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From: "pluto at agmk dot net" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/31170] cmpxchgq not emitted. Date: Wed, 14 Mar 2007 19:05:00 -0000 [thread overview] Message-ID: <20070314190524.2665.qmail@sourceware.org> (raw) In-Reply-To: <bug-31170-7667@http.gcc.gnu.org/bugzilla/> ------- Comment #2 from pluto at agmk dot net 2007-03-14 19:05 ------- (In reply to comment #1) > ifcvt could do this. But is cmpxchgq really faster with its atomictiy > guarantee? only `lock; cmpxchg' has atomicity guarantee on smp. > They are all vector-path instructions, a compare - cmov sequence looks > faster (8 cycle latency vs. 10 and also with less constraints on register > allocation). Even the code we emit now: > > emit_cmpxchg: > .LFB2: > movq (%rdi), %rax > cmpq %rsi, %rax > je .L6 > rep ; ret > .p2align 4,,7 > .L6: > movq %rdx, (%rdi) > ret > > could be faster dependent on branch probability. yes, it could be faster, but for -Os we could emit a small branchless code: movq %rsi, %rax cmpxchgq %rdx, (%rdi) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31170
prev parent reply other threads:[~2007-03-14 19:05 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2007-03-14 9:59 [Bug other/31170] New: " pluto at agmk dot net 2007-03-14 11:00 ` [Bug rtl-optimization/31170] " rguenth at gcc dot gnu dot org 2007-03-14 19:05 ` pluto at agmk dot net [this message]
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