[PATCH] fix incorrect codegen from instcombine (PR17827)

[Evan Cheng]

I looked at the analysis in the bug report and I think I agree with your assessment. Perhaps you can use Nuno's technique to verify the optimization with a SMT solver?

As for the patch itself, (assuming the analysis is correct), the patch is trivially correct. The only adjustment I would make is mechanical:

+        } else { // logical shift left or right
+          if (Shift->getOpcode() == Instruction::Shl) {
+            CanFold = Shift->hasNoSignedWrap() || !ICI.isSigned();
+          } else { // logical shift right
+            CanFold = true;
+          }
         }

I would have combined the else and the first if to
} else if (Shift->getOpcode() == Instruction::Shl) {

Evan

On Nov 21, 2013, at 11:46 AM, Kay Tiong Khoo <kkhoo at perfwizard.com> wrote:

> Detailed description of problem in instcombine's compare optimizer:
> http://llvm.org/bugs/show_bug.cgi?id=17827
> 
> Fix suggested by Henrique Santos via dev mailing list:
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-November/067822.html
> <pr17827.patch>_______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
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