[PATCH] D127189: [clang][AIX] Add option to control quadword lock free atomics ABI on AIX

[ChenZheng via Phabricator via cfe-commits]

shchenz added inline comments.


================
Comment at: clang/include/clang/Driver/Options.td:3611
   HelpText<"Enable the default Altivec ABI on AIX (AIX only). Uses only volatile vector registers.">;
+def maix_quadword_atomics : Flag<["-"], "maix64-quadword-atomics">,
+  Group<m_Group>, Flags<[CC1Option]>,
----------------
lkail wrote:
> shchenz wrote:
> > lkail wrote:
> > > shchenz wrote:
> > > > amyk wrote:
> > > > > Would it be better if we called this `maix64-quadword-atomics` instead? 
> > > > Do we need to change the backend check below too?
> > > > ```
> > > > bool PPCTargetLowering::shouldInlineQuadwordAtomics() const {
> > > >   // TODO: 16-byte atomic type support for AIX is in progress; we should be able
> > > >   // to inline 16-byte atomic ops on AIX too in the future.
> > > >   return Subtarget.isPPC64() &&
> > > >          (EnableQuadwordAtomics || !Subtarget.getTargetTriple().isOSAIX()) &&
> > > >          Subtarget.hasQuadwordAtomics();
> > > > }
> > > > ```
> > > We don't need to change this yet. When we are compiling a quadword lock free libatomic, we use options `-mabi=quadword-atomics -mllvm -ppc-quadword-atomics` to enforce generating quadword lock-free code on AIX.
> > This makes me confuse. We need to two different options to control the frontend and backend behavior?
> > 
> > Is it the final usage? Or we will add a follow up patch to map the backend one to the FE one? IMO finally we only need the driver option `-mabi=quadword-atomics` to control the final code generation for 128 bit atomic operations, right?
> > This makes me confuse. We need to two different options to control the frontend and backend behavior?
> 
> This is multi-lang support consideration. clang is not the only frontend we have using LLVM as backend on AIX. If other language frontend generates `store atomic i128, ...`, the backend is supposed to generate libcalls into libatomic currently.
> 
> > Is it the final usage?
> No. We finally want to achieve `-mabi=quadword-atomics` by default and generate inline atomic code for cpu above pwr7 by default(no need to take OS into consideration).
I know what you mean. But I assume the driver option `-mabi=quadword-atomics` will impact the assembly instead of just impact the frontend, right? Using `-mllvm` option is not right as the final solution.

There are some driver options example, like `-gstrict-dwarf`, Frontend can control the backend behavior and the backend can also change this option by `-strict-dwarf`.

Could you please explain:
1: how the backend will handle `-mabi=quadword-atomics` in future?
2: on what condition, we can start to remove below TODOs:
```
bool PPCTargetLowering::shouldInlineQuadwordAtomics() const {
// TODO: 16-byte atomic type support for AIX is in progress;
}
```

```
PPC64TargetInfo::setMaxAtomicWidth() {
    // TODO: We should allow AIX to inline quadword atomics in the future.
}
```


Repository:
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  https://reviews.llvm.org/D127189/new/

https://reviews.llvm.org/D127189