[llvm-commits] [llvm] r91675 - in /llvm/trunk: lib/Transforms/Scalar/InstructionCombining.cpp test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll

[Chris Lattner]

On Dec 18, 2009, at 12:22 AM, Eli Friedman wrote:
Author: efriedma
> Date: Fri Dec 18 02:22:35 2009
> New Revision: 91675
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=91675&view=rev
> Log:
> Optimize icmp of null and select of two constants even if the select has
> multiple uses.  (The construct in question was found in gcc.)

Nifty, this might be a good use for SimplifyCmpInst, instead of special casing constants.

-Chris

> 
> 
> Added:
>    llvm/trunk/test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll
> Modified:
>    llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
> 
> Modified: llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp?rev=91675&r1=91674&r2=91675&view=diff
> 
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp Fri Dec 18 02:22:35 2009
> @@ -6356,24 +6356,26 @@
>         // 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::getICmp(I.getPredicate(), C, RHSC);
> -            // Insert a new ICmp of the other select operand.
> -            Op2 = Builder->CreateICmp(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::getICmp(I.getPredicate(), C, RHSC);
> -            // Insert a new ICmp of the other select operand.
> +        if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(1)))
> +          Op1 = ConstantExpr::getICmp(I.getPredicate(), C, RHSC);
> +        if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(2)))
> +          Op2 = ConstantExpr::getICmp(I.getPredicate(), C, RHSC);
> +
> +        // We only want to perform this transformation if it will not lead to
> +        // additional code. This is true if either both sides of the select
> +        // fold to a constant (in which case the icmp is replaced with a select
> +        // which will usually simplify) or this is the only user of the
> +        // select (in which case we are trading a select+icmp for a simpler
> +        // select+icmp).
> +        if ((Op1 && Op2) || (LHSI->hasOneUse() && (Op1 || Op2))) {
> +          if (!Op1)
>             Op1 = Builder->CreateICmp(I.getPredicate(), LHSI->getOperand(1),
>                                       RHSC, I.getName());
> -          }
> -        }
> -
> -        if (Op1)
> +          if (!Op2)
> +            Op2 = Builder->CreateICmp(I.getPredicate(), LHSI->getOperand(2),
> +                                      RHSC, I.getName());
>           return SelectInst::Create(LHSI->getOperand(0), Op1, Op2);
> +        }
>         break;
>       }
>       case Instruction::Call:
> 
> Added: llvm/trunk/test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll?rev=91675&view=auto
> 
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll (added)
> +++ llvm/trunk/test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll Fri Dec 18 02:22:35 2009
> @@ -0,0 +1,16 @@
> +; RUN: opt < %s -instcombine -S | FileCheck %s
> +
> + at .str254 = internal constant [2 x i8] c".\00"
> + at .str557 = internal constant [3 x i8] c"::\00"
> +
> +define i8* @demangle_qualified(i32 %isfuncname) nounwind {
> +entry:
> +  %tobool272 = icmp ne i32 %isfuncname, 0
> +  %cond276 = select i1 %tobool272, i8* getelementptr inbounds ([2 x i8]* @.str254, i32 0, i32 0), i8* getelementptr inbounds ([3 x i8]* @.str557, i32 0, i32 0) ; <i8*> [#uses=4]
> +  %cmp.i504 = icmp eq i8* %cond276, null
> +  %rval = getelementptr i8* %cond276, i1 %cmp.i504
> +  ret i8* %rval
> +}
> +
> +; CHECK: %cond276 = select i1
> +; CHECK: ret i8* %cond276
> 
> 
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