[PATCH] D157187: [InstCombine] Propagate the nsw for instruction neg-sub

[Allen zhong via Phabricator via llvm-commits]

Allen added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp:235-242
       (I->hasOneUse() || match(I->getOperand(0), m_ImmConstant()))) {
     // `sub` is always negatible.
     // However, only do this either if the old `sub` doesn't stick around, or
     // it was subtracting from a constant. Otherwise, this isn't profitable.
-    return Builder.CreateSub(I->getOperand(1), I->getOperand(0),
-                             I->getName() + ".neg");
+    // Propagate flag nsw only when the single use of the I to avoid affecting
+    // other users.
+    bool Flag =
----------------
arsenm wrote:
> I think checking hasOneUse twice here is somewhat confusing
Thanks @arsenm for your idea. 
I double check the hasOneUse because it may be also here when the **I->getOperand(0)** is a const value.


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  https://reviews.llvm.org/D157187/new/

https://reviews.llvm.org/D157187