[llvm] [DAGCombiner] Eliminate fp casts if we have the right fast math flags (PR #131345)

[John Brawn via llvm-commits]

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@@ -18455,7 +18455,45 @@ SDValue DAGCombiner::visitFP_ROUND(SDNode *N) {
   return SDValue();
 }
 
+// Eliminate a floating-point widening of a narrowed value if the fast math
+// flags allow it.
+static SDValue eliminateFPCastPair(SDNode *N) {
+  SDValue N0 = N->getOperand(0);
+  EVT VT = N->getValueType(0);
+
+  unsigned NarrowingOp;
+  switch (N->getOpcode()) {
+  case ISD::FP16_TO_FP:
+    NarrowingOp = ISD::FP_TO_FP16;
+    break;
+  case ISD::BF16_TO_FP:
+    NarrowingOp = ISD::FP_TO_BF16;
+    break;
+  case ISD::FP_EXTEND:
+    NarrowingOp = ISD::FP_ROUND;
+    break;
+  default:
+    llvm_unreachable("Expected widening FP cast");
+  }
+
+  if (N0.getOpcode() == NarrowingOp && N0.getOperand(0).getValueType() == VT) {
+    const SDNodeFlags SrcFlags = N0->getFlags();
+    const SDNodeFlags DstFlags = N->getFlags();
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john-brawn-arm wrote:

I've done this, and also adjusted the comment.

https://github.com/llvm/llvm-project/pull/131345