[compiler-rt/profile] register __llvm_profile_write_file to run during quick_exit.

[pirama]

Summary

This change registers __llvm_profile_write_file with at_quick_exit
when it's available.

Some processes don't exit normally (via return from main or
exit()) to avoid the cost of calling destructors of static globals.
They call quick_exit or _exit instead. Registering with
at_quick_exit allows coverage data to be written when quick_exit is
called.

[davidxl]

compiler-rt/test/profile/instrprof-write-file-at-quick-exit-explicitly.c

13

Is this explicit call needed?

[pirama]

compiler-rt/test/profile/instrprof-write-file-at-quick-exit-explicitly.c

13

I based this test on compiler-rt/test/profile/instrprof-write-file-atexit-explicitly.c, which tests this for atexit. That test also has a similar structure.

We cannot set LLVM_PROFILE_FILE in this test (like RUN: %run LLVM_PROFILE_FILE=%t.profraw %t %t.profraw) because it may not work on all platforms. Instead, we want to set the file name via __llvm_profile_set_filename. But that call still creates an empty default.profraw when the runtime gets initialized. So we set __llvm_profile_runtime = 0 to skip initializing the runtime but explicitly have a call __llvm_profile_register_write_file_atexit.

[davidxl]

lgtm

[pirama]

Looks like quick_exit + dlclose interaction is not well-defined and this change may cause calls into dlclose-d libraries.

As @enh pointed out in an internal thread about An at_quick_exit() registered function is not called when the object defining the function is unloaded. Rather, it is simply removed from the at_quick_exit() registration list.:

it seems like glibc does this. no-one else (the BSDs, Android [which just reuses a BSD implementation], iOS/macOS [which haven't even implemented quick_exit yet], or musl [which has a trivial implementation like ours, but with more locking]) seems to do so.

"POSIX hasn't said anything about this yet, and the C standard doesn't have dlopen()/dlclose(), so C11 doesn't say anything"

I'm going to abandon this incarnation of the patch. Once Android has a well-defined behavior of this interaction, we can call at_quick_exit here under a #ifdef.

[enh]

In D102790#2774272, @pirama wrote:

I'm going to abandon this incarnation of the patch. Once Android has a well-defined behavior of this interaction, we can call at_quick_exit here under a #ifdef.

yeah, it should probably check the API level too, since it's no longer just whether the target _has_ at_quick_exit(), it's whether it's one that understands dlclose()...

compiler-rt/cmake/config-ix.cmake

 
 # Libraries.
 check_library_exists(dl dlopen "" COMPILER_RT_HAS_LIBDL)
 check_library_exists(rt shm_open "" COMPILER_RT_HAS_LIBRT)
 check_library_exists(m pow "" COMPILER_RT_HAS_LIBM)
 check_library_exists(pthread pthread_create "" COMPILER_RT_HAS_LIBPTHREAD)
 check_library_exists(execinfo backtrace "" COMPILER_RT_HAS_LIBEXECINFO)
 
+# system calls
+check_c_source_compiles(
+"
+#include <stdlib.h>
+void exit_handler() {}
+int main() {
+ at_quick_exit(exit_handler);
+ return 0;
+}
+" COMPILER_RT_TARGET_HAS_AT_QUICK_EXIT)
+
+
 # Look for terminfo library, used in unittests that depend on LLVMSupport.
 if(LLVM_ENABLE_TERMINFO STREQUAL FORCE_ON)
   set(MAYBE_REQUIRED REQUIRED)
 else()
   set(MAYBE_REQUIRED)
 endif()
 if(LLVM_ENABLE_TERMINFO)
   find_library(COMPILER_RT_TERMINFO_LIB NAMES terminfo tinfo curses ncurses ncursesw ${MAYBE_REQUIRED})

compiler-rt/lib/profile/CMakeLists.txt

 endif()
 
 if(COMPILER_RT_TARGET_HAS_UNAME)
  set(EXTRA_FLAGS
      ${EXTRA_FLAGS}
      -DCOMPILER_RT_HAS_UNAME=1)
 endif()
 
+if(COMPILER_RT_TARGET_HAS_AT_QUICK_EXIT)
+ set(EXTRA_FLAGS
+     ${EXTRA_FLAGS}
+     -DCOMPILER_RT_TARGET_HAS_AT_QUICK_EXIT=1)
+endif()
+
 # We don't use the C++ Standard Library here, so avoid including it by mistake.
 append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ EXTRA_FLAGS)
 # XRay uses C++ standard library headers.
 string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
 
 # This appears to be a C-only warning banning the use of locals in aggregate
 # initializers. All other compilers accept this, though.
 # nonstandard extension used : 'identifier' : cannot be initialized using address of automatic variable

compiler-rt/lib/profile/InstrProfilingFile.c

int __llvm_profile_register_write_file_atexit(void) {
   static int HasBeenRegistered = 0;
 
   if (HasBeenRegistered)
     return 0;
 
   lprofSetupValueProfiler();
 
   HasBeenRegistered = 1;
-  return atexit(writeFileWithoutReturn);
+  int rc = atexit(writeFileWithoutReturn);
+#if defined(COMPILER_RT_TARGET_HAS_AT_QUICK_EXIT)
+  rc = rc || at_quick_exit(writeFileWithoutReturn);
+#endif
+  return rc;
 }
 
 #endif

compiler-rt/test/lit.common.configured.in

 set_default("have_rpc_xdr_h", @HAVE_RPC_XDR_H@)
 set_default("gwp_asan", @COMPILER_RT_HAS_GWP_ASAN_PYBOOL@)
 set_default("expensive_checks", @LLVM_ENABLE_EXPENSIVE_CHECKS_PYBOOL@)
 set_default("test_standalone_build_libs", @COMPILER_RT_TEST_STANDALONE_BUILD_LIBS_PYBOOL@)
 # True iff the test suite supports ignoring the test compiler's runtime library path
 # and using `config.compiler_rt_libdir` instead. This only matters when the runtime
 # library paths differ.
 set_default("test_suite_supports_overriding_runtime_lib_path", False)
+set_default("quick_exit_available", @COMPILER_RT_TARGET_HAS_AT_QUICK_EXIT@)
 config.available_features.add('target-is-%s' % config.target_arch)
 
+if not config.quick_exit_available:
+  config.available_features.add("quick_exit_unavailable")
+
 if config.enable_per_target_runtime_dir:
   set_default("target_suffix", "")
 elif config.android:
   if config.target_arch == "i386":
     set_default("target_suffix", "-i686-android")
   else:
     set_default("target_suffix", "-%s-android" % config.target_arch)
 else:

compiler-rt/test/profile/instrprof-write-file-at-quick-exit-explicitly.c

+// RUN: %clang_profgen -o %t -O3 %s
+// RUN: %run %t %t.profraw
+// RUN: llvm-profdata merge -o %t.profdata %t.profraw
+// RUN: %clang_profuse=%t.profdata -o - -S -emit-llvm %s | FileCheck %s
+// UNSUPPORTED: quick_exit_unavailable
+
+#include <stdlib.h>
+
+int __llvm_profile_runtime = 0;
+int __llvm_profile_register_write_file_atexit(void);
+void __llvm_profile_set_filename(const char *);
+int main(int argc, const char *argv[]) {
+  __llvm_profile_register_write_file_atexit();
+  // CHECK: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}, !prof ![[PD1:[0-9]+]]
+  if (argc < 2)
+    return 1;
+  __llvm_profile_set_filename(argv[1]);
+  quick_exit(0);
+}
+// CHECK: ![[PD1]] = !{!"branch_weights", i32 1, i32 2}