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[libc] Change LIBC_THREAD_LOCAL to be dependent on LIBC_THREAD_MODE (#151527)

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When single-threaded mode is selected, all instances of the keyword
`LIBC_THREAD_LOCAL` are stubbed out, similar to how it currently works
on the GPU. This allows baremetal builds to avoid using thread_local.

However, libcxx uses shared headers, so we need to be careful there.
Thankfully, there is already an option to disable multithreading in
libcxx, so a flag is added such that single-threaded mode is propagated
down to libc.
This commit is contained in:
William Huynh 2025-08-06 15:04:26 +01:00 committed by GitHub
parent c4f6d34674
commit 0b1639581a
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GPG Key ID: B5690EEEBB952194
3 changed files with 29 additions and 23 deletions
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3
cmake/Modules/FindLibcCommonUtils.cmake
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@ -12,6 +12,9 @@ if(NOT TARGET llvm-libc-common-utilities)
add_library(llvm-libc-common-utilities INTERFACE)
# TODO: Reorganize the libc shared section so that it can be included without
# adding the root "libc" directory to the include path.
if (NOT(LIBCXX_ENABLE_THREADS))
target_compile_definitions(llvm-libc-common-utilities INTERFACE LIBC_THREAD_MODE=LIBC_THREAD_MODE_SINGLE)
endif()
target_include_directories(llvm-libc-common-utilities INTERFACE ${libc_path})
target_compile_definitions(llvm-libc-common-utilities INTERFACE LIBC_NAMESPACE=__llvm_libc_common_utils)
target_compile_features(llvm-libc-common-utilities INTERFACE cxx_std_17)

27
libc/src/__support/macros/attributes.h
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@ -28,7 +28,32 @@
#define LIBC_INLINE_ASM __asm__ __volatile__
#define LIBC_UNUSED __attribute__((unused))
#ifdef LIBC_TARGET_ARCH_IS_GPU
// Uses the platform specific specialization
#define LIBC_THREAD_MODE_PLATFORM 0
// Mutex guards nothing, used in single-threaded implementations
#define LIBC_THREAD_MODE_SINGLE 1
// Vendor provides implementation
#define LIBC_THREAD_MODE_EXTERNAL 2
// libcxx doesn't define LIBC_THREAD_MODE, unless that is passed in the command
// line in the CMake invocation. This defaults to the original implementation
// (before changes in https://github.com/llvm/llvm-project/pull/145358)
#ifndef LIBC_THREAD_MODE
#define LIBC_THREAD_MODE LIBC_THREAD_MODE_PLATFORM
#endif // LIBC_THREAD_MODE
#if LIBC_THREAD_MODE != LIBC_THREAD_MODE_PLATFORM && \
LIBC_THREAD_MODE != LIBC_THREAD_MODE_SINGLE && \
LIBC_THREAD_MODE != LIBC_THREAD_MODE_EXTERNAL
#error LIBC_THREAD_MODE must be one of the following values: \
LIBC_THREAD_MODE_PLATFORM, \
LIBC_THREAD_MODE_SINGLE, \
LIBC_THREAD_MODE_EXTERNAL.
#endif
#if LIBC_THREAD_MODE == LIBC_THREAD_MODE_SINGLE
#define LIBC_THREAD_LOCAL
#else
#define LIBC_THREAD_LOCAL thread_local

22
libc/src/__support/threads/mutex.h
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@ -12,28 +12,6 @@
#include "src/__support/macros/attributes.h"
#include "src/__support/macros/config.h"
// Uses the platform specific specialization
#define LIBC_THREAD_MODE_PLATFORM 0
// Mutex guards nothing, used in single-threaded implementations
#define LIBC_THREAD_MODE_SINGLE 1
// Vendor provides implementation
#define LIBC_THREAD_MODE_EXTERNAL 2
#if !defined(LIBC_THREAD_MODE)
#error LIBC_THREAD_MODE is undefined
#endif // LIBC_THREAD_MODE
#if LIBC_THREAD_MODE != LIBC_THREAD_MODE_PLATFORM && \
LIBC_THREAD_MODE != LIBC_THREAD_MODE_SINGLE && \
LIBC_THREAD_MODE != LIBC_THREAD_MODE_EXTERNAL
#error LIBC_THREAD_MODE must be one of the following values: \
LIBC_THREAD_MODE_PLATFORM, \
LIBC_THREAD_MODE_SINGLE, \
LIBC_THREAD_MODE_EXTERNAL.
#endif
#if LIBC_THREAD_MODE == LIBC_THREAD_MODE_PLATFORM
// Platform independent code will include this header file which pulls