[llvm] [InstCombine] Fold `(x < y) ? -1 : zext(x != y)` into `u/scmp(x,y)` (PR #101049)

[Nikita Popov via llvm-commits]

================
@@ -3529,6 +3529,62 @@ static Instruction *foldBitCeil(SelectInst &SI, IRBuilderBase &Builder) {
                                 Masked);
 }
 
+// This function tries to fold the following operations:
+//   (x < y) ? -1 : zext(x != y)
+//   (x > y) ? 1 : sext(x != y)
+//   (x >= y) ? zext(x != y) : -1
+// Into ucmp/scmp(x, y), where signedness is determined by the signedness
+// of the comparison in the original sequence
+Instruction *InstCombinerImpl::foldSelectToCmp(SelectInst &SI) {
+  Value *TV = SI.getTrueValue();
+  Value *FV = SI.getFalseValue();
+
+  ICmpInst::Predicate Pred;
+  Value *LHS, *RHS;
+  if (!match(SI.getCondition(), m_ICmp(Pred, m_Value(LHS), m_Value(RHS))))
+    return nullptr;
+
+  if (!LHS->getType()->isIntOrIntVectorTy())
+    return nullptr;
+
+  // Try to swap operands and the predicate. We need to be careful when doing
+  // so because two of the patterns have opposite predicates, so use the
+  // constant inside select to determine if swapping operands would be
+  // beneficial to us.
+  if ((ICmpInst::isGT(Pred) && match(TV, m_AllOnes())) ||
+      (ICmpInst::isLT(Pred) && match(TV, m_One())) || ICmpInst::isLE(Pred)) {
+    Pred = ICmpInst::getSwappedPredicate(Pred);
+    std::swap(LHS, RHS);
+  }
+
+  Intrinsic::ID IID =
+      ICmpInst::isSigned(Pred) ? Intrinsic::scmp : Intrinsic::ucmp;
+
+  CallInst *Intrinsic = nullptr;
+  ICmpInst::Predicate NEPred;
+  // (x < y) ? -1 : zext(x != y)
+  if (ICmpInst::isLT(Pred) && match(TV, m_AllOnes()) &&
+      match(FV, m_ZExt(m_c_ICmp(NEPred, m_Specific(LHS), m_Specific(RHS)))) &&
----------------
nikic wrote:

```suggestion
      match(FV, m_ZExt(m_c_SpecificICmp(ICmpInst::ICMP_NE, m_Specific(LHS), m_Specific(RHS)))) &&
```
This helper was recently added.

https://github.com/llvm/llvm-project/pull/101049