[libc][NFC] Detect host CPU features using try_compile instead of try_run.

[sivachandra]

Summary

This fixes scenarios like the runtime build wherein the clang is
built to use libcxx or libunwind by default which are either not yet
built or not yet installed in the places where the clang driver expects
them.

Fixes llvm/llvm-project#59762

[gchatelet]

Thx for the patch @sivachandra! It's definitely better than what I did, I was unhappy with the design but it was a first step.

There are still a few issues in my opinion:

• in msvc features have a different name, this means that the way we refer to features in CMake cannot be the same as what is put into the file,
• the way we detect features in CMake and in llvm libc can be different,
• what is actually compiled by CMake is hard to understand.

I think this can be improved further.

Here is what I have in mind :

• Add preprocessor feature detection to something like libc/src/__support/cpu_features.h (we already have LIBC_TARGET_HAS_FMA in libc/src/__support/architectures.h). This file would only depend on freestanding headers like compiler_features.h or architectures.h (sidenote we should probably make it clear in the filename that these headers are freestanding / only preprocessor)
• Create one cpp file per feature with a filename similar to the LIBC preprocessor definition. We don't have a lot of features so the number of files will stay low. This file would use the libc header so we have consistent detection between CMake (Bazel?) and the code.

WDYT? I can take a look at it if you want?

[sivachandra]

In D141997#4061578, @gchatelet wrote:

I think this can be improved further.

Here is what I have in mind :

• Add preprocessor feature detection to something like libc/src/__support/cpu_features.h (we already have LIBC_TARGET_HAS_FMA in libc/src/__support/architectures.h). This file would only depend on freestanding headers like compiler_features.h or architectures.h (sidenote we should probably make it clear in the filename that these headers are freestanding / only preprocessor)
• Create one cpp file per feature with a filename similar to the LIBC preprocessor definition. We don't have a lot of features so the number of files will stay low. This file would use the libc header so we have consistent detection between CMake (Bazel?) and the code.

WDYT? I can take a look at it if you want?

I think the essential idea you are proposing is to move feature detection from config time to build time. Is that correct? If yes, that sounds OK to me as long as we can do the detection without having to run fully linked executables (which is the core problem this patch is trying to solve.)

[gchatelet]

I've created https://reviews.llvm.org/D142108 as a replacement for this patch. Let me know what you think.

[gchatelet]

https://reviews.llvm.org/D142108 has been submitted, this patch can be marked as abandoned.

libc/cmake/modules/LLVMLibCCheckCpuFeatures.cmake

function(_intersection output_var list1 list2)
   foreach(element IN LISTS list1)
     if("${list2}" MATCHES "(^|;)${element}(;|$)")
       list(APPEND tmp "${element}")
     endif()
   endforeach()
   set(${output_var} ${tmp} PARENT_SCOPE)
 endfunction()
 
-# Generates a cpp file to introspect the compiler defined flags.
-function(_generate_check_code)
+# Generates cpp files to introspect the compiler defined flags.
+set(AVAILABLE_CPU_FEATURES "")
+if(LIBC_CROSSBUILD)
+  # If we are doing a cross build, we will just assume that all CPU features
+  # are available.
+  set(AVAILABLE_CPU_FEATURES ${ALL_CPU_FEATURES})
+else()
+  set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
   foreach(feature IN LISTS ALL_CPU_FEATURES)
-    set(DEFINITIONS
-        "${DEFINITIONS}
+    set(CPU_FEATURE_CHECK_SOURCE "
+        int main() {
         #ifdef __${feature}__
-    \"${feature}\",
-#endif")
-  endforeach()
+          return 0;
+        #else
+          __## syntax error ##__
+        #endif
+        }")
     configure_file(
       "${LIBC_SOURCE_DIR}/cmake/modules/cpu_features/check_cpu_features.cpp.in"
-    "cpu_features/check_cpu_features.cpp" @ONLY)
-endfunction()
-_generate_check_code()
+      "check_cpu_features/check_cpu_feature_${feature}.cpp" @ONLY
+    )
+    try_compile(
+      has_feature ${CMAKE_CURRENT_BINARY_DIR}/cpu_features
+      ${CMAKE_CURRENT_BINARY_DIR}/check_cpu_features/check_cpu_feature_${feature}.cpp
+      COMPILE_DEFINITIONS ${LIBC_COMPILE_OPTIONS_NATIVE}
+    )
+  if(has_feature)
+    list(APPEND AVAILABLE_CPU_FEATURES ${feature})
+  endif()
+endforeach()
+endif()
 
-set(LIBC_CPU_FEATURES "" CACHE PATH "Host supported CPU features")
+set(LIBC_CPU_FEATURES ${AVAILABLE_CPU_FEATURES} CACHE STRING "Host supported CPU features")
 
-if(LIBC_CROSSBUILD)
-  _intersection(cpu_features "${ALL_CPU_FEATURES}" "${LIBC_CPU_FEATURES}")
-  if(NOT "${cpu_features}" STREQUAL "${LIBC_CPU_FEATURES}")
-    message(FATAL_ERROR "Unsupported CPU features: ${cpu_features}")
-  endif()
+_intersection(cpu_features "${AVAILABLE_CPU_FEATURES}" "${LIBC_CPU_FEATURES}")
+else()
   message(STATUS "Set CPU features: ${cpu_features}")
   set(LIBC_CPU_FEATURES "${cpu_features}")
-else()
-  # Populates the LIBC_CPU_FEATURES list from host.
-  try_run(
-    run_result compile_result "${CMAKE_CURRENT_BINARY_DIR}/check_${feature}"
-    "${CMAKE_CURRENT_BINARY_DIR}/cpu_features/check_cpu_features.cpp"
-    COMPILE_DEFINITIONS ${LIBC_COMPILE_OPTIONS_NATIVE}
-    COMPILE_OUTPUT_VARIABLE compile_output
-    RUN_OUTPUT_VARIABLE run_output)
-  if("${run_result}" EQUAL 0)
-    message(STATUS "Set CPU features: ${run_output}")
-    set(LIBC_CPU_FEATURES "${run_output}")
-  elseif(NOT ${compile_result})
-    message(FATAL_ERROR "Failed to compile: ${compile_output}")
-  else()
-    message(FATAL_ERROR "Failed to run: ${run_output}")
-  endif()
 endif()

libc/cmake/modules/cpu_features/check_cpu_features.cpp.in

-#include <stdio.h>
-#include <stdlib.h>
-
-// This file is instantiated by CMake.
-// DEFINITIONS below is replaced with a set of lines like so:
-//   #ifdef __SSE2__
-//     "SSE2",
-//   #endif
-//
-// This allows for introspection of compiler definitions.
-// The output of the program is a single line of semi colon separated feature
-// names.
-
-// MSVC is using a different set of preprocessor definitions for 
-// SSE and SSE2, see _M_IX86_FP in
-// https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
-
-int main(int, char **) {
-  const char *strings[] = {
-      @DEFINITIONS@
-      // If DEFINITIONS turns out to be empty, we don't want to list
-      // an empty array. So, we add an end of list marker.
-      "<end_of_feature_list>"
-  };
-  const size_t size = sizeof(strings) / sizeof(strings[0]);
-  for (size_t i = 0; i < size; ++i) {
-    if (i)
-      putchar(';');
-    fputs(strings[i], stdout);
-  }
-  return EXIT_SUCCESS;
-}
+@CPU_FEATURE_CHECK_SOURCE@