[PATCH] Make ScalarEvolution less aggressive with respect to no-wrap flags.

[Sanjoy Das]

Replies inline.


================
Comment at: lib/Analysis/ScalarEvolution.cpp:3149-3160
@@ -3148,13 +3148,14 @@
 /// getMinusSCEV - Return LHS-RHS.  Minus is represented in SCEV as A+B*-1.
 const SCEV *ScalarEvolution::getMinusSCEV(const SCEV *LHS, const SCEV *RHS,
                                           SCEV::NoWrapFlags Flags) {
   assert(!maskFlags(Flags, SCEV::FlagNUW) && "subtraction does not have NUW");
 
   // Fast path: X - X --> 0.
   if (LHS == RHS)
     return getConstant(LHS->getType(), 0);
 
-  // X - Y --> X + -Y
-  return getAddExpr(LHS, getNegativeSCEV(RHS), Flags);
+  // X - Y --> X + -Y.
+  // X -(nsw || nuw) Y --> X + -Y.
+  return getAddExpr(LHS, getNegativeSCEV(RHS));
 }
 
----------------
majnemer wrote:
> `Flags` doesn't seem very useful now, perhaps it should be removed?
Yup, I'll do that in a follow-up refactoring change.

================
Comment at: lib/Analysis/ScalarEvolution.cpp:3464-3469
@@ -3463,8 +3464,2 @@
                 }
-              } else if (const SubOperator *OBO =
-                           dyn_cast<SubOperator>(BEValueV)) {
-                if (OBO->hasNoUnsignedWrap())
-                  Flags = setFlags(Flags, SCEV::FlagNUW);
-                if (OBO->hasNoSignedWrap())
-                  Flags = setFlags(Flags, SCEV::FlagNSW);
               }
----------------
atrick wrote:
> Add a short comment that subtract does not preserve nsw/nuw Flags.
Will do.

http://reviews.llvm.org/D7081

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