[InstCombine] Fold `sext(A < B) + zext(A > B)` into `ucmp/scmp(A, B)` (#103833)

This change also covers the fold of `zext(A > B) - zext(A < B)` since it
is already being canonicalized into the aforementioned pattern.

Proof: https://alive2.llvm.org/ce/z/AgnfMn

1 files changed, 20 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index dd4a640..d7758b5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1626,6 +1626,26 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
       A->getType()->isIntOrIntVectorTy(1))
     return replaceInstUsesWith(I, Constant::getNullValue(I.getType()));
 
+  // sext(A < B) + zext(A > B) => ucmp/scmp(A, B)
+  ICmpInst::Predicate LTPred, GTPred;
+  if (match(&I,
+            m_c_Add(m_SExt(m_c_ICmp(LTPred, m_Value(A), m_Value(B))),
+                    m_ZExt(m_c_ICmp(GTPred, m_Deferred(A), m_Deferred(B))))) &&
+      A->getType()->isIntOrIntVectorTy()) {
+    if (ICmpInst::isGT(LTPred)) {
+      std::swap(LTPred, GTPred);
+      std::swap(A, B);
+    }
+
+    if (ICmpInst::isLT(LTPred) && ICmpInst::isGT(GTPred) &&
+        ICmpInst::isSigned(LTPred) == ICmpInst::isSigned(GTPred))
+      return replaceInstUsesWith(
+          I, Builder.CreateIntrinsic(
+                 Ty,
+                 ICmpInst::isSigned(LTPred) ? Intrinsic::scmp : Intrinsic::ucmp,
+                 {A, B}));
+  }
+
   // A+B --> A|B iff A and B have no bits set in common.
   WithCache<const Value *> LHSCache(LHS), RHSCache(RHS);
   if (haveNoCommonBitsSet(LHSCache, RHSCache, SQ.getWithInstruction(&I)))