[llvm] [InstCombine] Fold `select (A &/| B), T, F` if `select B, T, F` is foldable (PR #76621)

[Nikita Popov via llvm-commits]

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@@ -3793,5 +3794,51 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   if (Instruction *I = foldBitCeil(SI, Builder))
     return I;
 
+  // Fold:
+  // (select A && B, T, F) -> (select A, (select B, T, F), F)
+  // (select A || B, T, F) -> (select A, T, (select B, T, F))
+  // if (select B, T, F) is foldable.
+  // TODO: preserve FMF flags
+  auto FoldSelectWithAndOrCond = [&](bool IsAnd, Value *A,
+                                     Value *B) -> Instruction * {
+    if (Value *V = simplifySelectInst(B, TrueVal, FalseVal,
+                                      SQ.getWithInstruction(&SI)))
+      return SelectInst::Create(A, IsAnd ? V : TrueVal, IsAnd ? FalseVal : V);
+
+    // Is (select B, T, F) a SPF?
+    if (CondVal->hasOneUse() && SelType->isIntOrIntVectorTy()) {
+      Value *LHS, *RHS;
+      if (ICmpInst *Cmp = dyn_cast<ICmpInst>(B))
+        if (Value *V = canonicalizeSPF(*Cmp, TrueVal, FalseVal, *this))
+          return SelectInst::Create(A, IsAnd ? V : TrueVal,
+                                    IsAnd ? FalseVal : V);
+    }
+
+    return nullptr;
+  };
+
+  Value *LHS, *RHS;
+  if (match(CondVal, m_And(m_Value(LHS), m_Value(RHS)))) {
+    if (Instruction *I = FoldSelectWithAndOrCond(/*IsAnd*/ true, LHS, RHS))
+      return I;
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nikic wrote:

Nevermind, we have to replace two operands, at which point this wouldn't really be better...

https://github.com/llvm/llvm-project/pull/76621