[llvm] d6de5f1 - [SVFS] Inject TLI Mappings in VFABI attribute.

[Benjamin Kramer via llvm-commits]

On Mon, Jan 27, 2020 at 3:29 PM Francesco Petrogalli
<Francesco.Petrogalli at arm.com> wrote:
>
>
>
> > On Jan 27, 2020, at 7:54 AM, Benjamin Kramer <benny.kra at gmail.com> wrote:
> >
> > On Sat, Nov 16, 2019 at 5:20 PM Francesco Petrogalli via llvm-commits
> > <llvm-commits at lists.llvm.org> wrote:
> >> +  // Make function declaration (without a body) "sticky" in the IR by
> >> +  // listing it in the @llvm.compiler.used intrinsic.
> >> +  assert(!VectorF->size() && "VFABI attribute requires `@llvm.compiler.used` "
> >> +                             "only on declarations.");
> >> +  appendToCompilerUsed(*M, {VectorF});
> >> +  LLVM_DEBUG(dbgs() << DEBUG_TYPE << ": Adding `" << VFName
> >> +                    << "` to `@llvm.compiler.used`.\n");
> >> +  ++NumCompUsedAdded;
> >
> > Sorry for resurrecting an old change, but what's the reason for making
> > functions sticky? In XLA I'm seeing a bunch of functions that get
> > inlined and then stick around unused because of being in
> > llvm.compiler.used without any other users.
> >
> > - Ben
>
>
>
> Hi Ben,
>
> It is needed for function declarations that need to be carried to the vectorizer even if they don’t have any use before vectorization.
>
> The use case is explained in this RFC: http://lists.llvm.org/pipermail/llvm-dev/2019-June/133484.html
>
> I have highlighted the paragraph that describes why we need that.
>
> ```
> The IR attribute is used in conjunction with the vector function
> declarations or definitions that are available in the module. Each
> mangled name in the `vector-function-abi-attribute` is be associated to
> a correspondent declaration/definition in the module. Such definition is
> provided by the front-end. The vector function declaration or definition
> is passed as an argument to the `llvm.compiler.used` intrinsic to
> prevent the compiler from removing it from the module (for example when
> the OpenMP mapping mechanism is used via C header file).
> ```
>
> If this is causing you problems, we might need to rethink the mechanism that ensures the availability of the vector declaration when auto-vectorizing loops.

Thanks for the explanation, but I'm still a bit confused on what's
actually happening. Is it that LLVM removes unused functions before
vectorization eagerly?

- Ben