[NFC] Add useFPRegsForHalfType().

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -505,12 +505,18 @@ class TargetLoweringBase {
     return TypePromoteInteger;
   }
 
-  // Return true if the half type should be passed around as i16, but promoted
-  // to float around arithmetic. The default behavior is to pass around as
-  // float and convert around loads/stores/bitcasts and other places where
-  // the size matters.
+  // Return true if the half type should be promoted using soft promotion rules
+  // where each operation is promoted to f32 individually, then converted to
+  // fp16. The default behavior is to promote chains of operations, keeping
+  // intermediate results in f32 precision and range.
   virtual bool softPromoteHalfType() const { return false; }
 
+  // Return true if, for soft-promoted half, the half type should be passed
+  // passed to and returned from functions as f32. The default behavior is to
+  // pass as i16. If soft-promoted half is not used, this function is ignored
+  // and values are always passed and returned as f32.
+  virtual bool useFPRegsForHalfType() const { return false; }
+
   // There are two general methods for expanding a BUILD_VECTOR node:
   //  1. Use SCALAR_TO_VECTOR on the defined scalar values and then shuffle
   //     them together.

llvm/lib/CodeGen/TargetLoweringBase.cpp

@@ -1430,15 +1430,20 @@ void TargetLoweringBase::computeRegisterProperties(
   // conversions).
   if (!isTypeLegal(MVT::f16)) {
     // Allow targets to control how we legalize half.
-    if (softPromoteHalfType()) {
+    bool SoftPromoteHalfType = softPromoteHalfType();
+    bool UseFPRegsForHalfType = !SoftPromoteHalfType || useFPRegsForHalfType();
+
+    if (!UseFPRegsForHalfType) {
       NumRegistersForVT[MVT::f16] = NumRegistersForVT[MVT::i16];
       RegisterTypeForVT[MVT::f16] = RegisterTypeForVT[MVT::i16];
-      TransformToType[MVT::f16] = MVT::f32;
-      ValueTypeActions.setTypeAction(MVT::f16, TypeSoftPromoteHalf);
     } else {
       NumRegistersForVT[MVT::f16] = NumRegistersForVT[MVT::f32];
       RegisterTypeForVT[MVT::f16] = RegisterTypeForVT[MVT::f32];
-      TransformToType[MVT::f16] = MVT::f32;
+    }
+    TransformToType[MVT::f16] = MVT::f32;
+    if (SoftPromoteHalfType) {
+      ValueTypeActions.setTypeAction(MVT::f16, TypeSoftPromoteHalf);
+    } else {
       ValueTypeActions.setTypeAction(MVT::f16, TypePromoteFloat);
     }
   }