[Clang][RISCV] Set native half type to legal when has zfh extension

[Yunzezhu]

Summary

Issue: When processing a f16 type value, a f16 value is extended to f32 after load and processed in f32 form, and truncated back to f16 before store, even when the target has zfh extension, which indicates the target has native half type float support, and such kind of extend/truncate actions are not necessary.

Some changes:
1.Set native half type to legal when has zfh extension, which prevent unnecessary promotion from f16 to f32, and unpromotion back to f16.
2.Add related test case to test generation of half type float.

[craig.topper]

Is this the same as https://reviews.llvm.org/D145071

[Yunzezhu]

In D149401#4304053, @craig.topper wrote:

Is this the same as https://reviews.llvm.org/D145071

Yes and I will abandon this one.

clang/lib/Basic/Targets/RISCV.cpp

handleAllErrors(ParseResult.takeError(), [&](llvm::StringError &ErrMsg) {
       OutputErrMsg << ErrMsg.getMessage();
     });
     Diags.Report(diag::err_invalid_feature_combination) << OutputErrMsg.str();
     return false;
   } else {
     ISAInfo = std::move(*ParseResult);
   }
 
+  // Turn on native half type support by set half type to legal.
+  if (ISAInfo->hasExtension("zfh")){
+    HasLegalHalfType = true;
+  }
+
   if (ABI.empty())
     ABI = ISAInfo->computeDefaultABI().str();
 
   return true;
 }
 
 bool RISCVTargetInfo::isValidCPUName(StringRef Name) const {
   bool Is64Bit = getTriple().isArch64Bit();

clang/test/CodeGen/RISCV/_Float16-add.c

+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py
+// RUN: %clang_cc1 -triple riscv64 -target-feature +zfh -emit-llvm %s -o - \
+// RUN:   | FileCheck %s
+
+_Float16 x;
+_Float16 y;
+_Float16 z;
+// CHECK-LABEL: @func(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[TMP0:%.*]] = load half, ptr @y, align 2
+// CHECK-NEXT:    [[TMP1:%.*]] = load half, ptr @z, align 2
+// CHECK-NEXT:    [[ADD:%.*]] = fadd half [[TMP0]], [[TMP1]]
+// CHECK-NEXT:    store half [[ADD]], ptr @x, align 2
+// CHECK-NEXT:    ret void
+//
+void func()
+{
+  x = y + z;
+}