From: Willy Tarreau <w@xxxxxx> [ Upstream commit 1bfbe1f3e96720daf185f03d101f072d69753f88 ] When building on ARM in thumb mode with gcc-11.3 at -O2 or -O3, nolibc-test segfaults during the select() tests. It turns out that at this level, gcc recognizes an opportunity for using memset() to zero the fd_set, but it miscompiles it because it also recognizes a memset pattern as well, and decides to call memset() from the memset() code: 000122bc <memset>: 122bc: b510 push {r4, lr} 122be: 0004 movs r4, r0 122c0: 2a00 cmp r2, #0 122c2: d003 beq.n 122cc <memset+0x10> 122c4: 23ff movs r3, #255 ; 0xff 122c6: 4019 ands r1, r3 122c8: f7ff fff8 bl 122bc <memset> 122cc: 0020 movs r0, r4 122ce: bd10 pop {r4, pc} Simply placing an empty asm() statement inside the loop suffices to avoid this. Signed-off-by: Willy Tarreau <w@xxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- tools/include/nolibc/string.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h index 0932db3817d2..fffdaf6ff467 100644 --- a/tools/include/nolibc/string.h +++ b/tools/include/nolibc/string.h @@ -88,8 +88,11 @@ void *memset(void *dst, int b, size_t len) { char *p = dst; - while (len--) + while (len--) { + /* prevent gcc from recognizing memset() here */ + asm volatile(""); *(p++) = b; + } return dst; } -- 2.39.0