[clang] fix half && bfloat16 convert node expr codegen (#89051)

Data type conversion between fp16 and bf16 will generate fptrunc and
fpextend nodes, but they are actually bitcast nodes.

Author: JinjinLi868

clang/lib/CodeGen/CGExprScalar.cpp

@@ -1455,6 +1455,10 @@ Value *ScalarExprEmitter::EmitScalarCast(Value *Src, QualType SrcType,
     return Builder.CreateFPToUI(Src, DstTy, "conv");
   }
 
+  if ((DstElementTy->is16bitFPTy() && SrcElementTy->is16bitFPTy())) {
+    Value *FloatVal = Builder.CreateFPExt(Src, Builder.getFloatTy(), "fpext");
+    return Builder.CreateFPTrunc(FloatVal, DstTy, "fptrunc");
+  }
   if (DstElementTy->getTypeID() < SrcElementTy->getTypeID())
     return Builder.CreateFPTrunc(Src, DstTy, "conv");
   return Builder.CreateFPExt(Src, DstTy, "conv");

clang/test/CodeGen/X86/bfloat16-convert-half.c

@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -disable-O0-optnone -emit-llvm \
+// RUN:   %s -o - | opt -S -passes=mem2reg | FileCheck %s
+
+// CHECK-LABEL: define dso_local half @test_convert_from_bf16_to_fp16(
+// CHECK-SAME: bfloat noundef [[A:%.*]]) #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[FPEXT:%.*]] = fpext bfloat [[A]] to float
+// CHECK-NEXT:    [[FPTRUNC:%.*]] = fptrunc float [[FPEXT]] to half
+// CHECK-NEXT:    ret half [[FPTRUNC]]
+//
+_Float16 test_convert_from_bf16_to_fp16(__bf16 a) {
+    return (_Float16)a;
+}
+
+// CHECK-LABEL: define dso_local bfloat @test_convert_from_fp16_to_bf16(
+// CHECK-SAME: half noundef [[A:%.*]]) #[[ATTR0]] {
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[FPEXT:%.*]] = fpext half [[A]] to float
+// CHECK-NEXT:    [[FPTRUNC:%.*]] = fptrunc float [[FPEXT]] to bfloat
+// CHECK-NEXT:    ret bfloat [[FPTRUNC]]
+//
+__bf16 test_convert_from_fp16_to_bf16(_Float16 a) {
+    return (__bf16)a;
+}
+