[libc-commits] [libc] [libc] Fix LLVM_LIBC_FUNCTION on 32-bit x86 Windows (PR #213456)
Nico Weber via libc-commits
libc-commits at lists.llvm.org
Sat Aug 1 09:39:54 PDT 2026
https://github.com/nico updated https://github.com/llvm/llvm-project/pull/213456
>From 06d24571c6e7d47ce54e6cc893a212395c5f9404 Mon Sep 17 00:00:00 2001
From: Nico Weber <thakis at chromium.org>
Date: Sat, 1 Aug 2026 11:03:28 -0400
Subject: [PATCH 1/2] [libc] Fix LLVM_LIBC_FUNCTION on 32-bit x86 Windows
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.
---
libc/src/__support/common.h | 31 +++++++++++++++++--------------
1 file changed, 17 insertions(+), 14 deletions(-)
diff --git a/libc/src/__support/common.h b/libc/src/__support/common.h
index d90fe7b8ae98d..fc6e093e5f3d1 100644
--- a/libc/src/__support/common.h
+++ b/libc/src/__support/common.h
@@ -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
>From 1271951b2c9f186d42ad20f98ed227066a4ba227 Mon Sep 17 00:00:00 2001
From: Nico Weber <thakis at chromium.org>
Date: Sat, 1 Aug 2026 12:39:42 -0400
Subject: [PATCH 2/2] format
---
libc/src/__support/common.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/libc/src/__support/common.h b/libc/src/__support/common.h
index fc6e093e5f3d1..8a8ffefd381ee 100644
--- a/libc/src/__support/common.h
+++ b/libc/src/__support/common.h
@@ -19,8 +19,8 @@
#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))
+#if defined(__APPLE__) || (defined(LIBC_TARGET_OS_IS_WINDOWS) && \
+ defined(LIBC_TARGET_ARCH_IS_X86_32))
#define LIBC_TARGET_USES_LEADING_UNDERSCORE
#endif
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