[llvm] [DAG] Support saturated truncate (PR #99418)

[David Green via llvm-commits]

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@@ -14915,6 +14920,171 @@ SDValue DAGCombiner::visitEXTEND_VECTOR_INREG(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitTRUNCATE_USAT(SDNode *N) {
+  EVT VT = N->getValueType(0);
+  SDValue N0 = N->getOperand(0);
+
+  std::function<SDValue(SDValue)> MatchFPTOINT = [&](SDValue Val) -> SDValue {
+    if (Val.getOpcode() == ISD::FP_TO_UINT)
+      return Val;
+    if (Val.getOpcode() == ISD::SMAX) {
+      for (unsigned I = 0; I < Val.getNumOperands(); I++)
+        if (SDValue Matched = MatchFPTOINT(Val.getOperand(I)))
+          return Matched;
+    }
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davemgreen wrote:

Remove this SMAX part, it is not very obvious why it would be correct and I don't believe it should be needed to get the basics working.

(MatchFPTOINT could then probably be removed, if the check for ISD::FP_TO_UINT was moved where it is currently called).

https://github.com/llvm/llvm-project/pull/99418