While looking at some asm I noticed we produce the most horrid code for
test_bit():
1a5e: 49 0f a3 30 bt %rsi,(%r8)
1a62: 45 19 c0 sbb %r8d,%r8d
1a65: 45 85 c0 test %r8d,%r8d
1a68: 75 a6 jne 1a10 <x86_schedule_events+0xc0>
Since test_bit() doesn't actually have any output variables, we can use
asm goto without having to add a memory clobber. This reduces the code
to something sensible:
1a12: 49 0f a3 30 bt %rsi,(%r8)
1a16: 72 68 jb 1a80 <x86_schedule_events+0x130>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
PS. should we kill the memory clobber for __test_and_change_bit()? It
seems inconsistent and out of place.
PPS. Jakub, I see gcc5.1 still hasn't got output operands for asm goto;
is this something we can get 'fixed' ?
arch/x86/include/asm/bitops.h | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index cfe3b954d5e4..bcf4fa77c04f 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -313,6 +313,15 @@ static __always_inline int constant_test_bit(long nr,
const volatile unsigned lo
static inline int variable_test_bit(long nr, volatile const unsigned long
*addr)
{
+#ifdef CC_HAVE_ASM_GOTO
+ asm_volatile_goto ("bt %1, %0\n\t"
+ "jc %l[cc_label]"
+ : : "m" (*(unsigned long *)addr), "Ir" (nr)
+ : : cc_label);
+ return 0;
+cc_label:
+ return 1;
+#else
int oldbit;
asm volatile("bt %2,%1\n\t"
@@ -321,6 +330,7 @@ static inline int variable_test_bit(long nr, volatile const
unsigned long *addr)
: "m" (*(unsigned long *)addr), "Ir" (nr));
return oldbit;
+#endif
}
#if 0 /* Fool kernel-doc since it doesn't do macros yet */
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