[llvm] r293537 - GlobalISel: permit unused vregs without a register-class after ISel.

[Quentin Colombet via llvm-commits]

+Ahmed.

I would have expected InstructionSelect to get rid of the unused vregs.

Do you know why this is not happening?

Cheers,
-Quentin
> On Jan 30, 2017, at 11:12 AM, Tim Northover via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Author: tnorthover
> Date: Mon Jan 30 13:12:50 2017
> New Revision: 293537
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=293537&view=rev
> Log:
> GlobalISel: permit unused vregs without a register-class after ISel.
> 
> This can happen if earlier combining has removed all uses of some VReg, which
> is fine and shouldn't flag an error.
> 
> Added:
>    llvm/trunk/test/CodeGen/AArch64/GlobalISel/no-regclass.mir
> Modified:
>    llvm/trunk/lib/CodeGen/GlobalISel/InstructionSelect.cpp
> 
> Modified: llvm/trunk/lib/CodeGen/GlobalISel/InstructionSelect.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/GlobalISel/InstructionSelect.cpp?rev=293537&r1=293536&r2=293537&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/GlobalISel/InstructionSelect.cpp (original)
> +++ llvm/trunk/lib/CodeGen/GlobalISel/InstructionSelect.cpp Mon Jan 30 13:12:50 2017
> @@ -141,15 +141,19 @@ bool InstructionSelect::runOnMachineFunc
>   for (auto &VRegToType : MRI.getVRegToType()) {
>     unsigned VReg = VRegToType.first;
>     auto *RC = MRI.getRegClassOrNull(VReg);
> -    auto *MI = MRI.def_instr_begin(VReg) == MRI.def_instr_end()
> -                   ? nullptr
> -                   : &*MRI.def_instr_begin(VReg);
> -    if (!RC) {
> +    MachineInstr *MI = nullptr;
> +    if (MRI.def_instr_begin(VReg) != MRI.def_instr_end())
> +      MI = &*MRI.def_instr_begin(VReg);
> +    else if (MRI.use_instr_begin(VReg) != MRI.use_instr_end())
> +      MI = &*MRI.use_instr_begin(VReg);
> +
> +    if (MI && !RC) {
>       if (TPC.isGlobalISelAbortEnabled())
>         reportSelectionError(MF, MI, "VReg has no regclass after selection");
>       Failed = true;
>       break;
> -    }
> +    } else if (!RC)
> +      continue;
> 
>     if (VRegToType.second.isValid() &&
>         VRegToType.second.getSizeInBits() > (RC->getSize() * 8)) {
> 
> Added: llvm/trunk/test/CodeGen/AArch64/GlobalISel/no-regclass.mir
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/GlobalISel/no-regclass.mir?rev=293537&view=auto
> ==============================================================================
> --- llvm/trunk/test/CodeGen/AArch64/GlobalISel/no-regclass.mir (added)
> +++ llvm/trunk/test/CodeGen/AArch64/GlobalISel/no-regclass.mir Mon Jan 30 13:12:50 2017
> @@ -0,0 +1,30 @@
> +# RUN: llc -O0 -mtriple=aarch64-apple-ios -global-isel -start-before=legalizer -stop-after=instruction-select %s -o - | FileCheck %s
> +
> +# We run the legalizer to combine the trivial EXTRACT_SEQ pair, leaving %1 and
> +# %2 orphaned after instruction-selection (no instructions define or use
> +# them). This shouldn't be a problem.
> +
> +--- |
> +  target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
> +
> +  define void @unused_reg() { ret void }
> +
> +---
> +# CHECK-LABEL: name: unused_reg
> +name:            unused_reg
> +legalized:       true
> +regBankSelected: true
> +tracksRegLiveness: true
> +
> +# CHECK:      body:
> +# CHECK:  %0 = COPY %w0
> +# CHECK:  %w0 = COPY %0
> +
> +body:             |
> +  bb.0:
> +    liveins: %w0
> +    %0:gpr(s32) = COPY %w0
> +    %1:gpr(s32) = G_SEQUENCE %0(s32), 0
> +    %2:gpr(s32) = G_EXTRACT %1(s32), 0
> +    %w0 = COPY %2(s32)
> +...
> 
> 
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