[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;
----------------
john-brawn-arm wrote:

FP16_TO_FP is tested in llvm/test/CodeGen/ARM/fp16_fast_math.ll - ARM uses FP16_TO_FP for fp16-to-float conversion as it doesn't have registers for fp16 types, AArch64 uses FP_EXTEND as it does have such registers. We don't have any BF16 tests though, so I've now added these.

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