[llvm] r323369 - [GlobalISel] Don't fall back to FastISel.

[Quentin Colombet via llvm-commits]

Hi Amara,

Falling back to FastISel was intended to get the best performance from the fallback path.

Why are we doing this change now?

Cheers,
-Quentin

> On Jan 24, 2018, at 11:59 AM, Amara Emerson via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Author: aemerson
> Date: Wed Jan 24 11:59:29 2018
> New Revision: 323369
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=323369&view=rev
> Log:
> [GlobalISel] Don't fall back to FastISel.
> 
> Apparently checking the pass structure isn't enough to ensure that we don't fall
> back to FastISel, as it's set up as part of the SelectionDAGISel.
> 
> Added:
>    llvm/trunk/test/CodeGen/AArch64/GlobalISel/fallback-nofastisel.ll
> Modified:
>    llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
>    llvm/trunk/lib/CodeGen/TargetPassConfig.cpp
> 
> Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp?rev=323369&r1=323368&r2=323369&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp (original)
> +++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp Wed Jan 24 11:59:29 2018
> @@ -1380,8 +1380,10 @@ void SelectionDAGISel::SelectAllBasicBlo
>   FastISelFailed = false;
>   // Initialize the Fast-ISel state, if needed.
>   FastISel *FastIS = nullptr;
> -  if (TM.Options.EnableFastISel)
> +  if (TM.Options.EnableFastISel) {
> +    DEBUG(dbgs() << "Enabling fast-isel\n");
>     FastIS = TLI->createFastISel(*FuncInfo, LibInfo);
> +  }
> 
>   setupSwiftErrorVals(Fn, TLI, FuncInfo);
> 
> 
> Modified: llvm/trunk/lib/CodeGen/TargetPassConfig.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/TargetPassConfig.cpp?rev=323369&r1=323368&r2=323369&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/TargetPassConfig.cpp (original)
> +++ llvm/trunk/lib/CodeGen/TargetPassConfig.cpp Wed Jan 24 11:59:29 2018
> @@ -708,6 +708,8 @@ bool TargetPassConfig::addCoreISelPasses
>   if (EnableGlobalISelOption == cl::BOU_TRUE ||
>       (EnableGlobalISelOption == cl::BOU_UNSET &&
>        TM->Options.EnableGlobalISel && EnableFastISelOption != cl::BOU_TRUE)) {
> +    TM->setFastISel(false);
> +
>     if (addIRTranslator())
>       return true;
> 
> 
> Added: llvm/trunk/test/CodeGen/AArch64/GlobalISel/fallback-nofastisel.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/GlobalISel/fallback-nofastisel.ll?rev=323369&view=auto
> ==============================================================================
> --- llvm/trunk/test/CodeGen/AArch64/GlobalISel/fallback-nofastisel.ll (added)
> +++ llvm/trunk/test/CodeGen/AArch64/GlobalISel/fallback-nofastisel.ll Wed Jan 24 11:59:29 2018
> @@ -0,0 +1,10 @@
> +; RUN: llc -mtriple=aarch64_be-- %s -o /dev/null -debug-only=isel -O0 2>&1 | FileCheck %s
> +
> +; This test uses big endian in order to force an abort since it's not currently supported for GISel.
> +; The purpose is to check that we don't fall back to FastISel. Checking the pass structure is insufficient
> +; because the FastISel is set up in the SelectionDAGISel, so it doesn't appear on the pass structure.
> +
> +; CHECK-NOT: Enabling fast-ise
> +define void @empty() {
> +  ret void
> +}
> 
> 
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