[llvm] [LLVM][SCEV] Look through common vscale multiplicand when simplifying compares. (PR #141798)

[Florian Hahn via llvm-commits]

================
@@ -10785,6 +10785,25 @@ bool ScalarEvolution::SimplifyICmpOperands(CmpPredicate &Pred, const SCEV *&LHS,
   if (Depth >= 3)
     return false;
 
+  const SCEV *NewLHS, *NewRHS;
+  if (match(LHS, m_scev_c_Mul(m_SCEV(NewLHS), m_SCEVVScale())) &&
+      match(RHS, m_scev_c_Mul(m_SCEV(NewRHS), m_SCEVVScale()))) {
+    const SCEVMulExpr *LMul = cast<SCEVMulExpr>(LHS);
+    const SCEVMulExpr *RMul = cast<SCEVMulExpr>(RHS);
+
+    // (X * vscale) uicmp/eq/ne (Y * vscale) ==> X uicmp/eq/ne Y
+    //     when neither multiply wraps.
+    // (X * vscale) sicmp (Y * vscale) ==> X sicmp Y
+    //     when neither multiply changes sign.
+    if ((LMul->hasNoSignedWrap() && RMul->hasNoSignedWrap()) ||
+        (LMul->hasNoUnsignedWrap() && RMul->hasNoUnsignedWrap() &&
+         !ICmpInst::isSigned(Pred))) {
----------------
fhahn wrote:

Would be good to reorder & update comment to match the condition?
```suggestion
    // (X * vscale) pred (Y * vscale) ==> X pred Y
    //     when both multiples are NSW.
    // (X * vscale) uicmp/eq/ne (Y * vscale) ==> X uicmp/eq/ne Y
    //     when both multiples are NUW.
    if ((LMul->hasNoSignedWrap() && RMul->hasNoSignedWrap()) ||
        (LMul->hasNoUnsignedWrap() && RMul->hasNoUnsignedWrap() &&
         !ICmpInst::isSigned(Pred))) {
```

https://github.com/llvm/llvm-project/pull/141798