[llvm] [ConstraintElim] Add NUW flag to sub if x >=u y. (PR #218685)

[Nikita Popov via llvm-commits]

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@@ -1291,6 +1291,40 @@ static bool getConstraintFromMemoryAccess(GetElementPtrInst &GEP,
   return true;
 }
 
+/// Returns true if \p I is a candidate whose poison-generating flags may be
+/// strengthened using the constraint systems.
+static bool canStrengthenFlags(Instruction *I) {
+  switch (I->getOpcode()) {
+  case Instruction::Sub:
+    // A - B does not wrap unsigned, if A >=u B. Constant operands are handled
+    // by CorrelatedValuePropagation using ranges.
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nikic wrote:

For sub specifically, a constant on the RHS will get canonicalized to add anyway.

https://github.com/llvm/llvm-project/pull/218685