[RISCV] Add an option to emit the Tag_RISCV_arch attribute based on the assembler's subtarget

This adds an option to emit the command line -mattr/-march into the
attributes of an object file. This can be useful to get objdump to
disassemble instructions that aren't in the base without forcing
users to add a .attribute to the assembly file.

The binutils assembler does this by default.

Similar option exists for ARM. I will wire it to a clang option in
another patch. Similar to https://reviews.llvm.org/D31813

Reviewed By: asb, kito-cheng

Differential Revision: https://reviews.llvm.org/D148782

llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp

@@ -35,6 +35,7 @@
 #include "llvm/MC/MCValue.h"
 #include "llvm/MC/TargetRegistry.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/RISCVAttributes.h"
 #include "llvm/Support/RISCVISAInfo.h"
@@ -48,6 +49,9 @@ using namespace llvm;
 STATISTIC(RISCVNumInstrsCompressed,
           "Number of RISC-V Compressed instructions emitted");
 
+static cl::opt<bool> AddBuildAttributes("riscv-add-build-attributes",
+                                        cl::init(false));
+
 namespace llvm {
 extern const SubtargetFeatureKV RISCVFeatureKV[RISCV::NumSubtargetFeatures];
 } // namespace llvm
@@ -240,6 +244,8 @@ public:
   RISCVAsmParser(const MCSubtargetInfo &STI, MCAsmParser &Parser,
                  const MCInstrInfo &MII, const MCTargetOptions &Options)
       : MCTargetAsmParser(Options, STI, MII) {
+    MCAsmParserExtension::Initialize(Parser);
+
     Parser.addAliasForDirective(".half", ".2byte");
     Parser.addAliasForDirective(".hword", ".2byte");
     Parser.addAliasForDirective(".word", ".4byte");
@@ -265,6 +271,9 @@ public:
 
     const MCObjectFileInfo *MOFI = Parser.getContext().getObjectFileInfo();
     ParserOptions.IsPicEnabled = MOFI->isPositionIndependent();
+
+    if (AddBuildAttributes)
+      getTargetStreamer().emitTargetAttributes(STI, /*EmitStackAlign*/ false);
   }
 };
 

llvm/lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.cpp

@@ -46,11 +46,13 @@ void RISCVTargetStreamer::setTargetABI(RISCVABI::ABI ABI) {
   TargetABI = ABI;
 }
 
-void RISCVTargetStreamer::emitTargetAttributes(const MCSubtargetInfo &STI) {
+void RISCVTargetStreamer::emitTargetAttributes(const MCSubtargetInfo &STI,
+                                               bool EmitStackAlign) {
   if (STI.hasFeature(RISCV::FeatureRVE))
     report_fatal_error("Codegen not yet implemented for RVE");
 
-  emitAttribute(RISCVAttrs::STACK_ALIGN, RISCVAttrs::ALIGN_16);
+  if (EmitStackAlign)
+    emitAttribute(RISCVAttrs::STACK_ALIGN, RISCVAttrs::ALIGN_16);
 
   auto ParseResult = RISCVFeatures::parseFeatureBits(
       STI.hasFeature(RISCV::Feature64Bit), STI.getFeatureBits());

llvm/lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.h

@@ -40,7 +40,7 @@ public:
   virtual void emitIntTextAttribute(unsigned Attribute, unsigned IntValue,
                                     StringRef StringValue);
 
-  void emitTargetAttributes(const MCSubtargetInfo &STI);
+  void emitTargetAttributes(const MCSubtargetInfo &STI, bool EmitStackAlign);
   void setTargetABI(RISCVABI::ABI ABI);
   RISCVABI::ABI getTargetABI() const { return TargetABI; }
 };

llvm/lib/Target/RISCV/RISCVAsmPrinter.cpp

@@ -229,7 +229,7 @@ void RISCVAsmPrinter::emitAttributes() {
   // Use MCSubtargetInfo from TargetMachine. Individual functions may have
   // attributes that differ from other functions in the module and we have no
   // way to know which function is correct.
-  RTS.emitTargetAttributes(*TM.getMCSubtargetInfo());
+  RTS.emitTargetAttributes(*TM.getMCSubtargetInfo(), /*EmitStackAlign*/ true);
 }
 
 void RISCVAsmPrinter::emitFunctionEntryLabel() {

llvm/test/MC/RISCV/attribute.s

@@ -2,6 +2,10 @@
 
 # RUN: llvm-mc %s -triple=riscv32 -filetype=asm | FileCheck %s
 # RUN: llvm-mc %s -triple=riscv64 -filetype=asm | FileCheck %s
+# RUN: llvm-mc %s -triple=riscv32 -filetype=asm -riscv-add-build-attributes \
+# RUN:   | FileCheck %s
+# RUN: llvm-mc %s -triple=riscv64 -filetype=asm -riscv-add-build-attributes \
+# RUN:   | FileCheck %s
 
 .attribute stack_align, 16
 # CHECK: attribute      4, 16

llvm/test/MC/RISCV/default-build-attributes.s

@@ -0,0 +1,20 @@
+# RUN: llvm-mc %s -triple=riscv32 -filetype=asm -riscv-add-build-attributes \
+# RUN:   | FileCheck %s --check-prefixes=RV32
+# RUN: llvm-mc %s -triple=riscv64 -filetype=asm -riscv-add-build-attributes \
+# RUN:   | FileCheck %s --check-prefixes=RV64
+# RUN: llvm-mc %s -triple=riscv32 -filetype=asm -riscv-add-build-attributes \
+# RUN:   -mattr=+m | FileCheck %s --check-prefixes=RV32M
+# RUN: llvm-mc %s -triple=riscv64 -filetype=asm -riscv-add-build-attributes \
+# RUN:   -mattr=+m | FileCheck %s --check-prefixes=RV64M
+
+# RV32-NOT: attribute 4
+# RV32: attribute 5, "rv32i2p1"
+
+# RV64-NOT: attribute 4
+# RV64: attribute 5, "rv64i2p1"
+
+# RV32M-NOT: attribute 4
+# RV32M: attribute 5, "rv32i2p1_m2p0"
+
+# RV64M-NOT: attribute 4
+# RV64M: attribute 5, "rv64i2p1_m2p0"