[SelectionDAG] Lower llvm.ldexp.f32 to ldexp() on Windows.

This reduces codesize. As discussed in llvm#92707.

Author: efriedma-quic

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -3650,6 +3650,27 @@ bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
     // FIXME: Use separate LibCall action.
     if (TLI.getLibcallName(LC))
       break;
+    if (Node->getOpcode() == ISD::FLDEXP && VT == MVT::f32 &&
+        TLI.isTypeLegal(MVT::f64) &&
+        TLI.getLibcallName(RTLIB::getLDEXP(MVT::f64))) {
+      // On Windows, it's common to be missing the 32-bit libcall, but have
+      // the 64-bit libcall. Expand to the 64-bit libcall. (Note that ldexp
+      // involves a rounding step if the result is subnormal, but that isn't
+      // relevant here because any subnormal result will round to zero when
+      // it's truncated.)
+      //
+      // FIXME: Consider doing something similar for f16/bf16. But be very
+      // careful handling bf16: expanding bf16->f64 is fine, but expanding
+      // bf16->f32 would produce incorrect subnormal results.
+      SDValue Extended =
+          DAG.getNode(ISD::FP_EXTEND, dl, MVT::f64, Node->getOperand(0));
+      SDValue LdExp =
+          DAG.getNode(ISD::FLDEXP, dl, MVT::f64, Extended, Node->getOperand(1));
+      Results.push_back(
+          DAG.getNode(ISD::FP_ROUND, dl, VT, LdExp,
+                      DAG.getIntPtrConstant(0, dl, /*isTarget=*/true)));
+      break;
+    }
 
     if (SDValue Expanded = expandLdexp(Node)) {
       Results.push_back(Expanded);