[llvm] [SCEV] Simplify SCEVExpr for PHI to SCEV for operand if operands are identical (PR #115945)

[Nikita Popov via llvm-commits]

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@@ -6019,6 +6019,34 @@ const SCEV *ScalarEvolution::createNodeFromSelectLikePHI(PHINode *PN) {
   return nullptr;
 }
 
+/// Returns SCEV for the first operand of a phi if all phi operands have
+/// identical opcodes and operands
+/// eg.
+/// a: %add = %a + %b
+///    br %c
+/// b: %add1 = %a + %b
+///    br %c
+/// c: %phi = phi [%add, a], [%add1, b]
+/// scev(%phi) => scev(%add)
+const SCEV *
+ScalarEvolution::createNodeForPHIWithIdenticalOperands(PHINode *PN) {
+  BinaryOperator *CommonInst = nullptr;
+  for (Value *Incoming : PN->incoming_values()) {
+    BinaryOperator *IncomingInst = dyn_cast<BinaryOperator>(Incoming);
+    if (CommonInst) {
+      if (!(IncomingInst && CommonInst->isIdenticalTo(IncomingInst)))
+        return nullptr; // Not identical, give up
+    } else if (IncomingInst) {
+      // Remember binary operator
+      CommonInst = IncomingInst;
+    } else {
+      // Not a binary operator, give up
+      return nullptr;
+    }
+  }
+  return CommonInst ? getSCEV(CommonInst) : nullptr;
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nikic wrote:

This looks dangerous to me. Even if the instructions are the same, I think that SCEV would be allowed to use context-sensitive reasoning when constructing the SCEV node. It would be safer to call getSCEV for each incoming value in a second loop and make sure they're all the same.

https://github.com/llvm/llvm-project/pull/115945