[LegalizeTypes][MSP430] Soften FAKE_USE operand

Adds support for softening FAKE_USE operands.
Adds MSP430 tests that exercise the new softening code.

Fixes #137572

Author: actinks

llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp

@@ -1143,6 +1143,9 @@ bool DAGTypeLegalizer::SoftenFloatOperand(SDNode *N, unsigned OpNo) {
     Res = SoftenFloatOp_ATOMIC_STORE(N, OpNo);
     break;
   case ISD::FCOPYSIGN:   Res = SoftenFloatOp_FCOPYSIGN(N); break;
+  case ISD::FAKE_USE:
+    Res = SoftenFloatOp_FAKE_USE(N);
+    break;
   }
 
   // If the result is null, the sub-method took care of registering results etc.
@@ -1477,6 +1480,12 @@ SDValue DAGTypeLegalizer::SoftenFloatOp_LLRINT(SDNode *N) {
                                              RTLIB::LLRINT_PPCF128));
 }
 
+SDValue DAGTypeLegalizer::SoftenFloatOp_FAKE_USE(SDNode *N) {
+  SDValue Op1 = BitConvertToInteger(N->getOperand(1));
+  return DAG.getNode(N->getOpcode(), SDLoc(N), N->getValueType(0),
+                     N->getOperand(0), Op1);
+}
+
 //===----------------------------------------------------------------------===//
 //  Float Result Expansion
 //===----------------------------------------------------------------------===//

llvm/lib/CodeGen/SelectionDAG/LegalizeTypes.h

@@ -651,6 +651,7 @@ class LLVM_LIBRARY_VISIBILITY DAGTypeLegalizer {
   SDValue SoftenFloatOp_STORE(SDNode *N, unsigned OpNo);
   SDValue SoftenFloatOp_ATOMIC_STORE(SDNode *N, unsigned OpNo);
   SDValue SoftenFloatOp_FCOPYSIGN(SDNode *N);
+  SDValue SoftenFloatOp_FAKE_USE(SDNode *N);
 
   //===--------------------------------------------------------------------===//
   // Float Expansion Support: LegalizeFloatTypes.cpp

llvm/test/CodeGen/MSP430/fake_use_float.ll

@@ -0,0 +1,31 @@
+; RUN: llc -stop-after=finalize-isel -filetype=asm -o - < %s | FileCheck %s
+target datalayout = "e-m:e-p:16:16-i32:16-i64:16-f32:16-f64:16-a:8-n8:16-S16"
+target triple = "msp430"
+
+; CHECK:       bb.0.entry:
+; CHECK:       %0:gr16 = MOV16ri
+; CHECK-DAG:   FAKE_USE killed %0
+; CHECK:       %1:gr16 = MOV16ri
+; CHECK-DAG:   FAKE_USE killed %1
+; CHECK:       %2:gr16 = MOV16ri
+; CHECK-DAG:   FAKE_USE killed %2
+; CHECK:       %3:gr16 = MOV16ri
+; CHECK-DAG:   FAKE_USE killed %3
+; CHECK:       RET
+define void @test-double() {
+entry:
+  call void (...) @llvm.fake.use(double -8.765430e+02)
+  ret void
+}
+
+; CHECK:       bb.0.entry:
+; CHECK:       %0:gr16 = MOV16ri
+; CHECK-DAG:   FAKE_USE killed %0
+; CHECK:       %1:gr16 = MOV16ri
+; CHECK-DAG:   FAKE_USE killed %1
+; CHECK:       RET
+define void @test-float() {
+entry:
+  call void (...) @llvm.fake.use(float -8.76e+02)
+  ret void
+}