Make 16-byte atomic type aligned to 16-byte on PPC64, thus consistent with GCC. Also enable inlining 16-byte atomics on non-AIX targets on PPC64.
Details
Details
- Reviewers
hubert.reinterpretcast jsji xingxue dim pkubaj efriedma - Group Reviewers
Restricted Project - Commits
- rG549e118e93c6: [PowerPC] Support 16-byte lock free atomics on pwr8 and up
Diff Detail
Diff Detail
- Repository
- rG LLVM Github Monorepo
Event Timeline
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 459 | Many isOSLinux() usage may be really isOSBinFormatELF() (to help FreeBSD, etc). Can you check whether that's the case? | |
Comment Actions
@dim @pkubaj We are modifying layout of 16-byte atomic type on Linux to be consistent with GCC, I don't know if it also applies to freebsd as @MaskRay pointed out.
To be more specific, on Linux, for arch level supporting 16-byte lock free atomics, 16-byte atomic type should be properly aligned.
#include <stdatomic.h>
int printf(const char *, ...);
typedef struct A {
char x[16];
} A;
typedef struct B {
char q;
_Atomic(A) a;
} B;
int main(void) {
_Atomic(A) *p = 0;
printf("aligned: %d\n", __builtin_offsetof(B, a) % 16 == 0);
#if __clang__
printf("lock free (size built-in): %d\n", __c11_atomic_is_lock_free(sizeof(*p)));
#endif
printf("lock free (type query using pointer): %d\n", atomic_is_lock_free(p));
}Current clang gives (-mcpu=power8)
aligned: 0 lock free (size built-in): 1 lock free (type query using pointer): 1
GCC gives
aligned: 1 lock free (type query using pointer): 1
This patch also modifies the query of __atomic_always_lock_free(16, 0) to return true for supported arch level.
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 461 | MaxAtomicPromoteWidth should not depend on whether quadword-atomics is present, only the target OS. It determines the layout of _Atomic(__int128_t). (MaxAtomicInlineWidth is allowed to adjust as necessary.) | |
| clang/lib/Driver/ToolChains/Clang.cpp | ||
| 2140 ↗ | (On Diff #418142) | Please avoid using "-mllvm" options like this; among other issues, it doesn't work with LTO. If we need to indicate this, please use a function attribute in the IR. (Yes, there are other cases where we translate flags to -mllvm options, but it's not something you should copy.) |
Comment Actions
Thanks @efriedma for pointing out. Updated guard code. Removed -mllvm while adjusting backend to reflect ABI change.
| llvm/lib/Target/PowerPC/PPCISelLowering.cpp | ||
|---|---|---|
| 1329 | You probably want an "else" here to call setMaxAtomicSizeInBitsSupported(64) or something like that. But you can leave that to a followup patch if you want. | |
| clang/test/CodeGen/PowerPC/atomic-alignment.c | ||
|---|---|---|
| 45 | Although it is a change that affects layout compatibility with the short history of 16-byte atomic types compiled with LLVM-based compilers on AIX, having this 16-byte atomic type aligned also on AIX for PPC64 would be reasonable. The consequences of not doing so is that operations on some types would be lock-free only on ELF-based platforms. | |
| clang/test/CodeGen/PowerPC/atomic-alignment.c | ||
|---|---|---|
| 35 | Just noting that GCC increases the alignment even for ppc32: typedef struct A8 { char x[8]; } A8;
typedef struct A16 { char x[16]; } A16;
extern char q8[_Alignof(_Atomic(A8))], q8[8]; // okay for GCC targeting ppc32
extern char q16[_Alignof(_Atomic(A16))], q16[16]; // okay for GCC targeting ppc32Apparently, the change for i686 in GCC occurred with version 11. | |
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 458 | I believe this should be presented more as this override being implemented currently only for ELF targets. The current presentation seems to imply more design intent for non-ELF targets than there is consensus for. For example: if (!getTriple().isOSBinFormatELF()) return PPCTargetInfo::setMaxAtomicWidth(); | |
| clang/test/Sema/atomic-ops.c | ||
| 13 | This will need a separate patch to cover ppc32 (likely with AIX). | |
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 458 | It looks a chance to adjust according to arch level to me(Considering we finally will make xcoff and elf targets consistent here). If you have strong opinion on this, I'll make a change here. | |
| clang/test/CodeGen/PowerPC/atomic-alignment.c | ||
| 35 | I'll post another patch to address ppc32 issue. | |
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 458 | This function currently modifies two different things. One that should be arch-level-sensitive (and, iiuc, is already so) and one that should not be arch-level-sensitive. So, the isOSBinFormatELF checks are here mainly to scope the patch (temporarily). I only suggested to use the call to the base class as a shorthand for "nothing special that this function is doing". The associated comment should read something like "Restrict 16-byte atomic changes to ELF targets for now. TODO: Consider other targets as well." At the very least (for this patch), I do not believe the isOSBinFormatELF should be checked twice in terms of control flow. | |
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 458 | The new version of the change already addresses this; thanks. | |
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 443–454 | Clarify that the statement is not for PPC64 in general. | |
| 445 | Clarify that the support up to 8 bytes is for "baseline" PPC64 (i.e., non-baseline implementations may support lock-free inline 16-byte atomic operations). | |
| llvm/lib/Target/PowerPC/PPCISelLowering.cpp | ||
| 18059 | Can we have a comment here to explain the isOSBinFormatELF check? | |
Comment Actions
I am not sure that the choice of isOSBinFormatELF to (afaik) primarily scope this change from affecting AIX (where we know the library calls are not implemented to be lock-free yet) is better than alternative where the condition is for little-endian mode or specifically for not AIX.
Comment Actions
I agree with we should exclude AIX explicitly(use !isOSAIX() rather than isOSBinFormatELF()) for current situation where AIX's support is on progress as we have known. What is unknown at present is the impact on freebsd.
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 440 | I don't understand what DataLayout does mean, but I'm wondering why FreeBSD doesn't append this as well. Is it expected to be AIX() and Linux only? | |
| clang/test/CodeGen/PowerPC/atomic-alignment.c | ||
|---|---|---|
| 15 | I was recently informed that, instead of using expected-no-diagnostics and -verify, using -Werror achieves the same with less processing cost. Sorry for the churn. Noting also that expected-no-diagnostics only needs to be added once the the file. | |
| clang/test/CodeGen/PowerPC/quadword-atomics.c | ||
| 6 | Can we add an AIX run line for pwr7? It should match the PPC64 check lines. | |
| 43–45 | The pointer cast is avoidable; let's avoid it. | |
| llvm/lib/Target/PowerPC/PPCISelLowering.cpp | ||
| 18057 | Typo fix; grammar fix. | |
| llvm/test/CodeGen/PowerPC/atomics-i128.ll | ||
| 17 | I think this case deserves an extra comment to explain the -ppc-quadword-atomics. | |
| 81 | What library is this expected to provide this symbol? | |
| clang/lib/Basic/Targets/PPC.h | ||
|---|---|---|
| 440 | As maskray has pointed out, usually using isOSBinFormatELF rather than isOSLinux can cover freebsd as well. This snippet of code might be beyond this patch, we might post another one to address this issue. | |
| llvm/test/CodeGen/PowerPC/atomics-i128.ll | ||
|---|---|---|
| 81 | As far as I know, should be none on AIX. This issue is fixed in https://reviews.llvm.org/D122868. | |
Comment Actions
LGTM with minor comments.
| clang/test/CodeGen/PowerPC/atomic-alignment.c | ||
|---|---|---|
| 1–13 | Use -Werror in place of -verify with no diagnostics. | |
| clang/test/CodeGen/PowerPC/quadword-atomics.c | ||
| 2–7 | Same comment as for the other file. | |
| llvm/lib/Target/PowerPC/PPCISelLowering.cpp | ||
| 18057 | Minor nit: Use semicolon. | |
| llvm/test/CodeGen/PowerPC/atomics-i128.ll | ||
| 160 | I have verified that the registers in the pairs are used correctly for this case. I've skimmed the other cases for the instructions applied to the loaded value (or, for non-inline cases, the functions called). I did not check that the set-up for the calls, etc. I also haven't looked into the memory barrier instruction usage (but that should be common with other widths). | |
I don't understand what DataLayout does mean, but I'm wondering why FreeBSD doesn't append this as well. Is it expected to be AIX() and Linux only?