[llvm] r201275 - Remove a very old instcombine where we would turn sequences of selects into

[Nadav Rotem]

On Feb 12, 2014, at 3:54 PM, Owen Anderson <resistor at mac.com> wrote:

> Author: resistor
> Date: Wed Feb 12 17:54:07 2014
> New Revision: 201275
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=201275&view=rev
> Log:
> Remove a very old instcombine where

For vectors this optimization is always a win.  Can you please bring it back and add a check that the comparison type is a vector?


> we would turn sequences of selects into
> logical operations on the i1's driving them.  This is a bad idea for every
> target I can think of (confirmed with micro tests on all of: x86-64, ARM,
> AArch64, Mips, and PowerPC) because it forces the i1 to be materialized into
> a general purpose register, whereas consuming it directly into a select generally
> allows it to exist only transiently in a predicate or flags register.
> 
> Chandler ran a set of performance tests with this change, and reported no
> measurable change on x86-64.
> 
> Added:
>    llvm/trunk/test/Transforms/InstCombine/select-select.ll
> Modified:
>    llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
> 
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?rev=201275&r1=201274&r2=201275&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Wed Feb 12 17:54:07 2014
> @@ -3313,31 +3313,6 @@ Instruction *InstCombiner::visitFCmpInst
>         if (Instruction *NV = FoldFCmp_IntToFP_Cst(I, LHSI, RHSC))
>           return NV;
>         break;
> -      case Instruction::Select: {
> -        // If either operand of the select is a constant, we can fold the
> -        // comparison into the select arms, which will cause one to be
> -        // constant folded and the select turned into a bitwise or.
> -        Value *Op1 = 0, *Op2 = 0;
> -        if (LHSI->hasOneUse()) {
> -          if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(1))) {
> -            // Fold the known value into the constant operand.
> -            Op1 = ConstantExpr::getCompare(I.getPredicate(), C, RHSC);
> -            // Insert a new FCmp of the other select operand.
> -            Op2 = Builder->CreateFCmp(I.getPredicate(),
> -                                      LHSI->getOperand(2), RHSC, I.getName());
> -          } else if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(2))) {
> -            // Fold the known value into the constant operand.
> -            Op2 = ConstantExpr::getCompare(I.getPredicate(), C, RHSC);
> -            // Insert a new FCmp of the other select operand.
> -            Op1 = Builder->CreateFCmp(I.getPredicate(), LHSI->getOperand(1),
> -                                      RHSC, I.getName());
> -          }
> -        }
> -
> -        if (Op1)
> -          return SelectInst::Create(LHSI->getOperand(0), Op1, Op2);
> -        break;
> -      }
>       case Instruction::FSub: {
>         // fcmp pred (fneg x), C -> fcmp swap(pred) x, -C
>         Value *Op;
> 
> Added: llvm/trunk/test/Transforms/InstCombine/select-select.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/select-select.ll?rev=201275&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/select-select.ll (added)
> +++ llvm/trunk/test/Transforms/InstCombine/select-select.ll Wed Feb 12 17:54:07 2014
> @@ -0,0 +1,24 @@
> +; RUN: opt -instcombine -S < %s | FileCheck %s
> +
> +; CHECK: @foo1
> +define float @foo1(float %a) #0 {
> +; CHECK-NOT: xor
> +  %b = fcmp ogt float %a, 0.000000e+00
> +  %c = select i1 %b, float %a, float 0.000000e+00
> +  %d = fcmp olt float %c, 1.000000e+00
> +  %f = select i1 %d, float %c, float 1.000000e+00
> +  ret float %f
> +}
> +
> +; CHECK: @foo2
> +define float @foo2(float %a) #0 {
> +; CHECK-NOT: xor
> +  %b = fcmp ogt float %a, 0.000000e+00
> +  %c = select i1 %b, float %a, float 0.000000e+00
> +  %d = fcmp olt float %c, 1.000000e+00
> +  %e = select i1 %b, float %a, float 0.000000e+00
> +  %f = select i1 %d, float %e, float 1.000000e+00
> +  ret float %f
> +}
> +
> +attributes #0 = { nounwind readnone ssp uwtable }
> 
> 
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