| Message ID | 20170905150559.1739140-1-arnd@arndb.de |
|---|---|
| State | Superseded |
| Headers | show |
| Series | ARM: make memzero optimization smarter | expand |
diff --git a/arch/arm/include/asm/string.h b/arch/arm/include/asm/string.h index fe1c6af3a1b1..d4f464b46eae 100644 --- a/arch/arm/include/asm/string.h +++ b/arch/arm/include/asm/string.h @@ -43,7 +43,7 @@ extern void __memzero(void *ptr, __kernel_size_t n); #define memset(p,v,n) \ ({ \ void *__p = (p); size_t __n = n; \ - if ((__n) != 0) { \ + if (!__builtin_constant_p(__n) || (__n) != 0) { \ if (__builtin_constant_p((v)) && (v) == 0) \ __memzero((__p),(__n)); \ else \
While testing with a gcc-8.0.0 snapshot, I ran into a harmless build warning: In file included from include/linux/string.h:18:0, ... from drivers/net/ethernet/hisilicon/hns/hns_ethtool.c:10: drivers/net/ethernet/hisilicon/hns/hns_ethtool.c: In function '__lb_other_process': arch/arm/include/asm/string.h:50:5: error: 'memset' specified size 4294967295 exceeds maximum object size 2147483647 [-Werror=stringop-overflow=] memset((__p),(v),(__n)); \ ^~~~~~~~~~~~~~~~~~~~~~~ drivers/net/ethernet/hisilicon/hns/hns_ethtool.c:394:3: note: in expansion of macro 'memset' memset(&skb->data[frame_size / 2], 0xAA, frame_size / 2 - 1); ^~~~~~ I think the warning is unintentional here, and gcc should not actually warn, so I reported this in the gcc bugzilla as pr82103. The problem here is that testing the 'frame_size' variable for non-zero in the first memset() macro invocation leads to a code path in which gcc thinks it may be zero, and that code path would lead to an overly large length for the following memset that is now "(u32)-1". We know this won't happen as the skb len is already guaranteed to be nonzero when we get here (it has just been allocated with a nonzero size). However, we can avoid this class of bogus warnings for the memset() macro by only doing the micro-optimization for zero-length arguments when the length is a compile-time constant. This should also reduce code size by a few bytes, and avoid an extra branch for the cases that a variable-length argument is always nonzero, which is probably the common case anyway. I have made sure that the __memzero implementation can safely handle a zero length argument. Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82103 Signed-off-by: Arnd Bergmann <arnd@arndb.de> --- arch/arm/include/asm/string.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) -- 2.9.0