[llvm-commits] [llvm] r133022 - in /llvm/trunk: lib/CodeGen/SelectionDAG/DAGCombiner.cpp test/CodeGen/X86/shl_undef.ll

[Evan Cheng]

Hi Chad,

I'm not sure I understand this. It seems like the test cases are dependent on undefined behavior? I don't think anything can assume shl undef, 1 lsb is zero. Anything arithmetics involving undef should be safely folded into undef. 

Evan

On Jun 14, 2011, at 3:29 PM, Chad Rosier <mcrosier at apple.com> wrote:

> Author: mcrosier
> Date: Tue Jun 14 17:29:10 2011
> New Revision: 133022
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=133022&view=rev
> Log:
> When pattern matching during instruction selection make sure shl x,1 is not
> converted to add x,x if x is a undef.  add undef, undef does not guarantee
> that the resulting low order bit is zero.
> Fixes <rdar://problem/9453156> and <rdar://problem/9487392>.
> 
> Added:
>    llvm/trunk/test/CodeGen/X86/shl_undef.ll
> Modified:
>    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> 
> Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=133022&r1=133021&r2=133022&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
> +++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Tue Jun 14 17:29:10 2011
> @@ -3030,6 +3030,9 @@
>   // fold (shl x, 0) -> x
>   if (N1C && N1C->isNullValue())
>     return N0;
> +  // fold (shl undef, x) -> 0
> +  if (N0.getOpcode() == ISD::UNDEF)
> +    return DAG.getConstant(0, VT);
>   // if (shl x, c) is known to be zero, return 0
>   if (DAG.MaskedValueIsZero(SDValue(N, 0),
>                             APInt::getAllOnesValue(OpSizeInBits)))
> 
> Added: llvm/trunk/test/CodeGen/X86/shl_undef.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/shl_undef.ll?rev=133022&view=auto
> ==============================================================================
> --- llvm/trunk/test/CodeGen/X86/shl_undef.ll (added)
> +++ llvm/trunk/test/CodeGen/X86/shl_undef.ll Tue Jun 14 17:29:10 2011
> @@ -0,0 +1,53 @@
> +; RUN: llc < %s -O1 -mtriple=i386-apple-darwin | FileCheck %s
> +;
> +; Interesting test case where %tmp1220 = xor i32 %tmp862, %tmp592 and
> +; %tmp1676 = xor i32 %tmp1634, %tmp1530 have zero demanded bits after
> +; TargetLoweringOpt pass.  These are changed to undef and in turn
> +; the successor shl(s) become shl undef, 1.  This pattern then matches
> +; shl x, 1 -> add x, x.  add undef, undef doesn't guarentee the low
> +; order bit is zero and is incorrect.
> +;
> +; See rdar://9453156 and rdar://9487392.
> +;
> +
> +; CHECK-NOT: shl
> +define i32 @foo(i8* %a0, i32* %a2) nounwind {
> +entry:
> +  %tmp0 = alloca i8
> +  %tmp1 = alloca i32
> +  store i8 1, i8* %tmp0
> +  %tmp921.i7845 = load i8* %a0, align 1
> +  %tmp309 = xor i8 %tmp921.i7845, 104
> +  %tmp592 = zext i8 %tmp309 to i32
> +  %tmp862 = xor i32 1293461297, %tmp592
> +  %tmp1220 = xor i32 %tmp862, %tmp592
> +  %tmp1506 = shl i32 %tmp1220, 1
> +  %tmp1530 = sub i32 %tmp592, %tmp1506
> +  %tmp1557 = sub i32 %tmp1530, 542767629
> +  %tmp1607 = and i32 %tmp1557, 1
> +  store i32 %tmp1607, i32* %tmp1
> +  %tmp1634 = and i32 %tmp1607, 2080309246
> +  %tmp1676 = xor i32 %tmp1634, %tmp1530
> +  %tmp1618 = shl i32 %tmp1676, 1
> +  %tmp1645 = sub i32 %tmp862, %tmp1618
> +  %tmp1697 = and i32 %tmp1645, 1
> +  store i32 %tmp1697, i32* %a2
> +  ret i32 %tmp1607
> +}
> +
> +; CHECK-NOT: shl
> +; shl undef, 0 -> undef
> +define i32 @foo2_undef() nounwind {
> +entry:
> +  %tmp2 = shl i32 undef, 0;
> +  ret i32 %tmp2
> +}
> +
> +; CHECK-NOT: shl
> +; shl undef, x -> 0
> +define i32 @foo1_undef(i32* %a0) nounwind {
> +entry:
> +  %tmp1 = load i32* %a0, align 1
> +  %tmp2 = shl i32 undef, %tmp1;
> +  ret i32 %tmp2
> +}
> 
> 
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