Unlike BUG_ON(x), WARN_ON(x) uses !!(x) as the trigger
of the t(d/w)nei instruction instead of using directly the
value of x.
This leads to GCC adding unnecessary pair of addic/subfe. This was
revealed after adding a WARN_ON() on top of clear_page() in order
to detect misaligned destination:
@@ -49,6 +51,8 @@ static inline void clear_page(void *addr)
{
unsigned int i;
+ WARN_ON((unsigned long)addr & (L1_CACHE_BYTES - 1));
+
for (i = 0; i < PAGE_SIZE / L1_CACHE_BYTES; i++, addr += L1_CACHE_BYTES)
dcbz(addr);
}
This resulted on:
0000019c <clear_user_page>:
19c: 54 68 06 fe clrlwi r8,r3,27
1a0: 31 48 ff ff addic r10,r8,-1
1a4: 7d 4a 41 10 subfe r10,r10,r8
1a8: 0f 0a 00 00 twnei r10,0
1ac: 39 20 00 80 li r9,128
1b0: 7d 29 03 a6 mtctr r9
1b4: 7c 00 1f ec dcbz 0,r3
1b8: 38 63 00 20 addi r3,r3,32
1bc: 42 00 ff f8 bdnz 1b4 <clear_user_page+0x18>
1c0: 7c a3 2b 78 mr r3,r5
1c4: 48 00 00 00 b 1c4 <clear_user_page+0x28>
1c4: R_PPC_REL24 flush_dcache_page
By using (x) instead of !!(x) like BUG_ON() does, the additional
instructions go away:
0000019c <clear_user_page>:
19c: 54 6a 06 fe clrlwi r10,r3,27
1a0: 0f 0a 00 00 twnei r10,0
1a4: 39 20 00 80 li r9,128
1a8: 7d 29 03 a6 mtctr r9
1ac: 7c 00 1f ec dcbz 0,r3
1b0: 38 63 00 20 addi r3,r3,32
1b4: 42 00 ff f8 bdnz 1ac <clear_user_page+0x10>
1b8: 7c a3 2b 78 mr r3,r5
1bc: 48 00 00 00 b 1bc <clear_user_page+0x20>
1bc: R_PPC_REL24 flush_dcache_page
Signed-off-by: Christophe Leroy <christophe.le...@c-s.fr>
---
arch/powerpc/include/asm/bug.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
index fed7e6241349..77074582fe65 100644
--- a/arch/powerpc/include/asm/bug.h
+++ b/arch/powerpc/include/asm/bug.h
@@ -107,7 +107,7 @@
: : "i" (__FILE__), "i" (__LINE__), \
"i" (BUGFLAG_WARNING|BUGFLAG_TAINT(TAINT_WARN)),\
"i" (sizeof(struct bug_entry)), \
- "r" (__ret_warn_on)); \
+ "r" ((__force long)(x))); \
} \
unlikely(__ret_warn_on); \
})
--
2.17.1
