[llvm] [SelectionDAG] Fold constant min/max vector reductions (PR #209190)

[via llvm-commits]

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@@ -7808,16 +7833,23 @@ SDValue SelectionDAG::FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL,
     if (Opcode == ISD::BITCAST)
       return SDValue();
 
-    // Constant fold VECREDUCE_ADD with a BUILD_VECTOR of integer constants.
-    if (Opcode == ISD::VECREDUCE_ADD && ISD::isBuildVectorOfConstantSDNodes(N1.getNode())) {
+    // Constant fold integer vector reductions with constant BUILD_VECTORs.
+    if ((Opcode == ISD::VECREDUCE_ADD || Opcode == ISD::VECREDUCE_SMAX ||
+         Opcode == ISD::VECREDUCE_SMIN || Opcode == ISD::VECREDUCE_UMAX ||
+         Opcode == ISD::VECREDUCE_UMIN) &&
+        ISD::isBuildVectorOfConstantSDNodes(N1.getNode())) {
       unsigned EltBits = N1.getValueType().getScalarSizeInBits();
-      APInt Acc = APInt::getZero(EltBits);
+      unsigned BaseOpcode = ISD::getVecReduceBaseOpcode(Opcode);
+      APInt Acc = getIntegerIdentity(BaseOpcode, EltBits);
       for (SDValue Elt : N1->op_values()) {
         if (Elt.getOpcode() == ISD::POISON)
           return getPOISON(VT);
         if (Elt.isUndef() || cast<ConstantSDNode>(Elt)->isOpaque())
           return SDValue();
-        Acc += cast<ConstantSDNode>(Elt)->getAPIntValue().trunc(EltBits);
+        APInt Value = cast<ConstantSDNode>(Elt)->getAPIntValue().trunc(EltBits);
+        std::optional<APInt> Folded = FoldValue(BaseOpcode, Acc, Value);
+        assert(Folded && "Unexpected vector reduction opcode");
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mygitljf wrote:

Would "Expected vector reduction base opcode to be foldable" better describe the invariant here, since every opcode admitted by the outer condition maps to a base opcode handled by FoldValue?

https://github.com/llvm/llvm-project/pull/209190