[Clang][RISCV] Add CMake options to configure default CPU

[• pcwang-thead]

Summary

For GCC, we can configure default target with with-{tune/arch/abi}
options. While for Clang, we don't have the similar way to configure.

Here we add two CMake options CLANG_RISCV32_DEFAULT_CPU and
CLANG_RISCV64_DEFAULT_CPU to configure default CPU for riscv32
and riscv64.

By specifying these two options when compiling Clang/LLVM, we can
achieve the same goal just like GCC's with-{tune/arch/abi}:

• Tune/arch info can be inferred from CPU info.
• Default ABI can be computed from arch string.

And we can do native detection via specifying default CPU to native.

Test clang/test/Driver/riscv-march-mcpu-mtune.c will failed if we
specify different default CPU.

[asb]

If this is a useful thing to have, would it make more sense as a target-independent option?

[craig.topper]

clang/lib/Driver/ToolChains/Arch/RISCV.cpp

322

Can we initialize CPU to the default CPU at the top? Remove the CPU.empty() check. Then we don't need a second "native" check.

[• pcwang-thead]

Simplify code.

[• pcwang-thead]

In D144853#4155482, @asb wrote:

If this is a useful thing to have, would it make more sense as a target-independent option?

Yes, I agree.

I need to do some researches here to figure out if all targets have the same behavior for CPU generic and some other stuffs.

[tstellar]

Would this make more sense in a config file rather than a CMake option? @MaskRay

[MaskRay]

I object to this change which further complicates the build system and makes the difference difficult to observe by users.
If your clang executable is at bin/clang, just create bin/riscv64-unknown-linux-gnu.cfg with -mcpu=xxx.
You may configure LLVM_DEFAULT_TARGET_TRIPLE=riscv64-unknown-linux-gnu so that the driver doesn't need extra --target=.

[• pcwang-thead]

In D144853#4157629, @MaskRay wrote:

I object to this change which further complicates the build system and makes the difference difficult to observe by users.
If your clang executable is at bin/clang, just create bin/riscv64-unknown-linux-gnu.cfg with -mcpu=xxx.
You may configure LLVM_DEFAULT_TARGET_TRIPLE=riscv64-unknown-linux-gnu so that the driver doesn't need extra --target=.

Thanks! I almost forget about config file.
I think this change is unnecessary now and thanks all for your time to review this patch. :-)

clang/CMakeLists.txt

 set(CLANG_DEFAULT_OBJCOPY "objcopy" CACHE STRING
   "Default objcopy executable to use.")
 
 set(CLANG_DEFAULT_OPENMP_RUNTIME "libomp" CACHE STRING
   "Default OpenMP runtime used by -fopenmp.")
 
 set(CLANG_SYSTEMZ_DEFAULT_ARCH "z10" CACHE STRING "SystemZ Default Arch")
 
+set(CLANG_RISCV32_DEFAULT_CPU "generic-rv32" CACHE STRING "RISCV32 Default CPU")
+set(CLANG_RISCV64_DEFAULT_CPU "generic-rv64" CACHE STRING "RISCV64 Default CPU")
+
 set(CLANG_VENDOR ${PACKAGE_VENDOR} CACHE STRING
   "Vendor-specific text for showing with version information.")
 
 set(CLANG_REPOSITORY_STRING "" CACHE STRING
   "Vendor-specific text for showing the repository the source is taken from.")
 
 if(CLANG_REPOSITORY_STRING)
   add_definitions(-DCLANG_REPOSITORY_STRING="${CLANG_REPOSITORY_STRING}")

clang/include/clang/Config/config.h.cmake

 #define CLANG_DEFAULT_OBJCOPY "${CLANG_DEFAULT_OBJCOPY}"
 
 /* Default OpenMP runtime used by -fopenmp. */
 #define CLANG_DEFAULT_OPENMP_RUNTIME "${CLANG_DEFAULT_OPENMP_RUNTIME}"
 
 /* Default architecture for SystemZ. */
 #define CLANG_SYSTEMZ_DEFAULT_ARCH "${CLANG_SYSTEMZ_DEFAULT_ARCH}"
 
+/* Default CPU for RISCV. */
+#define CLANG_RISCV32_DEFAULT_CPU "${CLANG_RISCV32_DEFAULT_CPU}"
+#define CLANG_RISCV64_DEFAULT_CPU "${CLANG_RISCV64_DEFAULT_CPU}"
+
 /* Multilib basename for libdir. */
 #define CLANG_INSTALL_LIBDIR_BASENAME "${CLANG_INSTALL_LIBDIR_BASENAME}"
 
 /* Relative directory for resource files */
 #define CLANG_RESOURCE_DIR "${CLANG_RESOURCE_DIR}"
 
 /* Directories clang will search for headers */
 #define C_INCLUDE_DIRS "${C_INCLUDE_DIRS}"

clang/lib/Driver/ToolChains/Arch/RISCV.cpp

 //===--- 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 "../Clang.h"
 #include "ToolChains/CommonArgs.h"
 #include "clang/Basic/CharInfo.h"
+#include "clang/Config/config.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/RISCVISAInfo.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TargetParser/Host.h"
if (Triple.getOS() == llvm::Triple::UnknownOS)
       return "rv64imac";
     else
       return "rv64imafdc";
   }
 }
 
 std::string riscv::getRISCVTargetCPU(const llvm::opt::ArgList &Args,
                                      const llvm::Triple &Triple) {
-  std::string CPU;
+  std::string CPU = Triple.isRISCV64() ? CLANG_RISCV64_DEFAULT_CPU
+                                       : CLANG_RISCV32_DEFAULT_CPU;
   // If we have -mcpu, use that.
   if (const Arg *A = Args.getLastArg(options::OPT_mcpu_EQ))
     CPU = A->getValue();
 
   // Handle CPU name is 'native'.
   if (CPU == "native")
     CPU = llvm::sys::getHostCPUName();
 
-  if (!CPU.empty())
-    return CPU;
-
-  return Triple.isRISCV64() ? "generic-rv64" : "generic-rv32";
 }