[llvm-commits] Patch review for 9216 - Endless loop in Instcombine

[Chris Lattner]

On Feb 14, 2011, at 12:37 PM, Rotem, Nadav wrote:

> Hi,
>  
> This patch reproduces and fixes an endless loop in Instcombine.  The cause of the problem is described below.  Please review this patch.

Looks good, but please use "!isa<Constant>(A)" instead of dyn_cast, and no extra spaces before dyn_cast and between the ))'s.

Thanks Nadav!

-Chris

>  
> Cheers,
> Nadav
>  
> Index: llvm/test/Transforms/InstCombine/2011-02-14-InfLoop.ll
> ===================================================================
> --- llvm/test/Transforms/InstCombine/2011-02-14-InfLoop.ll      (revision 0)
> +++ llvm/test/Transforms/InstCombine/2011-02-14-InfLoop.ll   (revision 0)
> @@ -0,0 +1,17 @@
> +; This testcase causes an infinite loop in the instruction combiner,
> +; because it changes a pattern and the original pattern is almost
> +; identical to the newly-generated pattern.
> +; RUN: opt < %s -instcombine -disable-output
> +
> +target triple = "x86_64-unknown-linux-gnu"
> +
> +define <4 x float> @m_387(i8* noalias nocapture %A, i8* nocapture %B, <4 x i1> %C) nounwind {
> +entry:
> +  %movcsext20 = sext <4 x i1> %C to <4 x i32>
> +  %tmp2389 = xor <4 x i32> %movcsext20, <i32 -1, i32 -1, i32 -1, i32 -1>
> +  %movcand25 = and <4 x i32> %tmp2389, <i32 undef, i32 undef, i32 undef, i32 -1>
> +  %movcor26 = or <4 x i32> %movcand25, zeroinitializer
> +  %L2 = bitcast <4 x i32> %movcor26 to <4 x float>
> +  %L3 = shufflevector <4 x float> zeroinitializer, <4 x float> %L2, <4 x i32> <i32 0, i32 1, i32 2, i32 7>
> +  ret <4 x float> %L3
> +}
>  
> Property changes on: llvm/test/Transforms/InstCombine/2011-02-14-InfLoop.ll
> ___________________________________________________________________
> Added: svn:eol-style
>    + native
>  
> Index: llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
> ===================================================================
> --- llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp             (revision 125511)
> +++ llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp          (working copy)
> @@ -1138,7 +1138,11 @@
>          cast<BinaryOperator>(Op1)->swapOperands();
>          std::swap(A, B);
>        }
> -      if (A == Op0)                                // A&(A^B) -> A & ~B
> +      // Notice that the patten (A&(~B)) is actually (A&(-1^B)), so if
> +      // A is originally -1 (or a vector of -1 and undefs), then we enter
> +      // an endless loop. By checking that A is non-constant we ensure that
> +      // we will never get to the loop.
> +      if (A == Op0 && ! dyn_cast<Constant>(A) ) // A&(A^B) -> A & ~B
>          return BinaryOperator::CreateAnd(A, Builder->CreateNot(B, "tmp"));
>      }
>  
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