[llvm-commits] [llvm] r78714 - in /llvm/trunk: lib/Transforms/Scalar/InstructionCombining.cpp test/Transforms/InstCombine/exact-sdiv.ll

[Chris Lattner]

On Aug 11, 2009, at 1:47 PM, Dan Gohman wrote:
> Author: djg
> Date: Tue Aug 11 15:47:47 2009
> New Revision: 78714
>
> URL: http://llvm.org/viewvc/llvm-project?rev=78714&view=rev
> Log:
> Optimize exact sdiv by a constant power of 2 to ashr.

Nice, once llvm-gcc/clang generate these, we should be able to remove  
the "power of two" hack for pointer differences, and this should fix  
an abstraction penalty bugzilla.

-Chris

>
> Added:
>    llvm/trunk/test/Transforms/InstCombine/exact-sdiv.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=78714&r1=78713&r2=78714&view=diff
>
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> ======================================================================
> --- llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp  
> (original)
> +++ llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp Tue  
> Aug 11 15:47:47 2009
> @@ -3064,6 +3064,15 @@
>     // sdiv X, -1 == -X
>     if (RHS->isAllOnesValue())
>       return BinaryOperator::CreateNeg(*Context, Op0);
> +
> +    // sdiv X, C  --> ashr X, log2(C)
> +    if (cast<SDivOperator>(&I)->isExact() &&
> +        RHS->getValue().isNonNegative() &&
> +        RHS->getValue().isPowerOf2()) {
> +      Value *ShAmt = llvm::ConstantInt::get(RHS->getType(),
> +                                            RHS- 
> >getValue().exactLogBase2());
> +      return BinaryOperator::CreateAShr(Op0, ShAmt, I.getName());
> +    }
>   }
>
>   // If the sign bits of both operands are zero (i.e. we can prove  
> they are
>
> Added: llvm/trunk/test/Transforms/InstCombine/exact-sdiv.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/exact-sdiv.ll?rev=78714&view=auto
>
> = 
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> ======================================================================
> --- llvm/trunk/test/Transforms/InstCombine/exact-sdiv.ll (added)
> +++ llvm/trunk/test/Transforms/InstCombine/exact-sdiv.ll Tue Aug 11  
> 15:47:47 2009
> @@ -0,0 +1,15 @@
> +; RUN: llvm-as < %s | opt -instcombine | llvm-dis | FileCheck %s
> +
> +; CHECK: define i32 @foo
> +; CHECK: sdiv i32 %x, 8
> +define i32 @foo(i32 %x) {
> +  %y = sdiv i32 %x, 8
> +  ret i32 %y
> +}
> +
> +; CHECK: define i32 @bar
> +; CHECK: ashr i32 %x, 3
> +define i32 @bar(i32 %x) {
> +  %y = sdiv exact i32 %x, 8
> +  ret i32 %y
> +}
>
>
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