[llvm] Simplify `(a % b) lt/ge (b-1)` into `(a % b) eq/ne (b-1)` (PR #72504)

[Yingwei Zheng via llvm-commits]

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@@ -6855,6 +6855,52 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
   if (Value *V = simplifyICmpInst(I.getPredicate(), Op0, Op1, Q))
     return replaceInstUsesWith(I, V);
 
+  {
+    Value *X;
+    const APInt *C1, *C2;
+    ICmpInst::Predicate Pred;
+    if ((match(&I, m_ICmp(Pred, m_SRem(m_Value(X), m_NonNegative(C1)),
+                          m_APInt(C2))) &&
+         ((Pred == ICmpInst::ICMP_SLT && *C2 == *C1 - 1) ||
+          (Pred == ICmpInst::ICMP_SGT && *C2 == *C1 - 2))) ||
+        (match(&I,
+               m_ICmp(Pred, m_SRem(m_Value(X), m_Negative(C1)), m_APInt(C2))) &&
+         *C2 == *C1 + 1 &&
+         (Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SLE))) {
+      // icmp slt (X s% C), (C - 1) --> icmp ne (X s% C), (C - 1), if C >= 0
+      // icmp sgt (X s% C), (C - 2) --> icmp eq (X s% C), (C - 1), if C >= 0
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dtcxzyw wrote:

Please update your alive2 proofs as well (use `sgt` instead of `sge`).


https://github.com/llvm/llvm-project/pull/72504