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

[John Brawn via llvm-commits]

================
@@ -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");
+  }
----------------
john-brawn-arm wrote:

There doesn't seem to be one as far as I can tell. I could move this out into a separate function like e.g. ISD::getInverseMinMaxOpcode, but that would just be moving the switch to a separate location.

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