[llvm] Fold (a % b) lt/ge (b-1) where b is a power of 2 (PR #72504)

[Yingwei Zheng via llvm-commits]

================
@@ -6837,6 +6837,22 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
     Changed = true;
   }
 
+  {
+    Value *X, *Y;
+    ICmpInst::Predicate Pred = I.getPredicate();
+    if ((Pred == ICmpInst::ICMP_SGE || Pred == ICmpInst::ICMP_SLT) &&
+        match(Op0, m_OneUse(m_SRem(m_Value(X), m_Value(Y)))) &&
+        match(Op1, m_OneUse(m_c_Add(m_Deferred(Y), m_AllOnes()))) &&
+        isKnownNonNegative(Y, DL, 0, Q.AC, Q.CxtI, Q.DT)) {
+      // icmp slt (X % C), (C - 1) --> icmp ne (X % C), (C - 1)
+      auto *NewCmp = Builder.CreateICmpNE(Op0, Op1);
+      // icmp sge (X % C), (C - 1) --> icmp eq (X % C), (C - 1)
+      if (Pred == ICmpInst::ICMP_SGE)
+        NewCmp = Builder.CreateICmpEQ(Op0, Op1);
+      return replaceInstUsesWith(I, NewCmp);
----------------
dtcxzyw wrote:

```suggestion
      // icmp slt (X % C), (C - 1) --> icmp ne (X % C), (C - 1)
      // icmp sge (X % C), (C - 1) --> icmp eq (X % C), (C - 1)
      auto *NewCmp = Builder.CreateICmp(Pred == ICmpInst::ICMP_SLT ? ICmpInst::ICMP_NE : ICmpInst::ICMP_EQ, Op0, Op1);
      return replaceInstUsesWith(I, NewCmp);
```

Don't create instructions that may be unused.


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