I was on vacation during this this thread, sorry. I'm the one who has been
(sporadically) fixing this in llvm-gcc. As of 119392 I believe local asm
variables are correct there.
On Jan 1, 2011, at 12:15 PM, Jakob Stoklund Olesen wrote:
> On Dec 31, 2010, at 7:05 PM, Chris Lattner wrote:
>> On Dec 31, 2010, at 12:43 PM, Jakob Stoklund Olesen wrote:
>>> 1. Pinned local variables are treated no differently than normal local
>>> variables except when they are used as operands for inline asm. This is
>>> what Rafael's patch implements. I suppose you could warn if such a variable
>>> is never used for inline asm.
>> […]
Also, the register is not pinned to the variable. It is available for normal
allocation except around asm statements.
>>> 2. Global variables pinned to a reserved register. This is just an odd way
>>> of accessing a register, and it is fairly easy to support in the front end
>>> by emitting empty inline asm instead of reads and writes to the global.
>>
>> Right, llvm-gcc does this. This is vaguely bogus but it is (obviously) used
>> and useful to support.
>>
>>> 3. Global variables pinned to an allocatable register. For this to work,
>>> the backend must treat the pinned register as reserved, effectively
>>> changing the calling convention of all functions in the compilation unit.
>>> Other compilation units would have to be built with -ffixed-reg. I really
>>> don't want to support this if it can at all be avoided.
>>
>> […]
Currently, the IR doesn't support a way of representing this. It doesn't work
in llvm-gcc, and I agree it would be good to avoid doing it.
I haven't looked at the patches, but clang should generate the same thing
llvm-gcc generates for the same constructs. If you do that it will work; if
you do anything else you'll probably hit subtle bugs.
>> Interestingly enough, GCC emits a warning:
>>
>> $ cat t.c
>> register int foo asm("rax");
>> void f() {
>> foo = 42;
>> }
>>
>> t.c:1: warning: call-clobbered register used for global register variable
>
> Yes, the documentation says that you should only use callee-saved registers,
> otherwise any library function you call will clobber your global.
>
>> My opinion is that the warning is a (really low priority) QoI issue and that
>> we should implement #2 and forget about people who get burned with #3.
>
> I'm fine with that, but it does mean that we silently miscompile #3 because
> the register can be allocated normally.
>
>> We should not implement -ffixed-reg if possible IMO, though it has been
>> discussed on llvmdev as useful for people who have GC'd languages etc, and I
>> vaguely recall the Haskell folks doing some work in this area. To do this
>> right we'd have to do something like this:
>> http://nondot.org/sabre/LLVMNotes/GlobalRegisterVariables.txt
>
> I would prefer that approach too. It lets the register allocator decide if
> the register global is really more important than all the locals it
> interferes with.
>
>>> We could permit:
>>> asm ("" : "={esp}"(foo));
>>> The curly brace syntax for inline asm constraints also obsoletes pinned
>>> local variables.
>>
>> Lets not extend GCC assembly syntax anymore! :)
>
> Yeah, fair enough.
>
> /jakob
>
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