[RISCV] Support predefined marcro __riscv_misaligned_{fast,avoid}.

RISC-V C API introduce predefined macro to achieve hints about unaligned accesses [0].
This defines __riscv_misaligned_fast when using -mno-strict-align, otherwise,
defines __riscv_misaligned_avoid.

Note: This ignores __riscv_misaligned_slow which is also defined by spec.

The spec has mentioned riscv-non-isa/riscv-c-api-doc#40

[0]: riscv-non-isa/riscv-c-api-doc#40

Author: Yeting Kuo

clang/lib/Basic/Targets/RISCV.cpp

@@ -204,6 +204,11 @@ void RISCVTargetInfo::getTargetDefines(const LangOptions &Opts,
   if (VScale && VScale->first && VScale->first == VScale->second)
     Builder.defineMacro("__riscv_v_fixed_vlen",
                         Twine(VScale->first * llvm::RISCV::RVVBitsPerBlock));
+
+  if (FastUnalignedAccess)
+    Builder.defineMacro("__riscv_misaligned_fast");
+  else
+    Builder.defineMacro("__riscv_misaligned_avoid");
 }
 
 static constexpr Builtin::Info BuiltinInfo[] = {
@@ -322,6 +327,8 @@ bool RISCVTargetInfo::handleTargetFeatures(std::vector<std::string> &Features,
   if (ISAInfo->hasExtension("zfh") || ISAInfo->hasExtension("zhinx"))
     HasLegalHalfType = true;
 
+  FastUnalignedAccess = llvm::is_contained(Features, "+unaligned-scalar-mem");
+
   return true;
 }
 

clang/lib/Basic/Targets/RISCV.h

@@ -29,6 +29,9 @@ class RISCVTargetInfo : public TargetInfo {
   std::string ABI, CPU;
   std::unique_ptr<llvm::RISCVISAInfo> ISAInfo;
 
+private:
+  bool FastUnalignedAccess;
+
 public:
   RISCVTargetInfo(const llvm::Triple &Triple, const TargetOptions &)
       : TargetInfo(Triple) {

clang/test/Preprocessor/riscv-target-features.c

@@ -1228,3 +1228,15 @@
 // RUN: -march=rv64i_zve32x_zvkt1p0 -x c -E -dM %s \
 // RUN: -o - | FileCheck --check-prefix=CHECK-ZVKT-EXT %s
 // CHECK-ZVKT-EXT: __riscv_zvkt 1000000{{$}}
+
+// RUN: %clang --target=riscv32-unknown-linux-gnu -march=rv32i -x c -E -dM %s \
+// RUN: -o - | FileCheck %s --check-prefix=CHECK-MISALIGNED-AVOID
+// RUN: %clang --target=riscv64-unknown-linux-gnu -march=rv64i -x c -E -dM %s \
+// RUN: -o - | FileCheck %s --check-prefix=CHECK-MISALIGNED-AVOID
+// CHECK-MISALIGNED-AVOID: __riscv_misaligned_avoid 1
+
+// RUN: %clang --target=riscv32-unknown-linux-gnu -march=rv32i -x c -E -dM %s \
+// RUN: -munaligned-access -o - | FileCheck %s --check-prefix=CHECK-MISALIGNED-FAST
+// RUN: %clang --target=riscv64-unknown-linux-gnu -march=rv64i -x c -E -dM %s \
+// RUN: -munaligned-access -o - | FileCheck %s --check-prefix=CHECK-MISALIGNED-FAST
+// CHECK-MISALIGNED-FAST: __riscv_misaligned_fast 1