[libc-commits] [libc] [libc] Suppress GCC's <mm_malloc.h> in freestanding builds (PR #234863)

[Pavel Labath via libc-commits]

https://github.com/labath created https://github.com/llvm/llvm-project/pull/234863

Unlike Clang, GCC's <xmmintrin.h> (included via <immintrin.h>) unconditionally includes <mm_malloc.h> even with -ffreestanding, which in turn includes <stdlib.h>.

In full-build mode, targets that use x86 intrinsics (such as op_x86.h) don't declare a dependency on libc.include.stdlib (and they don't really need it). If these targets are built before our stdlib.h header is generated, this will end up including the system stdlib.h -- and fail miserably because of mixing of standard libraries. This can happen already, but the problem gets more acute if we remove (#234150) transitive full-header dependencies from the proxy headers (as it lets us build these targets much sooner).

Since we don't use _mm_malloc or _mm_free, I'm suppressing <mm_malloc.h> by defining its include guard (_MM_MALLOC_H_INCLUDED) on x86/x86_64 GCC full builds.

Assisted-by: Gemini

>From da75f00a46be65894639bb9c504a18c87c9e9520 Mon Sep 17 00:00:00 2001
From: Pavel Labath <pavel at labath.sk>
Date: Fri, 23 Oct 2026 07:01:27 +0000
Subject: [PATCH] [libc] Suppress GCC's <mm_malloc.h> in freestanding builds

Unlike Clang, GCC's <xmmintrin.h> (included via <immintrin.h>)
unconditionally includes <mm_malloc.h> even with -ffreestanding, which
in turn includes <stdlib.h>.

In full-build mode, targets that use x86 intrinsics (such as
op_x86.h) don't declare a dependency on libc.include.stdlib (and
they don't really need it). If these targets are built before our
stdlib.h header is generated, this will end up including the system
stdlib.h -- and fail miserably because of mixing of standard
libraries. This can happen already, but the problem gets more acute
if we remove (#234150) transitive full-header dependencies from the
proxy headers (as it lets us build these targets much sooner).

Since we don't use _mm_malloc or _mm_free, I'm suppressing <mm_malloc.h>
by defining its include guard (_MM_MALLOC_H_INCLUDED) on x86/x86_64 GCC
full builds.

Assisted-by: Gemini
---
 libc/cmake/modules/LLVMLibCCompileOptionRules.cmake | 7 +++++++
 libc/cmake/modules/LLVMLibCTestRules.cmake          | 4 ++++
 2 files changed, 11 insertions(+)

diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
index 98aab039a63621..dda44f23daf5bc 100644
--- a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
+++ b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -242,6 +242,13 @@ function(_get_common_compile_options_impl output_var)
       # Only add -ffreestanding flag in non-GPU full build mode.
       if(NOT LIBC_TARGET_OS_IS_GPU)
         list(APPEND compile_options "-ffreestanding")
+        # GCC's <xmmintrin.h> unconditionally includes <mm_malloc.h> even with
+        # -ffreestanding, which in turn includes <stdlib.h>. Suppress it by
+        # defining its header guard.
+        if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND
+           (LIBC_TARGET_ARCHITECTURE_IS_X86_64 OR LIBC_TARGET_ARCHITECTURE_IS_X86))
+          list(APPEND compile_options "-D_MM_MALLOC_H_INCLUDED")
+        endif()
       endif()
       list(APPEND compile_options "-DLIBC_FULL_BUILD")
       # Manually disable standard include paths to prevent system headers from
diff --git a/libc/cmake/modules/LLVMLibCTestRules.cmake b/libc/cmake/modules/LLVMLibCTestRules.cmake
index 1c7a4252a69d6b..2cca0def2f2f48 100644
--- a/libc/cmake/modules/LLVMLibCTestRules.cmake
+++ b/libc/cmake/modules/LLVMLibCTestRules.cmake
@@ -53,6 +53,10 @@ function(_get_common_test_compile_options_impl output_var c_test)
       list(APPEND compile_options "-DLIBC_FULL_BUILD")
       # Only add -ffreestanding flag in full build mode.
       list(APPEND compile_options "-ffreestanding")
+      if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND
+         (LIBC_TARGET_ARCHITECTURE_IS_X86_64 OR LIBC_TARGET_ARCHITECTURE_IS_X86))
+        list(APPEND compile_options "-D_MM_MALLOC_H_INCLUDED")
+      endif()
       list(APPEND compile_options "-fno-exceptions")
       list(APPEND compile_options "-fno-unwind-tables")
       list(APPEND compile_options "-fno-asynchronous-unwind-tables")