[Bug middle-end/112711] New: [14 Regression] possibly wrong code in bswap32(int) on llvm-16.0.6 test suite

["slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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)