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[Bug rtl-optimization/31170] cmpxchgq not emitted.
- From: "pluto at agmk dot net" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 14 Mar 2007 19:05:24 -0000
- Subject: [Bug rtl-optimization/31170] cmpxchgq not emitted.
- References: <bug-31170-7667@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- 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