[llvm] [SCEVExpander] Attempt to reinfer flags dropped due to CSE (PR #72431)

[Nikita Popov via llvm-commits]

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@@ -1532,8 +1532,28 @@ Value *SCEVExpander::expand(const SCEV *S) {
     V = visit(S);
     V = fixupLCSSAFormFor(V);
   } else {
-    for (Instruction *I : DropPoisonGeneratingInsts)
+    for (Instruction *I : DropPoisonGeneratingInsts) {
       I->dropPoisonGeneratingFlagsAndMetadata();
+      // See if we can re-infer from first principles any of the flags we just
+      // dropped.
+      if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(I))
+        if (auto Flags = SE.getStrengthenedNoWrapFlagsFromBinOp(OBO)) {
+          auto *BO = cast<BinaryOperator>(I);
+          BO->setHasNoUnsignedWrap(
+            ScalarEvolution::maskFlags(*Flags, SCEV::FlagNUW) == SCEV::FlagNUW);
+          BO->setHasNoSignedWrap(
+            ScalarEvolution::maskFlags(*Flags, SCEV::FlagNSW) == SCEV::FlagNSW);
+        }
+      if (auto *NNI = dyn_cast<PossiblyNonNegInst>(I)) {
+        auto *Src = NNI->getOperand(0);
+        if (SE.isKnownNonNegative(SE.getSCEV(Src)) ||
+            isKnownNonNegative(Src, DL, 0, &SE.AC, I, &SE.DT) ||
+            isImpliedByDomCondition(ICmpInst::ICMP_SGE, Src,
+                                    Constant::getNullValue(Src->getType()), I,
+                                    DL).value_or(false))
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nikic wrote:

Generally I'd expect ValueTracking `isKnownNonNegative` to not give any useful additional information here, with the exception of `assume` s.

https://github.com/llvm/llvm-project/pull/72431