[llvm] [InstCombine] Fold `(icmp eq/ne (or (select cond, 0/NZ, 0/NZ), X), 0)` (PR #88183)

[Nikita Popov via llvm-commits]

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@@ -3487,6 +3487,67 @@ Instruction *InstCombinerImpl::foldICmpBinOpEqualityWithConstant(
       Value *And = Builder.CreateAnd(BOp0, NotBOC);
       return new ICmpInst(Pred, And, NotBOC);
     }
+    // (icmp eq (or (select cond, 0, NonZero), Other))
+    //  -> (and cond, (icmp eq Other, 0))
+    // (icmp ne (or (select cond, NonZero, 0), Other))
+    //  -> (or cond, (icmp ne Other, 0))
+    // (icmp ne (or (select cond, 0, NonZero), Other))
+    //  -> (or (not cond), (icmp ne Other, 0))
+    // (icmp eq (or (select cond, NonZero, 0), Other))
+    //  -> (and (not cond), (icmp eq Other, 0))
+    Value *Cond, *TV, *FV, *Other;
+    if (C.isZero() &&
+        match(BO, m_c_Or(m_Select(m_Value(Cond), m_Value(TV), m_Value(FV)),
+                         m_Value(Other)))) {
+      const SimplifyQuery Q = SQ.getWithInstruction(&Cmp);
+      auto IsNonZero = [&](Value *V) {
+        return isKnownNonZero(V, Q.DL, /*Depth=*/0, Q.AC, Q.CxtI, Q.DT);
+      };
+      // Easy case is if eq/ne matches whether 0 is trueval/falseval.
+      if (Pred == ICmpInst::ICMP_EQ
+              ? (match(TV, m_SpecificInt(C)) && IsNonZero(FV))
+              : (match(FV, m_SpecificInt(C)) && IsNonZero(TV))) {
+        Value *Cmp = Builder.CreateICmp(
+            Pred, Other, Constant::getNullValue(Other->getType()));
+        return BinaryOperator::Create(
+            Pred == ICmpInst::ICMP_EQ ? Instruction::And : Instruction::Or, Cmp,
+            Cond);
+      }
+      // Harder case is if eq/ne matches whether 0 is falseval/trueval. In this
+      // case we need to invert the select condition so we need to be careful to
+      // avoid creating extra instructions.
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nikic wrote:

Is all this complexity needed for motivating patterns? Can we only handle one-use BO + select instead?

https://github.com/llvm/llvm-project/pull/88183