[Clang][AArch64] Introduce CLANG_AARCH64_DEFAULT_CPU

[SjoerdMeijer]

Summary

We would like to default to a particular CPU when -mcpu is not provided on the Clang command line. This introduces a CMake variable CLANG_AARCH64_DEFAULT_CPU, so that a default CPU can be specified at CMake build time.

If CLANG_AARCH64_DEFAULT_CPU is not provided, it will default to "generic", so this will make sure we keep the current behaviour.

If CLANG_AARCH64_DEFAULT_CPU is changed, a companion downstream patch is required at the moment for the tests that check "target-cpu generic" when -mcpu is omitted. Ideally we could somehow parameterize that with something like "target-cpu ${CLANG_AARCH64_DEFAULT_CPU}" in the tests. Maybe that is possible in Lit, in the config files, but I don't know and am still looking into that.

[tschuett]

Clang configuration files are insufficient for this task?
https://clang.llvm.org/docs/UsersManual.html#configuration-files

[SjoerdMeijer]

In D155201#4497585, @tschuett wrote:

Clang configuration files are insufficient for this task?
https://clang.llvm.org/docs/UsersManual.html#configuration-files

I don't know, but will look into it.
Ideally, we could change this:

CHECK:  "-cc1"{{.*}} "-triple" "aarch64{{.*}}" "-target-cpu" "generic"

into:

CHECK:  "-cc1"{{.*}} "-triple" "aarch64{{.*}}" "-target-cpu" "$(CLANG_AARCH64_DEFAULT_CPU)"

So I feel that this is more of a lit thing. Lit has to understand the value of CLANG_AARCH64_DEFAULT_CPU.

[madhur13490]

Other approach worth looking at is to push args in Clang.cpp. We can look at AddAArch64TargetArgs() and pass default CPU if -mcpu or mtune is not provided.

[SjoerdMeijer]

In D155201#4497960, @madhur13490 wrote:

Other approach worth looking at is to push args in Clang.cpp. We can look at AddAArch64TargetArgs() and pass default CPU if -mcpu or mtune is not provided.

yes, but I think I prefer this approach if I am able find a solution for the test cases. This approach defines a variable, which we can use to define a "substitution" in the lit configuration file (https://llvm.org/docs/TestingGuide.html#substitutions).

In lit.cfg.py, I am looking to add something along the lines of:

config.substitutions.append(("%AARCH64_DEFAULT_CPU", @CLANG_AARCH64_DEFAULT_CPU@))

And then in the test files we can check for:

"-target-cpu" %(AARCH64_DEFAULT_CPU)

I don't have it working yet, but am working on it.
I think this is the more elegant solution, which pushing an -mcpu value doesn't provide, I think.

[MaskRay]

I missed the CLANG_SYSTEMZ_DEFAULT_ARCH introduction, but adding this kind of customization is what we strive to avoid and there is trend to remove more CLANG_*_DEFAULT.
Can you use https://clang.llvm.org/docs/UsersManual.html#configuration-files instead?

[michaelplatings]

clang/CMakeLists.txt

241–243

No need for if(NOT... for cache variables.

[SjoerdMeijer]

In D155201#4498039, @MaskRay wrote:

I missed the CLANG_SYSTEMZ_DEFAULT_ARCH introduction, but adding this kind of customization is what we strive to avoid and there is trend to remove more CLANG_*_DEFAULT.
Can you use https://clang.llvm.org/docs/UsersManual.html#configuration-files instead?

Ha, I didn't know about these configuration files!
Sorry @tschuett , you also suggested that, I had not yet looked closely enough.

Thanks for the suggestions. These config files look exactly what I need, so am happy to abandon this.

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")
 
+if (NOT (CLANG_AARCH64_DEFAULT_CPU))
+  set(CLANG_AARCH64_DEFAULT_CPU "generic" CACHE STRING "AArch64 Default CPU")
+endif()
+
 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 AArch64. */
+#define CLANG_AARCH64_DEFAULT_CPU "${CLANG_AARCH64_DEFAULT_CPU}"
+
 /* Multilib basename for libdir. */
 #define CLANG_INSTALL_LIBDIR_BASENAME "${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/AArch64.cpp

 //===--- AArch64.cpp - AArch64 (not ARM) 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 "AArch64.h"
 #include "../CommonArgs.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/TargetParser/AArch64TargetParser.h"
 #include "llvm/TargetParser/Host.h"
 
 using namespace clang::driver;
if (Triple.isArm64e())
     return "apple-a12";
 
   // Make sure we pick the appropriate Apple CPU if -arch is used or when
   // targetting a Darwin OS.
   if (Args.getLastArg(options::OPT_arch) || Triple.isOSDarwin())
     return Triple.getArch() == llvm::Triple::aarch64_32 ? "apple-s4"
                                                         : "apple-a7";
 
-  return "generic";
+  return CLANG_AARCH64_DEFAULT_CPU;
 }
 
 // Decode AArch64 features from string like +[no]featureA+[no]featureB+...
 static bool DecodeAArch64Features(const Driver &D, StringRef text,
                                   std::vector<StringRef> &Features,
                                   const llvm::AArch64::ArchInfo &ArchInfo) {
   SmallVector<StringRef, 8> Split;
   text.split(Split, StringRef("+"), -1, false);