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[libc] Fix LLVM_LIBC_FUNCTION on 32-bit x86 Windows (#213456)

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Previously, trying to compile libm with LIBC_COPT_PUBLIC_PACKAGING
defined failed on 32-bit Windows with

    error: alias must point to a defined variable or function

This is because 32-bit Windows adds a leading underscore to __cdecl C
functions, making the alias declaration not find its target symbol name.
The same problem exists on Apple platforms, which as solution don't emit
the alias for the C++ LIBC_NAMESPACE:: symbol.

Do the same on 32-bit Windows as on Apple platforms: Emit only
the underscore-prefixed symbol, not the LIBC_NAMESPACE:: alias.
This commit is contained in:
Nico Weber 2026-08-01 14:48:20 -04:00 committed by GitHub
parent 007c684208
commit 350089d0cb
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GPG key ID: B5690EEEBB952194
1 changed files with 17 additions and 14 deletions
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31
libc/src/__support/common.h
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@ -17,6 +17,12 @@
#include "src/__support/macros/config.h"
#include "src/__support/macros/properties/architectures.h"
#include "src/__support/macros/properties/compiler.h"
#include "src/__support/macros/properties/os.h"
#if defined(__APPLE__) || (defined(LIBC_TARGET_OS_IS_WINDOWS) && \
defined(LIBC_TARGET_ARCH_IS_X86_32))
#define LIBC_TARGET_USES_LEADING_UNDERSCORE
#endif
#ifndef LLVM_LIBC_FUNCTION_ATTR
#define LLVM_LIBC_FUNCTION_ATTR
@ -44,25 +50,24 @@
#define LLVM_LIBC_ATTR(name) EXPAND_THEN_SECOND(LLVM_LIBC_FUNCTION_ATTR_##name)
// At the moment, [[gnu::alias()]] is not supported on MacOS, and it is needed
// to cleanly export and alias the C++ symbol `LIBC_NAMESPACE::func` with the C
// symbol `func`. So for public packaging on MacOS, we will only export the C
// symbol. Moreover, a C symbol `func` in macOS is mangled as `_func`.
// Export both `func` and `LIBC_NAMESPACE::func` using an alias symbol.
// This does not work on platfors with LIBC_TARGET_USES_LEADING_UNDERSCORE
// so there this only exports `_func`.
#if defined(LIBC_COPT_PUBLIC_PACKAGING) && !defined(LIBC_COMPILER_IS_MSVC)
#ifndef __APPLE__
#ifndef LIBC_TARGET_USES_LEADING_UNDERSCORE
#define LLVM_LIBC_FUNCTION_IMPL_4(type, name, arglist, c_alias) \
LLVM_LIBC_ATTR(name) \
LLVM_LIBC_FUNCTION_ATTR decltype(LIBC_NAMESPACE::name) \
__##name##_impl__ asm(c_alias); \
decltype(LIBC_NAMESPACE::name) name [[gnu::alias(c_alias)]]; \
type __##name##_impl__ arglist
#else // __APPLE__
#else // LIBC_TARGET_USES_LEADING_UNDERSCORE
#define LLVM_LIBC_FUNCTION_IMPL_4(type, name, arglist, c_alias) \
LLVM_LIBC_ATTR(name) \
LLVM_LIBC_FUNCTION_ATTR decltype(LIBC_NAMESPACE::name) name asm( \
"_" c_alias); \
type name arglist
#endif // __APPLE__
#endif // LIBC_TARGET_USES_LEADING_UNDERSCORE
#else // LIBC_COPT_PUBLIC_PACKAGING
#define LLVM_LIBC_FUNCTION_IMPL_4(type, name, arglist, c_alias) \
@ -78,25 +83,23 @@
GET_FIFTH(__VA_ARGS__, LLVM_LIBC_FUNCTION_IMPL_4, LLVM_LIBC_FUNCTION_IMPL_3, \
GET_NOTHING)(__VA_ARGS__)
// At the moment, [[gnu::alias()]] is not supported on MacOS, and it is needed
// to cleanly export and alias the C++ symbol `LIBC_NAMESPACE::func` with the C
// symbol `func`. So for public packaging on MacOS, we will only export the C
// symbol. Moreover, a C symbol `func` in macOS is mangled as `_func`.
// See comment on LLVM_LIBC_FUNCTION_IMPL_4 for why this checks
// LIBC_TARGET_USES_LEADING_UNDERSCORE.
#if defined(LIBC_COPT_PUBLIC_PACKAGING) && !defined(LIBC_COMPILER_IS_MSVC)
#ifndef __APPLE__
#ifndef LIBC_TARGET_USES_LEADING_UNDERSCORE
#define LLVM_LIBC_VARIABLE_IMPL(type, name) \
LLVM_LIBC_ATTR(name) \
extern LLVM_LIBC_VARIABLE_ATTR decltype(LIBC_NAMESPACE::name) \
__##name##_impl__ asm(#name); \
extern decltype(LIBC_NAMESPACE::name) name [[gnu::alias(#name)]]; \
type __##name##_impl__
#else // __APPLE__
#else // LIBC_TARGET_USES_LEADING_UNDERSCORE
#define LLVM_LIBC_VARIABLE_IMPL(type, name) \
LLVM_LIBC_ATTR(name) \
extern LLVM_LIBC_VARIABLE_ATTR decltype(LIBC_NAMESPACE::name) name asm( \
"_" #name); \
type name
#endif // __APPLE__
#endif // LIBC_TARGET_USES_LEADING_UNDERSCORE
#else // LIBC_COPT_PUBLIC_PACKAGING
#define LLVM_LIBC_VARIABLE_IMPL(type, name) type name
#endif // LIBC_COPT_PUBLIC_PACKAGING