From 1591747bf692d10c3b2fd92c9dc8ba931626fd84 Mon Sep 17 00:00:00 2001 From: Lasse Collin Date: Mon, 24 Jun 2024 22:41:10 +0300 Subject: [PATCH] CMake: Fix three checks if building with -flto In CMake, check_c_source_compiles() always links too. With link-time optimization, unused functions may get omitted if main() doesn't depend on them. Consider the following which tries to check if somefunction() is available when has been included: #include int foo(void) { return somefunction(); } int main(void) { return 0; } LTO may omit foo() completely because the program as a whole doesn't need it and then the program will link even if the symbol somefunction isn't available in libc or other library being linked in, and then the test may pass when it shouldn't. What happens if doesn't declare somefunction()? Shouldn't the test fail in the compilation phase already? It should but many compilers don't follow the C99 and later standards that prohibit implicit function declarations. Instead such compilers assume that somefunction() exists, compilation succeeds (with a warning), and then linker with LTO omits the call to somefunction(). Change the tests so that they are part of main(). If compiler accepts implicitly declared functions, LTO cannot omit them because it has to assume that they might have side effects and thus linking will fail. On the other hand, if the functions/intrinsics being used are supported, they might get optimized away but in that case it's fine because they really are supported. It is fine to use __attribute__((target(...))) for main(). At least it works with GCC 4.9 to 14.1 on x86-64. Reported-by: Sam James (cherry picked from commit 114cba69dbb96003e676c8c87a2e9943b12d065f) --- CMakeLists.txt | 19 ++++++++----------- 1 file changed, 8 insertions(+), 11 deletions(-) diff --git a/CMakeLists.txt b/CMakeLists.txt index 3c24e541..95589b8d 100644 --- a/CMakeLists.txt +++ b/CMakeLists.txt @@ -1038,16 +1038,14 @@ if(NOT SANDBOX_FOUND AND ENABLE_SANDBOX MATCHES "^ON$|^landlock$") #include #include - void my_sandbox(void) + int main(void) { (void)prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); (void)SYS_landlock_create_ruleset; (void)SYS_landlock_restrict_self; (void)LANDLOCK_CREATE_RULESET_VERSION; - return; + return 0; } - - int main(void) { return 0; } " HAVE_LINUX_LANDLOCK) @@ -1171,12 +1169,12 @@ calculation if supported by the system" ON) && !defined(__EDG__) __attribute__((__target__(\"ssse3,sse4.1,pclmul\"))) #endif - __m128i my_clmul(__m128i a) + int main(void) { - const __m128i b = _mm_set_epi64x(1, 2); - return _mm_clmulepi64_si128(a, b, 0); + __m128i a = _mm_set_epi64x(1, 2); + a = _mm_clmulepi64_si128(a, a, 0); + return 0; } - int main(void) { return 0; } " HAVE_USABLE_CLMUL) tuklib_add_definition_if(liblzma HAVE_USABLE_CLMUL) @@ -1201,11 +1199,10 @@ if(ALLOW_ARM64_CRC32) #if (defined(__GNUC__) || defined(__clang__)) && !defined(__EDG__) __attribute__((__target__(\"+crc\"))) #endif - uint32_t my_crc(uint32_t a, uint64_t b) + int main(void) { - return __crc32d(a, b); + return __crc32d(1, 2) != 0; } - int main(void) { return 0; } " HAVE_ARM64_CRC32)