash debugging.
>>> 
>>> Any thoughts?
>>> 
>>> Mark
>>> 
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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Mark Lam]

Oh yeah, I forgot about that.  I think the register state is more important for 
crash analysis, especially if we can make sure that the compiler does not 
aggregate the int3s.  I’ll explore alternatives.

> On Feb 21, 2017, at 5:54 PM, Saam barati  wrote:
> 
> I thought the main point of moving to SIGTRAP was to preserve register state?
> 
> That said, there are probably places where we care more about the message 
> than the registers.
> 
> - Saam
> 
>> On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
>> 
>> Is there a reason why RELEASE_ASSERT (and friends) does not call 
>> WTFReportAssertionFailure() to report where the assertion occur?  Is this 
>> purely to save memory?  svn blame tells me that it has been this way since 
>> the introduction of RELEASE_ASSERT in r140577 many years ago.
>> 
>> Would anyone object to adding a call to WTFReportAssertionFailure() in 
>> RELEASE_ASSERT() like we do for ASSERT()?  One of the upside (side-effect) 
>> of adding this call is that it appears to stop the compiler from aggregating 
>> all the RELEASE_ASSERTS into a single code location, and this will help with 
>> post-mortem crash debugging.
>> 
>> Any thoughts?
>> 
>> Mark
>> 
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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Saam barati]

I thought the main point of moving to SIGTRAP was to preserve register state?

That said, there are probably places where we care more about the message than 
the registers.

- Saam

> On Feb 21, 2017, at 5:43 PM, Mark Lam  wrote:
> 
> Is there a reason why RELEASE_ASSERT (and friends) does not call 
> WTFReportAssertionFailure() to report where the assertion occur?  Is this 
> purely to save memory?  svn blame tells me that it has been this way since 
> the introduction of RELEASE_ASSERT in r140577 many years ago.
> 
> Would anyone object to adding a call to WTFReportAssertionFailure() in 
> RELEASE_ASSERT() like we do for ASSERT()?  One of the upside (side-effect) of 
> adding this call is that it appears to stop the compiler from aggregating all 
> the RELEASE_ASSERTS into a single code location, and this will help with 
> post-mortem crash debugging.
> 
> Any thoughts?
> 
> Mark
> 
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Re: [webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Filip Pizlo]

+1

-Filip

> On Feb 21, 2017, at 17:43, Mark Lam  wrote:
> 
> Is there a reason why RELEASE_ASSERT (and friends) does not call 
> WTFReportAssertionFailure() to report where the assertion occur?  Is this 
> purely to save memory?  svn blame tells me that it has been this way since 
> the introduction of RELEASE_ASSERT in r140577 many years ago.
> 
> Would anyone object to adding a call to WTFReportAssertionFailure() in 
> RELEASE_ASSERT() like we do for ASSERT()?  One of the upside (side-effect) of 
> adding this call is that it appears to stop the compiler from aggregating all 
> the RELEASE_ASSERTS into a single code location, and this will help with 
> post-mortem crash debugging.
> 
> Any thoughts?
> 
> Mark
> 
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[webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Mark Lam]

Is there a reason why RELEASE_ASSERT (and friends) does not call 
WTFReportAssertionFailure() to report where the assertion occur?  Is this 
purely to save memory?  svn blame tells me that it has been this way since the 
introduction of RELEASE_ASSERT in r140577 many years ago.

Would anyone object to adding a call to WTFReportAssertionFailure() in 
RELEASE_ASSERT() like we do for ASSERT()?  One of the upside (side-effect) of 
adding this call is that it appears to stop the compiler from aggregating all 
the RELEASE_ASSERTS into a single code location, and this will help with 
post-mortem crash debugging.

Any thoughts?

Mark

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