aaron.ballman added inline comments.
================
Comment at: clang/include/clang/Basic/AttrDocs.td:1416-1417
+not significant. This allows global constants with the same contents to be
+merged. This can break global pointer identity, i.e. two different globals have
+the same address.
+
----------------
aeubanks wrote:
> rnk wrote:
> > aaron.ballman wrote:
> > > aeubanks wrote:
> > > > rnk wrote:
> > > > > erichkeane wrote:
> > > > > > aeubanks wrote:
> > > > > > > erichkeane wrote:
> > > > > > > > aeubanks wrote:
> > > > > > > > > aaron.ballman wrote:
> > > > > > > > > > aeubanks wrote:
> > > > > > > > > > > aaron.ballman wrote:
> > > > > > > > > > > > aeubanks wrote:
> > > > > > > > > > > > > aaron.ballman wrote:
> > > > > > > > > > > > > > What happens for tentative definitions where the
> > > > > > > > > > > > > > value isn't known? e.g.,
> > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > [[clang::unnamed_addr]] int i1, i2;
> > > > > > > > > > > > > > ```
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > What happens if the types are similar but not the
> > > > > > > > > > > > > > same?
> > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > [[clang::unnamed_addr]] signed int i1 = 32;
> > > > > > > > > > > > > > [[clang::unnamed_addr]] unsigned int i2 = 32;
> > > > > > > > > > > > > > ```
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Should we diagnose taking the address of such an
> > > > > > > > > > > > > > attributed variable so users have some hope of
> > > > > > > > > > > > > > spotting the non-conforming situations?
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Does this attribute have impacts across translation
> > > > > > > > > > > > > > unit boundaries (perhaps only when doing LTO) or
> > > > > > > > > > > > > > only within a single TU?
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > What does this attribute do in C++ in the presence
> > > > > > > > > > > > > > of constructors and destructors? e.g.,
> > > > > > > > > > > > > > ```
> > > > > > > > > > > > > > struct S {
> > > > > > > > > > > > > > S();
> > > > > > > > > > > > > > ~S();
> > > > > > > > > > > > > > };
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > [[clang::unnamed_addr]] S s1, s2; // Are these
> > > > > > > > > > > > > > merged and there's only one ctor/dtor call?
> > > > > > > > > > > > > > ```
> > > > > > > > > > > > > globals are only mergeable if they're known to be
> > > > > > > > > > > > > constant and have the same value/size. this can be
> > > > > > > > > > > > > done at compile time only if the optimizer can see
> > > > > > > > > > > > > the constant values, or at link time
> > > > > > > > > > > > >
> > > > > > > > > > > > > so nothing would happen in any of the cases you've
> > > > > > > > > > > > > given.
> > > > > > > > > > > > >
> > > > > > > > > > > > > but yeah that does imply that we should warn when the
> > > > > > > > > > > > > attribute is used on non const, non-POD globals. I'll
> > > > > > > > > > > > > update this patch to do that
> > > > > > > > > > > > >
> > > > > > > > > > > > > as mentioned in the description, we actually do want
> > > > > > > > > > > > > to take the address of these globals for table-driven
> > > > > > > > > > > > > parsing, but we don't care about identity equality
> > > > > > > > > > > > > globals are only mergeable if they're known to be
> > > > > > > > > > > > > constant and have the same value/size. this can be
> > > > > > > > > > > > > done at compile time only if the optimizer can see
> > > > > > > > > > > > > the constant values, or at link time
> > > > > > > > > > > > >
> > > > > > > > > > > > > so nothing would happen in any of the cases you've
> > > > > > > > > > > > > given.
> > > > > > > > > > > >
> > > > > > > > > > > > Ahhhh that's good to know. So I assume we *will* merge
> > > > > > > > > > > > these?
> > > > > > > > > > > >
> > > > > > > > > > > > ```
> > > > > > > > > > > > struct S {
> > > > > > > > > > > > int i, j;
> > > > > > > > > > > > float f;
> > > > > > > > > > > > };
> > > > > > > > > > > >
> > > > > > > > > > > > [[clang::unnamed_addr]] const S s1 = { 1, 2, 3.0f };
> > > > > > > > > > > > [[clang::unnamed_addr]] const S s2 = { 1, 2, 3.0f };
> > > > > > > > > > > > [[clang::unnamed_addr]] const S s3 = s2;
> > > > > > > > > > > > ```
> > > > > > > > > > > >
> > > > > > > > > > > > > but yeah that does imply that we should warn when the
> > > > > > > > > > > > > attribute is used on non const, non-POD globals. I'll
> > > > > > > > > > > > > update this patch to do that
> > > > > > > > > > > >
> > > > > > > > > > > > Thank you, I think that will be more user-friendly
> > > > > > > > > > > >
> > > > > > > > > > > > > as mentioned in the description, we actually do want
> > > > > > > > > > > > > to take the address of these globals for table-driven
> > > > > > > > > > > > > parsing, but we don't care about identity equality
> > > > > > > > > > > >
> > > > > > > > > > > > Yeah, I still wonder if we want to diagnose just the
> > > > > > > > > > > > same -- if the address is never taken, there's not
> > > > > > > > > > > > really a way to notice the optimization, but if the
> > > > > > > > > > > > address is taken, you basically get UB (and I think we
> > > > > > > > > > > > should explicitly document it as such). Given how easy
> > > > > > > > > > > > it is to accidentally take the address of something
> > > > > > > > > > > > (like via a reference in C++), I think we should warn
> > > > > > > > > > > > by default, but still have a warning group for folks
> > > > > > > > > > > > who want to live life dangerously.
> > > > > > > > > > > > > globals are only mergeable if they're known to be
> > > > > > > > > > > > > constant and have the same value/size. this can be
> > > > > > > > > > > > > done at compile time only if the optimizer can see
> > > > > > > > > > > > > the constant values, or at link time
> > > > > > > > > > > > >
> > > > > > > > > > > > > so nothing would happen in any of the cases you've
> > > > > > > > > > > > > given.
> > > > > > > > > > > >
> > > > > > > > > > > > Ahhhh that's good to know. So I assume we *will* merge
> > > > > > > > > > > > these?
> > > > > > > > > > > >
> > > > > > > > > > > > ```
> > > > > > > > > > > > struct S {
> > > > > > > > > > > > int i, j;
> > > > > > > > > > > > float f;
> > > > > > > > > > > > };
> > > > > > > > > > > >
> > > > > > > > > > > > [[clang::unnamed_addr]] const S s1 = { 1, 2, 3.0f };
> > > > > > > > > > > > [[clang::unnamed_addr]] const S s2 = { 1, 2, 3.0f };
> > > > > > > > > > > > [[clang::unnamed_addr]] const S s3 = s2;
> > > > > > > > > > > > ```
> > > > > > > > > > > yeah those are merged even just by clang
> > > > > > > > > > >
> > > > > > > > > > > ```
> > > > > > > > > > > struct S {
> > > > > > > > > > > int i, j;
> > > > > > > > > > > float f;
> > > > > > > > > > > };
> > > > > > > > > > >
> > > > > > > > > > > [[clang::unnamed_addr]] const S s1 = { 1, 2, 3.0f };
> > > > > > > > > > > [[clang::unnamed_addr]] const S s2 = { 1, 2, 3.0f };
> > > > > > > > > > > [[clang::unnamed_addr]] const S s3 = s2;
> > > > > > > > > > >
> > > > > > > > > > > const void * f1() {
> > > > > > > > > > > return &s1;
> > > > > > > > > > > }
> > > > > > > > > > >
> > > > > > > > > > > const void * f2() {
> > > > > > > > > > > return &s2;
> > > > > > > > > > > }
> > > > > > > > > > >
> > > > > > > > > > > const void * f3() {
> > > > > > > > > > > return &s3;
> > > > > > > > > > > }
> > > > > > > > > > >
> > > > > > > > > > > $ ./build/rel/bin/clang++ -S -emit-llvm -o - -O2 /tmp/a.cc
> > > > > > > > > > > ```
> > > > > > > > > > > >
> > > > > > > > > > > > > but yeah that does imply that we should warn when the
> > > > > > > > > > > > > attribute is used on non const, non-POD globals. I'll
> > > > > > > > > > > > > update this patch to do that
> > > > > > > > > > > >
> > > > > > > > > > > > Thank you, I think that will be more user-friendly
> > > > > > > > > > > >
> > > > > > > > > > > > > as mentioned in the description, we actually do want
> > > > > > > > > > > > > to take the address of these globals for table-driven
> > > > > > > > > > > > > parsing, but we don't care about identity equality
> > > > > > > > > > > >
> > > > > > > > > > > > Yeah, I still wonder if we want to diagnose just the
> > > > > > > > > > > > same -- if the address is never taken, there's not
> > > > > > > > > > > > really a way to notice the optimization, but if the
> > > > > > > > > > > > address is taken, you basically get UB (and I think we
> > > > > > > > > > > > should explicitly document it as such). Given how easy
> > > > > > > > > > > > it is to accidentally take the address of something
> > > > > > > > > > > > (like via a reference in C++), I think we should warn
> > > > > > > > > > > > by default, but still have a warning group for folks
> > > > > > > > > > > > who want to live life dangerously.
> > > > > > > > > > >
> > > > > > > > > > > I don't understand how there would be any UB. Especially
> > > > > > > > > > > if the user only dereferences them, and isn't comparing
> > > > > > > > > > > pointers, which is the requested use case.
> > > > > > > > > > > yeah those are merged even just by clang
> > > > > > > > > >
> > > > > > > > > > Nice, thank you for the confirmation!
> > > > > > > > > >
> > > > > > > > > > > I don't understand how there would be any UB. Especially
> > > > > > > > > > > if the user only dereferences them, and isn't comparing
> > > > > > > > > > > pointers, which is the requested use case.
> > > > > > > > > >
> > > > > > > > > > That's just it -- nothing prevents the user from taking the
> > > > > > > > > > address and comparing the pointers, which is no longer
> > > > > > > > > > defined behavior with this attribute. It would require a
> > > > > > > > > > static analysis check to catch this problem unless the
> > > > > > > > > > compiler statically warns on taking the address in the
> > > > > > > > > > first place (IMO, we shouldn't assume users will use the
> > > > > > > > > > attribute properly and thus need no help to do so). I was
> > > > > > > > > > also thinking about things like accidental sharing across
> > > > > > > > > > thread boundaries -- but perhaps that's fine because the
> > > > > > > > > > data is constant. I was also thinking that this potentially
> > > > > > > > > > breaks `restrict` semantics but on reflection... that seems
> > > > > > > > > > almost intentional given the goal of the attribute. But
> > > > > > > > > > things along these lines are what have me worried -- the
> > > > > > > > > > language assumes unique locations for objects, so I expect
> > > > > > > > > > there's going to be fallout when object locations are no
> > > > > > > > > > longer unique. If we can remove sharp edges for users
> > > > > > > > > > without compromising the utility of the attribute, I think
> > > > > > > > > > that's beneficial. Or are you saying that warning like this
> > > > > > > > > > would basically compromise the utility?
> > > > > > > > > > > I don't understand how there would be any UB. Especially
> > > > > > > > > > > if the user only dereferences them, and isn't comparing
> > > > > > > > > > > pointers, which is the requested use case.
> > > > > > > > > >
> > > > > > > > > > That's just it -- nothing prevents the user from taking the
> > > > > > > > > > address and comparing the pointers, which is no longer
> > > > > > > > > > defined behavior with this attribute. It would require a
> > > > > > > > > > static analysis check to catch this problem unless the
> > > > > > > > > > compiler statically warns on taking the address in the
> > > > > > > > > > first place (IMO, we shouldn't assume users will use the
> > > > > > > > > > attribute properly and thus need no help to do so). I was
> > > > > > > > > > also thinking about things like accidental sharing across
> > > > > > > > > > thread boundaries -- but perhaps that's fine because the
> > > > > > > > > > data is constant. I was also thinking that this potentially
> > > > > > > > > > breaks `restrict` semantics but on reflection... that seems
> > > > > > > > > > almost intentional given the goal of the attribute. But
> > > > > > > > > > things along these lines are what have me worried -- the
> > > > > > > > > > language assumes unique locations for objects, so I expect
> > > > > > > > > > there's going to be fallout when object locations are no
> > > > > > > > > > longer unique. If we can remove sharp edges for users
> > > > > > > > > > without compromising the utility of the attribute, I think
> > > > > > > > > > that's beneficial. Or are you saying that warning like this
> > > > > > > > > > would basically compromise the utility?
> > > > > > > > >
> > > > > > > > > when you say "undefined behavior" do you mean "it's
> > > > > > > > > unspecified what happens" or literally the C/C++ "undefined
> > > > > > > > > behavior" where the compiler can assume it doesn't happen?
> > > > > > > > >
> > > > > > > > > I don't think there's any UB in the C/C++ "undefined
> > > > > > > > > behavior" sense, we're just dropping a C/C++ guarantee of
> > > > > > > > > unique pointer identity for certain globals.
> > > > > > > > >
> > > > > > > > > Yes I believe the warning would compromise the utility since
> > > > > > > > > the underlying request behind this is a case where the user
> > > > > > > > > explicitly wants to take the address of these globals for
> > > > > > > > > table driven parsing but does not care about unique global
> > > > > > > > > identity. i.e. it's fine if we have duplicate addresses in
> > > > > > > > > the table as long as each entry points to the proper data.
> > > > > > > > I think this IS causing undefined behavior, any program that
> > > > > > > > assumes the addresses aren't the same (such as inserting
> > > > > > > > addresses into a map, explicitly destructing/initializing/etc),
> > > > > > > > or are comparing addresses are now exhibiting UB (in the purest
> > > > > > > > of C++ senses). It isn't the 'taking the address' that is UB,
> > > > > > > > it is comparing them, but unfortunately we don't have a good
> > > > > > > > way to diagnose THAT. I believe what Aaron is getting at is
> > > > > > > > that the taking of the addresses should be diagnosed,
> > > > > > > > particularly if we end up taking said address less-obviously.
> > > > > > > >
> > > > > > > > It DOES have to be a Static Analysis type diagnostic however,
> > > > > > > > since I don't think it is accurate enough.
> > > > > > > >
> > > > > > > perhaps I'm arguing too literally (or am just wrong), but even if
> > > > > > > the program behaves incorrectly due to comparisons of addresses
> > > > > > > now giving different results, that's not necessarily UB. even if
> > > > > > > a program were using pointers as a key into a map and two globals
> > > > > > > that previously were separate entries are now one, that's not
> > > > > > > necessarily UB, that's just a program behaving incorrectly.
> > > > > > >
> > > > > > > unless there's a specific C/C++ rule that you have in mind that
> > > > > > > I'm missing?
> > > > > > >
> > > > > > > I'm happy to add a warning that we can turn off internally if
> > > > > > > people agree that taking the address of an `unnamed_addr` global
> > > > > > > is dangerous in general, not sure if that should be default on or
> > > > > > > off
> > > > > > The simplest one I can think of is something like a :
> > > > > >
> > > > > > ```swap_containers(global1, global2); //swap that takes by ref, but
> > > > > > doesn't check addresses```
> > > > > >
> > > > > > Warnings are obviously trivially disable-able, and I'd be fine with
> > > > > > making it a really fine-grained warning group, but I think the
> > > > > > dangers of this attribute are significant. MANY operations can
> > > > > > have a precondition of "my parameters are different objects" that
> > > > > > this can now cause you to trivially violate in a way that was never
> > > > > > a problem before (since we give them different names!).
> > > > > I think, to Aaron's earlier question, warning on taking the address
> > > > > of such a global kind of defeats the point of the attribute. If you
> > > > > *don't* take the address of the constant global, LLVM will already
> > > > > mark it with `unnamed_addr` or `local_unnamed_addr`, and the
> > > > > attribute is usually unnecessary. You would only reach for this
> > > > > attribute if you are already taking the address of the global.
> > > > >
> > > > > And, to try to elaborate more on the UB situation, I believe LLVM
> > > > > does perform optimizations like `&gv1 == &gv2` -> `false`:
> > > > > https://gcc.godbolt.org/z/3f5qd8vKr
> > > > > This does present a soundness issue, but it's already a bit of an
> > > > > existing issue, since there are other ways to make globals alias,
> > > > > such as aliases.
> > > > ah I misunderstood how ICF worked in regards to `local_unnamed_addr`.
> > > > right now we will ICF globals that don't have their address taken
> > > > across any translation unit.
> > > >
> > > > ```
> > > > $ cat /tmp/a.c
> > > > extern const int h;
> > > > extern const int g;
> > > > const int h = 42;
> > > > const int g = 42;
> > > >
> > > > int f() {
> > > > return h;
> > > > }
> > > >
> > > > $ cat /tmp/b.c
> > > > extern const int g;
> > > >
> > > > int f();
> > > >
> > > > int main() {
> > > > return g + f();
> > > > }
> > > >
> > > > $ clang -o /tmp/a /tmp/a.c /tmp/b.c -O2 -fdata-sections
> > > > -ffunction-sections -fuse-ld=lld -Wl,--icf=safe
> > > > -Wl,--print-icf-sections -Wl,--verbose
> > > > ...
> > > > ld.lld: ICF needed 2 iterations
> > > > selected section
> > > > /usr/local/google/home/aeubanks/tmp/a-da430f.o:(.rodata.h)
> > > > removing identical section
> > > > /usr/local/google/home/aeubanks/tmp/a-da430f.o:(.rodata.g)
> > > > ...
> > > > ```
> > > >
> > > > so yeah +1 to @rnk's comment. this change would only benefit globals
> > > > that have their address taken.
> > > > I think, to Aaron's earlier question, warning on taking the address of
> > > > such a global kind of defeats the point of the attribute. If you
> > > > *don't* take the address of the constant global, LLVM will already mark
> > > > it with unnamed_addr or local_unnamed_addr, and the attribute is
> > > > usually unnecessary. You would only reach for this attribute if you are
> > > > already taking the address of the global.
> > >
> > > Thanks for the information! That somehow makes me even *less* comfortable
> > > with the attribute because the only times you'd reach for it are
> > > precisely the times when it can bite you. Having some sort of static
> > > analysis check paired with it to help give it some guard rails for
> > > problematic uses would make me more comfortable. However, we certainly
> > > have other attributes that are "use at your own peril" without such guard
> > > rails, so I don't insist (but I still strongly encourage), but then I
> > > would insist on more comprehensive documentation calling out what misuse
> > > looks like and what potential problems come from misuse.
> > >
> > > > perhaps I'm arguing too literally (or am just wrong), but even if the
> > > > program behaves incorrectly due to comparisons of addresses now giving
> > > > different results, that's not necessarily UB. even if a program were
> > > > using pointers as a key into a map and two globals that previously were
> > > > separate entries are now one, that's not necessarily UB, that's just a
> > > > program behaving incorrectly.
> > >
> > > The reason why I believe it is UB is because everything else in the
> > > language model believes object addresses are distinct. So there's no way
> > > to determine what the impact is of two objects whose addresses which
> > > should be distinct but aren't, when that's observed by the program. It's
> > > undefined because we have no definition of what happens in this case. The
> > > program could be incorrect and just behave poorly, crash, cause data
> > > loss, etc. (As a concrete example, the `restrict` keyword effectively
> > > boils down to reasoning about whether two pointers overlap; if you pass
> > > the address of two distinct objects, they won't overlap but if they've
> > > been merged with this attribute, now they will overlap and the `restrict`
> > > requirements are broken.)
> > >
> > > Regardless, I wasn't trying to get us hung up on UB as a standards term
> > > of art. I am worried about the user experience with the attribute.
> > > Optimization attributes that silently break program correctness do not
> > > provide a particularly good user experience and so if there's a practical
> > > way we can help users out without negatively impacting correct uses of
> > > the attribute, I think we should implement it. C and C++ already get a
> > > bad rap for safety and security because tooling is unhelpful, so I think
> > > we need to do better than we've done in the past in terms of diagnostic
> > > behavior.
> > So, I think we disagree on the substance of the issue here, and perhaps we
> > should try to get more opinions on Discourse. The feature request really is
> > to have an optimization attribute to mark global constant data as mergeable
> > by the linker, and for the user to promise that the address of the global
> > is not significant (named).
> I'll post something to discourse
Sure, getting more feedback sounds reasonable to me. I should be clear -- I
think I'm raising the bar on this attribute compared to previous optimization
attributes, and that might be where some of this friction comes from. We have
plenty of other optimization-related attributes that are "use at your own risk"
without safety rails. However, I don't think that precedent is reasonable any
longer in the face of growing resistance to using C and C++ for new projects
specifically because tooling doesn't help catch safety and security related
concerns, which are often the result of optimizer assumptions on incorrect
code. So I'm asking for (what I think are) reasonable diagnostics to help
people catch misuse, not trying to ask for heroics.
Repository:
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https://reviews.llvm.org/D158223
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