From b18b6daf65554fd9e1b6008513bfd50a7ef5d3e9 Mon Sep 17 00:00:00 2001 From: Jan Beulich Date: Mon, 26 Jan 2015 12:50:21 +0100 Subject: [PATCH] x86: use tzcnt instead of bsf Following a compiler change done in 2012, make use of the fact that for non-zero input BSF and TZCNT produce the same numeric result (EFLAGS setting differs), and that CPUs not knowing of TZCNT will treat the instruction as BSF (i.e. ignore what looks like a REP prefix to them). The assumption here is that TZCNT would never have worse performance than BSF. Also extend the asm() input in find_first_set_bit() to allow memory operands. Signed-off-by: Jan Beulich Reviewed-by: Andrew Cooper --- xen/arch/x86/bitops.c | 2 +- xen/arch/x86/hvm/vpic.c | 2 +- xen/include/asm-x86/bitops.h | 2 +- 3 files changed, 3 insertions(+), 3 deletions(-) diff --git a/xen/arch/x86/bitops.c b/xen/arch/x86/bitops.c index c037567c6e..f6ee71512c 100644 --- a/xen/arch/x86/bitops.c +++ b/xen/arch/x86/bitops.c @@ -62,7 +62,7 @@ unsigned int __find_first_zero_bit( " je 2f\n\t" " xor -"STR(BITS_PER_LONG/8)"(%2),%3\n\t" " jz 1b\n\t" - " bsf %3,%0\n\t" + " rep; bsf %3,%0\n\t" " lea -"STR(BITS_PER_LONG/8)"(%2),%2\n\t" "2: sub %%ebx,%%edi\n\t" " shl $3,%%edi\n\t" diff --git a/xen/arch/x86/hvm/vpic.c b/xen/arch/x86/hvm/vpic.c index 2c6e6e5f3f..d72b6ff3fa 100644 --- a/xen/arch/x86/hvm/vpic.c +++ b/xen/arch/x86/hvm/vpic.c @@ -56,7 +56,7 @@ static int vpic_get_priority(struct hvm_hw_vpic *vpic, uint8_t mask) return VPIC_PRIO_NONE; /* prio = ffs(mask ROR vpic->priority_add); */ - asm ( "ror %%cl,%b1 ; bsf %1,%0" + asm ( "ror %%cl,%b1 ; rep; bsf %1,%0" : "=r" (prio) : "q" ((uint32_t)mask), "c" (vpic->priority_add) ); return prio; } diff --git a/xen/include/asm-x86/bitops.h b/xen/include/asm-x86/bitops.h index 099a74c373..6f0f34bc57 100644 --- a/xen/include/asm-x86/bitops.h +++ b/xen/include/asm-x86/bitops.h @@ -391,7 +391,7 @@ static inline unsigned int __scanbit(unsigned long val, unsigned long max) */ static inline unsigned int find_first_set_bit(unsigned long word) { - asm ( "bsf %1,%0" : "=r" (word) : "r" (word) ); + asm ( "rep; bsf %1,%0" : "=r" (word) : "rm" (word) ); return (unsigned int)word; } -- 2.30.2