clang/lib/Driver/ToolChains/Arch/RISCV.cpp - llvm-project - Git at Google

 //===--- RISCV.cpp - RISCV Helpers for Tools --------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//

 #include "RISCV.h"
 #include "ToolChains/CommonArgs.h"
 #include "clang/Basic/CharInfo.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/RISCVISAInfo.h"
 #include "llvm/Support/TargetParser.h"
 #include "llvm/Support/raw_ostream.h"

 using namespace clang::driver;
 using namespace clang::driver::tools;
 using namespace clang;
 using namespace llvm::opt;

 // Returns false if an error is diagnosed.
 static bool getArchFeatures(const Driver &D, StringRef Arch,
                             std::vector<StringRef> &Features,
                             const ArgList &Args) {
   bool EnableExperimentalExtensions =
       Args.hasArg(options::OPT_menable_experimental_extensions);
   auto ISAInfo =
       llvm::RISCVISAInfo::parseArchString(Arch, EnableExperimentalExtensions);
   if (!ISAInfo) {
     handleAllErrors(ISAInfo.takeError(), [&](llvm::StringError &ErrMsg) {
       D.Diag(diag::err_drv_invalid_riscv_arch_name)
           << Arch << ErrMsg.getMessage();
     });

     return false;
   }

   (*ISAInfo)->toFeatures(
       Features, [&Args](const Twine &Str) { return Args.MakeArgString(Str); });
   return true;
 }

 // Get features except standard extension feature
 static void getRISCFeaturesFromMcpu(const Driver &D, const llvm::Triple &Triple,
                                     const llvm::opt::ArgList &Args,
                                     const llvm::opt::Arg *A, StringRef Mcpu,
                                     std::vector<StringRef> &Features) {
   bool Is64Bit = (Triple.getArch() == llvm::Triple::riscv64);
   llvm::RISCV::CPUKind CPUKind = llvm::RISCV::parseCPUKind(Mcpu);
   if (!llvm::RISCV::checkCPUKind(CPUKind, Is64Bit) ||
       !llvm::RISCV::getCPUFeaturesExceptStdExt(CPUKind, Features)) {
     D.Diag(clang::diag::err_drv_clang_unsupported) << A->getAsString(Args);
   }
 }

 void riscv::getRISCVTargetFeatures(const Driver &D, const llvm::Triple &Triple,
                                    const ArgList &Args,
                                    std::vector<StringRef> &Features) {
   StringRef MArch = getRISCVArch(Args, Triple);

   if (!getArchFeatures(D, MArch, Features, Args))
     return;

   // If users give march and mcpu, get std extension feature from MArch
   // and other features (ex. mirco architecture feature) from mcpu
   if (Arg *A = Args.getLastArg(options::OPT_mcpu_EQ))
     getRISCFeaturesFromMcpu(D, Triple, Args, A, A->getValue(), Features);

   // Handle features corresponding to "-ffixed-X" options
   if (Args.hasArg(options::OPT_ffixed_x1))
     Features.push_back("+reserve-x1");
   if (Args.hasArg(options::OPT_ffixed_x2))
     Features.push_back("+reserve-x2");
   if (Args.hasArg(options::OPT_ffixed_x3))
     Features.push_back("+reserve-x3");
   if (Args.hasArg(options::OPT_ffixed_x4))
     Features.push_back("+reserve-x4");
   if (Args.hasArg(options::OPT_ffixed_x5))
     Features.push_back("+reserve-x5");
   if (Args.hasArg(options::OPT_ffixed_x6))
     Features.push_back("+reserve-x6");
   if (Args.hasArg(options::OPT_ffixed_x7))
     Features.push_back("+reserve-x7");
   if (Args.hasArg(options::OPT_ffixed_x8))
     Features.push_back("+reserve-x8");
   if (Args.hasArg(options::OPT_ffixed_x9))
     Features.push_back("+reserve-x9");
   if (Args.hasArg(options::OPT_ffixed_x10))
     Features.push_back("+reserve-x10");
   if (Args.hasArg(options::OPT_ffixed_x11))
     Features.push_back("+reserve-x11");
   if (Args.hasArg(options::OPT_ffixed_x12))
     Features.push_back("+reserve-x12");
   if (Args.hasArg(options::OPT_ffixed_x13))
     Features.push_back("+reserve-x13");
   if (Args.hasArg(options::OPT_ffixed_x14))
     Features.push_back("+reserve-x14");
   if (Args.hasArg(options::OPT_ffixed_x15))
     Features.push_back("+reserve-x15");
   if (Args.hasArg(options::OPT_ffixed_x16))
     Features.push_back("+reserve-x16");
   if (Args.hasArg(options::OPT_ffixed_x17))
     Features.push_back("+reserve-x17");
   if (Args.hasArg(options::OPT_ffixed_x18))
     Features.push_back("+reserve-x18");
   if (Args.hasArg(options::OPT_ffixed_x19))
     Features.push_back("+reserve-x19");
   if (Args.hasArg(options::OPT_ffixed_x20))
     Features.push_back("+reserve-x20");
   if (Args.hasArg(options::OPT_ffixed_x21))
     Features.push_back("+reserve-x21");
   if (Args.hasArg(options::OPT_ffixed_x22))
     Features.push_back("+reserve-x22");
   if (Args.hasArg(options::OPT_ffixed_x23))
     Features.push_back("+reserve-x23");
   if (Args.hasArg(options::OPT_ffixed_x24))
     Features.push_back("+reserve-x24");
   if (Args.hasArg(options::OPT_ffixed_x25))
     Features.push_back("+reserve-x25");
   if (Args.hasArg(options::OPT_ffixed_x26))
     Features.push_back("+reserve-x26");
   if (Args.hasArg(options::OPT_ffixed_x27))
     Features.push_back("+reserve-x27");
   if (Args.hasArg(options::OPT_ffixed_x28))
     Features.push_back("+reserve-x28");
   if (Args.hasArg(options::OPT_ffixed_x29))
     Features.push_back("+reserve-x29");
   if (Args.hasArg(options::OPT_ffixed_x30))
     Features.push_back("+reserve-x30");
   if (Args.hasArg(options::OPT_ffixed_x31))
     Features.push_back("+reserve-x31");

   // -mrelax is default, unless -mno-relax is specified.
   if (Args.hasFlag(options::OPT_mrelax, options::OPT_mno_relax, true))
     Features.push_back("+relax");
   else
     Features.push_back("-relax");

   // GCC Compatibility: -mno-save-restore is default, unless -msave-restore is
   // specified.
   if (Args.hasFlag(options::OPT_msave_restore, options::OPT_mno_save_restore, false))
     Features.push_back("+save-restore");
   else
     Features.push_back("-save-restore");

   // Now add any that the user explicitly requested on the command line,
   // which may override the defaults.
   handleTargetFeaturesGroup(Args, Features, options::OPT_m_riscv_Features_Group);
 }

 StringRef riscv::getRISCVABI(const ArgList &Args, const llvm::Triple &Triple) {
   assert((Triple.getArch() == llvm::Triple::riscv32 ||
           Triple.getArch() == llvm::Triple::riscv64) &&
          "Unexpected triple");

   // GCC's logic around choosing a default `-mabi=` is complex. If GCC is not
   // configured using `--with-abi=`, then the logic for the default choice is
   // defined in config.gcc. This function is based on the logic in GCC 9.2.0.
   //
   // The logic used in GCC 9.2.0 is the following, in order:
   // 1. Explicit choices using `--with-abi=`
   // 2. A default based on `--with-arch=`, if provided
   // 3. A default based on the target triple's arch
   //
   // The logic in config.gcc is a little circular but it is not inconsistent.
   //
   // Clang does not have `--with-arch=` or `--with-abi=`, so we use `-march=`
   // and `-mabi=` respectively instead.
   //
   // In order to make chosing logic more clear, Clang uses the following logic,
   // in order:
   // 1. Explicit choices using `-mabi=`
   // 2. A default based on the architecture as determined by getRISCVArch
   // 3. Choose a default based on the triple

   // 1. If `-mabi=` is specified, use it.
   if (const Arg *A = Args.getLastArg(options::OPT_mabi_EQ))
     return A->getValue();

   // 2. Choose a default based on the target architecture.
   //
   // rv32g | rv32*d -> ilp32d
   // rv32e -> ilp32e
   // rv32* -> ilp32
   // rv64g | rv64*d -> lp64d
   // rv64* -> lp64
   StringRef Arch = getRISCVArch(Args, Triple);

   auto ParseResult = llvm::RISCVISAInfo::parseArchString(
       Arch, /* EnableExperimentalExtension */ true);
   if (!ParseResult) {
     // Ignore parsing error, just go 3rd step.
     consumeError(ParseResult.takeError());
   } else {
     auto &ISAInfo = *ParseResult;
     bool HasD = ISAInfo->hasExtension("d");
     unsigned XLen = ISAInfo->getXLen();
     if (XLen == 32) {
       bool HasE = ISAInfo->hasExtension("e");
       if (HasD)
         return "ilp32d";
       if (HasE)
         return "ilp32e";
       return "ilp32";
     } else if (XLen == 64) {
       if (HasD)
         return "lp64d";
       return "lp64";
     }
     llvm_unreachable("unhandled XLen");
   }

   // 3. Choose a default based on the triple
   //
   // We deviate from GCC's defaults here:
   // - On `riscv{XLEN}-unknown-elf` we use the integer calling convention only.
   // - On all other OSs we use the double floating point calling convention.
   if (Triple.getArch() == llvm::Triple::riscv32) {
     if (Triple.getOS() == llvm::Triple::UnknownOS)
       return "ilp32";
     else
       return "ilp32d";
   } else {
     if (Triple.getOS() == llvm::Triple::UnknownOS)
       return "lp64";
     else
       return "lp64d";
   }
 }

 StringRef riscv::getRISCVArch(const llvm::opt::ArgList &Args,
                               const llvm::Triple &Triple) {
   assert((Triple.getArch() == llvm::Triple::riscv32 ||
           Triple.getArch() == llvm::Triple::riscv64) &&
          "Unexpected triple");

   // GCC's logic around choosing a default `-march=` is complex. If GCC is not
   // configured using `--with-arch=`, then the logic for the default choice is
   // defined in config.gcc. This function is based on the logic in GCC 9.2.0. We
   // deviate from GCC's default on additional `-mcpu` option (GCC does not
   // support `-mcpu`) and baremetal targets (UnknownOS) where neither `-march`
   // nor `-mabi` is specified.
   //
   // The logic used in GCC 9.2.0 is the following, in order:
   // 1. Explicit choices using `--with-arch=`
   // 2. A default based on `--with-abi=`, if provided
   // 3. A default based on the target triple's arch
   //
   // The logic in config.gcc is a little circular but it is not inconsistent.
   //
   // Clang does not have `--with-arch=` or `--with-abi=`, so we use `-march=`
   // and `-mabi=` respectively instead.
   //
   // Clang uses the following logic, in order:
   // 1. Explicit choices using `-march=`
   // 2. Based on `-mcpu` if the target CPU has a default ISA string
   // 3. A default based on `-mabi`, if provided
   // 4. A default based on the target triple's arch
   //
   // Clang does not yet support MULTILIB_REUSE, so we use `rv{XLEN}imafdc`
   // instead of `rv{XLEN}gc` though they are (currently) equivalent.

   // 1. If `-march=` is specified, use it.
   if (const Arg *A = Args.getLastArg(options::OPT_march_EQ))
     return A->getValue();

   // 2. Get march (isa string) based on `-mcpu=`
   if (const Arg *A = Args.getLastArg(options::OPT_mcpu_EQ)) {
     StringRef MArch = llvm::RISCV::getMArchFromMcpu(A->getValue());
     // Bypass if target cpu's default march is empty.
     if (MArch != "")
       return MArch;
   }

   // 3. Choose a default based on `-mabi=`
   //
   // ilp32e -> rv32e
   // ilp32 | ilp32f | ilp32d -> rv32imafdc
   // lp64 | lp64f | lp64d -> rv64imafdc
   if (const Arg *A = Args.getLastArg(options::OPT_mabi_EQ)) {
     StringRef MABI = A->getValue();

     if (MABI.equals_insensitive("ilp32e"))
       return "rv32e";
     else if (MABI.startswith_insensitive("ilp32"))
       return "rv32imafdc";
     else if (MABI.startswith_insensitive("lp64"))
       return "rv64imafdc";
   }

   // 4. Choose a default based on the triple
   //
   // We deviate from GCC's defaults here:
   // - On `riscv{XLEN}-unknown-elf` we default to `rv{XLEN}imac`
   // - On all other OSs we use `rv{XLEN}imafdc` (equivalent to `rv{XLEN}gc`)
   if (Triple.getArch() == llvm::Triple::riscv32) {
     if (Triple.getOS() == llvm::Triple::UnknownOS)
       return "rv32imac";
     else
       return "rv32imafdc";
   } else {
     if (Triple.getOS() == llvm::Triple::UnknownOS)
       return "rv64imac";
     else
       return "rv64imafdc";
   }
 }
	//===--- RISCV.cpp - RISCV Helpers for Tools --------------------- C++ --===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//

	#include "RISCV.h"
	#include "ToolChains/CommonArgs.h"
	#include "clang/Basic/CharInfo.h"
	#include "clang/Driver/Driver.h"
	#include "clang/Driver/DriverDiagnostic.h"
	#include "clang/Driver/Options.h"
	#include "llvm/ADT/Optional.h"
	#include "llvm/Option/ArgList.h"
	#include "llvm/Support/Error.h"
	#include "llvm/Support/RISCVISAInfo.h"
	#include "llvm/Support/TargetParser.h"
	#include "llvm/Support/raw_ostream.h"

	using namespace clang::driver;
	using namespace clang::driver::tools;
	using namespace clang;
	using namespace llvm::opt;

	// Returns false if an error is diagnosed.
	static bool getArchFeatures(const Driver &D, StringRef Arch,
	std::vector<StringRef> &Features,
	const ArgList &Args) {
	bool EnableExperimentalExtensions =
	Args.hasArg(options::OPT_menable_experimental_extensions);
	auto ISAInfo =
	llvm::RISCVISAInfo::parseArchString(Arch, EnableExperimentalExtensions);
	if (!ISAInfo) {
	handleAllErrors(ISAInfo.takeError(), [&](llvm::StringError &ErrMsg) {
	D.Diag(diag::err_drv_invalid_riscv_arch_name)
	<< Arch << ErrMsg.getMessage();
	});

	return false;
	}

	(*ISAInfo)->toFeatures(
	Features, [&Args](const Twine &Str) { return Args.MakeArgString(Str); });
	return true;
	}

	// Get features except standard extension feature
	static void getRISCFeaturesFromMcpu(const Driver &D, const llvm::Triple &Triple,
	const llvm::opt::ArgList &Args,
	const llvm::opt::Arg *A, StringRef Mcpu,
	std::vector<StringRef> &Features) {
	bool Is64Bit = (Triple.getArch() == llvm::Triple::riscv64);
	llvm::RISCV::CPUKind CPUKind = llvm::RISCV::parseCPUKind(Mcpu);
	if (!llvm::RISCV::checkCPUKind(CPUKind, Is64Bit) \|\|
	!llvm::RISCV::getCPUFeaturesExceptStdExt(CPUKind, Features)) {
	D.Diag(clang::diag::err_drv_clang_unsupported) << A->getAsString(Args);
	}
	}

	void riscv::getRISCVTargetFeatures(const Driver &D, const llvm::Triple &Triple,
	const ArgList &Args,
	std::vector<StringRef> &Features) {
	StringRef MArch = getRISCVArch(Args, Triple);

	if (!getArchFeatures(D, MArch, Features, Args))
	return;

	// If users give march and mcpu, get std extension feature from MArch
	// and other features (ex. mirco architecture feature) from mcpu
	if (Arg *A = Args.getLastArg(options::OPT_mcpu_EQ))
	getRISCFeaturesFromMcpu(D, Triple, Args, A, A->getValue(), Features);

	// Handle features corresponding to "-ffixed-X" options
	if (Args.hasArg(options::OPT_ffixed_x1))
	Features.push_back("+reserve-x1");
	if (Args.hasArg(options::OPT_ffixed_x2))
	Features.push_back("+reserve-x2");
	if (Args.hasArg(options::OPT_ffixed_x3))
	Features.push_back("+reserve-x3");
	if (Args.hasArg(options::OPT_ffixed_x4))
	Features.push_back("+reserve-x4");
	if (Args.hasArg(options::OPT_ffixed_x5))
	Features.push_back("+reserve-x5");
	if (Args.hasArg(options::OPT_ffixed_x6))
	Features.push_back("+reserve-x6");
	if (Args.hasArg(options::OPT_ffixed_x7))
	Features.push_back("+reserve-x7");
	if (Args.hasArg(options::OPT_ffixed_x8))
	Features.push_back("+reserve-x8");
	if (Args.hasArg(options::OPT_ffixed_x9))
	Features.push_back("+reserve-x9");
	if (Args.hasArg(options::OPT_ffixed_x10))
	Features.push_back("+reserve-x10");
	if (Args.hasArg(options::OPT_ffixed_x11))
	Features.push_back("+reserve-x11");
	if (Args.hasArg(options::OPT_ffixed_x12))
	Features.push_back("+reserve-x12");
	if (Args.hasArg(options::OPT_ffixed_x13))
	Features.push_back("+reserve-x13");
	if (Args.hasArg(options::OPT_ffixed_x14))
	Features.push_back("+reserve-x14");
	if (Args.hasArg(options::OPT_ffixed_x15))
	Features.push_back("+reserve-x15");
	if (Args.hasArg(options::OPT_ffixed_x16))
	Features.push_back("+reserve-x16");
	if (Args.hasArg(options::OPT_ffixed_x17))
	Features.push_back("+reserve-x17");
	if (Args.hasArg(options::OPT_ffixed_x18))
	Features.push_back("+reserve-x18");
	if (Args.hasArg(options::OPT_ffixed_x19))
	Features.push_back("+reserve-x19");
	if (Args.hasArg(options::OPT_ffixed_x20))
	Features.push_back("+reserve-x20");
	if (Args.hasArg(options::OPT_ffixed_x21))
	Features.push_back("+reserve-x21");
	if (Args.hasArg(options::OPT_ffixed_x22))
	Features.push_back("+reserve-x22");
	if (Args.hasArg(options::OPT_ffixed_x23))
	Features.push_back("+reserve-x23");
	if (Args.hasArg(options::OPT_ffixed_x24))
	Features.push_back("+reserve-x24");
	if (Args.hasArg(options::OPT_ffixed_x25))
	Features.push_back("+reserve-x25");
	if (Args.hasArg(options::OPT_ffixed_x26))
	Features.push_back("+reserve-x26");
	if (Args.hasArg(options::OPT_ffixed_x27))
	Features.push_back("+reserve-x27");
	if (Args.hasArg(options::OPT_ffixed_x28))
	Features.push_back("+reserve-x28");
	if (Args.hasArg(options::OPT_ffixed_x29))
	Features.push_back("+reserve-x29");
	if (Args.hasArg(options::OPT_ffixed_x30))
	Features.push_back("+reserve-x30");
	if (Args.hasArg(options::OPT_ffixed_x31))
	Features.push_back("+reserve-x31");

	// -mrelax is default, unless -mno-relax is specified.
	if (Args.hasFlag(options::OPT_mrelax, options::OPT_mno_relax, true))
	Features.push_back("+relax");
	else
	Features.push_back("-relax");

	// GCC Compatibility: -mno-save-restore is default, unless -msave-restore is
	// specified.
	if (Args.hasFlag(options::OPT_msave_restore, options::OPT_mno_save_restore, false))
	Features.push_back("+save-restore");
	else
	Features.push_back("-save-restore");

	// Now add any that the user explicitly requested on the command line,
	// which may override the defaults.
	handleTargetFeaturesGroup(Args, Features, options::OPT_m_riscv_Features_Group);
	}

	StringRef riscv::getRISCVABI(const ArgList &Args, const llvm::Triple &Triple) {
	assert((Triple.getArch() == llvm::Triple::riscv32 \|\|
	Triple.getArch() == llvm::Triple::riscv64) &&
	"Unexpected triple");

	// GCC's logic around choosing a default `-mabi=` is complex. If GCC is not
	// configured using `--with-abi=`, then the logic for the default choice is
	// defined in config.gcc. This function is based on the logic in GCC 9.2.0.
	//
	// The logic used in GCC 9.2.0 is the following, in order:
	// 1. Explicit choices using `--with-abi=`
	// 2. A default based on `--with-arch=`, if provided
	// 3. A default based on the target triple's arch
	//
	// The logic in config.gcc is a little circular but it is not inconsistent.
	//
	// Clang does not have `--with-arch=` or `--with-abi=`, so we use `-march=`
	// and `-mabi=` respectively instead.
	//
	// In order to make chosing logic more clear, Clang uses the following logic,
	// in order:
	// 1. Explicit choices using `-mabi=`
	// 2. A default based on the architecture as determined by getRISCVArch
	// 3. Choose a default based on the triple

	// 1. If `-mabi=` is specified, use it.
	if (const Arg *A = Args.getLastArg(options::OPT_mabi_EQ))
	return A->getValue();

	// 2. Choose a default based on the target architecture.
	//
	// rv32g \| rv32*d -> ilp32d
	// rv32e -> ilp32e
	// rv32* -> ilp32
	// rv64g \| rv64*d -> lp64d
	// rv64* -> lp64
	StringRef Arch = getRISCVArch(Args, Triple);

	auto ParseResult = llvm::RISCVISAInfo::parseArchString(
	Arch, /* EnableExperimentalExtension */ true);
	if (!ParseResult) {
	// Ignore parsing error, just go 3rd step.
	consumeError(ParseResult.takeError());
	} else {
	auto &ISAInfo = *ParseResult;
	bool HasD = ISAInfo->hasExtension("d");
	unsigned XLen = ISAInfo->getXLen();
	if (XLen == 32) {
	bool HasE = ISAInfo->hasExtension("e");
	if (HasD)
	return "ilp32d";
	if (HasE)
	return "ilp32e";
	return "ilp32";
	} else if (XLen == 64) {
	if (HasD)
	return "lp64d";
	return "lp64";
	}
	llvm_unreachable("unhandled XLen");
	}

	// 3. Choose a default based on the triple
	//
	// We deviate from GCC's defaults here:
	// - On `riscv{XLEN}-unknown-elf` we use the integer calling convention only.
	// - On all other OSs we use the double floating point calling convention.
	if (Triple.getArch() == llvm::Triple::riscv32) {
	if (Triple.getOS() == llvm::Triple::UnknownOS)
	return "ilp32";
	else
	return "ilp32d";
	} else {
	if (Triple.getOS() == llvm::Triple::UnknownOS)
	return "lp64";
	else
	return "lp64d";
	}
	}

	StringRef riscv::getRISCVArch(const llvm::opt::ArgList &Args,
	const llvm::Triple &Triple) {
	assert((Triple.getArch() == llvm::Triple::riscv32 \|\|
	Triple.getArch() == llvm::Triple::riscv64) &&
	"Unexpected triple");

	// GCC's logic around choosing a default `-march=` is complex. If GCC is not
	// configured using `--with-arch=`, then the logic for the default choice is
	// defined in config.gcc. This function is based on the logic in GCC 9.2.0. We
	// deviate from GCC's default on additional `-mcpu` option (GCC does not
	// support `-mcpu`) and baremetal targets (UnknownOS) where neither `-march`
	// nor `-mabi` is specified.
	//
	// The logic used in GCC 9.2.0 is the following, in order:
	// 1. Explicit choices using `--with-arch=`
	// 2. A default based on `--with-abi=`, if provided
	// 3. A default based on the target triple's arch
	//
	// The logic in config.gcc is a little circular but it is not inconsistent.
	//
	// Clang does not have `--with-arch=` or `--with-abi=`, so we use `-march=`
	// and `-mabi=` respectively instead.
	//
	// Clang uses the following logic, in order:
	// 1. Explicit choices using `-march=`
	// 2. Based on `-mcpu` if the target CPU has a default ISA string
	// 3. A default based on `-mabi`, if provided
	// 4. A default based on the target triple's arch
	//
	// Clang does not yet support MULTILIB_REUSE, so we use `rv{XLEN}imafdc`
	// instead of `rv{XLEN}gc` though they are (currently) equivalent.

	// 1. If `-march=` is specified, use it.
	if (const Arg *A = Args.getLastArg(options::OPT_march_EQ))
	return A->getValue();

	// 2. Get march (isa string) based on `-mcpu=`
	if (const Arg *A = Args.getLastArg(options::OPT_mcpu_EQ)) {
	StringRef MArch = llvm::RISCV::getMArchFromMcpu(A->getValue());
	// Bypass if target cpu's default march is empty.
	if (MArch != "")
	return MArch;
	}

	// 3. Choose a default based on `-mabi=`
	//
	// ilp32e -> rv32e
	// ilp32 \| ilp32f \| ilp32d -> rv32imafdc
	// lp64 \| lp64f \| lp64d -> rv64imafdc
	if (const Arg *A = Args.getLastArg(options::OPT_mabi_EQ)) {
	StringRef MABI = A->getValue();

	if (MABI.equals_insensitive("ilp32e"))
	return "rv32e";
	else if (MABI.startswith_insensitive("ilp32"))
	return "rv32imafdc";
	else if (MABI.startswith_insensitive("lp64"))
	return "rv64imafdc";
	}

	// 4. Choose a default based on the triple
	//
	// We deviate from GCC's defaults here:
	// - On `riscv{XLEN}-unknown-elf` we default to `rv{XLEN}imac`
	// - On all other OSs we use `rv{XLEN}imafdc` (equivalent to `rv{XLEN}gc`)
	if (Triple.getArch() == llvm::Triple::riscv32) {
	if (Triple.getOS() == llvm::Triple::UnknownOS)
	return "rv32imac";
	else
	return "rv32imafdc";
	} else {
	if (Triple.getOS() == llvm::Triple::UnknownOS)
	return "rv64imac";
	else
	return "rv64imafdc";
	}
	}