[llvm] 1eda9bf - [SCEV] ScalarEvolution::createSCEV(): Instruction::Or: drop bogus no-wrap flag detection

[Roman Lebedev via llvm-commits]

Author: Roman Lebedev
Date: 2020-06-06T13:02:07+03:00
New Revision: 1eda9bfd6129a0da4cb6e4619d230ab0e08d33f9

URL: https://github.com/llvm/llvm-project/commit/1eda9bfd6129a0da4cb6e4619d230ab0e08d33f9
DIFF: https://github.com/llvm/llvm-project/commit/1eda9bfd6129a0da4cb6e4619d230ab0e08d33f9.diff

LOG: [SCEV] ScalarEvolution::createSCEV(): Instruction::Or: drop bogus no-wrap flag detection

Summary:
That's just really wrong. While sure, if LHS is AddRec, and we could
propagate it's no-wrap flags, that doesn't make, because as long as
the operands of `or` had no common bits set, then the `add`
of these operands will never overflow: http://volta.cs.utah.edu:8080/z/gmt7Sy
IOW we need no propagation/detection, we are free to just set NUW+NSW.

But as rG39e3683534c83573da5c8b70c8adfb43948f601f shows,
even when the old code failed to "deduce" flags,
we'd eventually re-deduce them somewhere, later.

So let's just set them.

Reviewers: mkazantsev, reames, sanjoy, efriedma

Reviewed By: efriedma

Subscribers: efriedma, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81246

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 4d7def2e4d0e..85f9794aa8d8 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -6389,15 +6389,8 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
         if (GetMinTrailingZeros(LHS) >=
             (CIVal.getBitWidth() - CIVal.countLeadingZeros())) {
           // Build a plain add SCEV.
-          const SCEV *S = getAddExpr(LHS, getSCEV(CI));
-          // If the LHS of the add was an addrec and it has no-wrap flags,
-          // transfer the no-wrap flags, since an or won't introduce a wrap.
-          if (const SCEVAddRecExpr *NewAR = dyn_cast<SCEVAddRecExpr>(S)) {
-            const SCEVAddRecExpr *OldAR = cast<SCEVAddRecExpr>(LHS);
-            const_cast<SCEVAddRecExpr *>(NewAR)->setNoWrapFlags(
-                OldAR->getNoWrapFlags());
-          }
-          return S;
+          return getAddExpr(LHS, getSCEV(CI),
+                            (SCEV::NoWrapFlags)(SCEV::FlagNUW | SCEV::FlagNSW));
         }
       }
       break;