https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112711
Bug ID: 112711 Summary: [14 Regression] possibly wrong code in bswap32(int) on llvm-16.0.6 test suite Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: slyfox at gcc dot gnu.org Target Milestone: --- Initially noticed possibly wrong code on llvm-16.0.6 test suite when building with gcc-master from r14-5844-g9c26c91b94eb72: Failed Tests (2): LLVM-Unit :: Support/./SupportTests/Endian/Write LLVM-Unit :: Support/./SupportTests/Endian/WriteBitAligned I extracted the following self-contained example: // $ cat EndianTest.cpp typedef int i32; typedef unsigned int u32; static inline void write_i32(void *memory, i32 value) { // swap i32 bytes as if it was u32: u32 u_value = value; value = __builtin_bswap32(u_value); // llvm infers '1' alignment from destination type __builtin_memcpy(__builtin_assume_aligned(memory, 1), &value, sizeof(value)); } __attribute__((noipa)) static void bug (void) { #define assert_eq(lhs, rhs) if (lhs != rhs) __builtin_trap() unsigned char data[5]; write_i32(data, -1362446643); assert_eq(data[0], 0xAE); assert_eq(data[1], 0xCA); write_i32(data + 1, -1362446643); assert_eq(data[1], 0xAE); } int main() { bug(); } Fails as: $ gcc/xg++ -Bgcc EndianTest.cpp -o bug -O0 && ./bug $ gcc/xg++ -Bgcc EndianTest.cpp -o bug -O2 && ./bug Illegal instruction (core dumped) $ gcc/xg++ -Bgcc -v Reading specs from gcc/specs COLLECT_GCC=gcc/xg++ COLLECT_LTO_WRAPPER=gcc/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: /home/slyfox/dev/git/gcc/configure --disable-multilib --disable-bootstrap --disable-lto --disable-libsanitizer --disable-libstdcxx-pch --enable-languages=c,c++ --disable-libgomp --disable-libquadmath --disable-libvtv CFLAGS='-O1 -g0' CXXFLAGS='-O1 -g0' LDFLAGS='-O1 -g0' Thread model: posix Supported LTO compression algorithms: zlib gcc version 14.0.0 20231125 (experimental) (GCC)