Messages in this thread | | | Date | Thu, 4 Jan 2007 07:34:52 -0800 (PST) | From | Linus Torvalds <> | Subject | RE: kernel + gcc 4.1 = several problems |
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On Thu, 4 Jan 2007, Zou, Nanhai wrote: > > cmov will stall on eflags in your test program.
And that is EXACTLY my point.
CMOV is a piece of CRAP for most things, exactly because it serializes three streams of data: the two inputs, and the conditional.
My test-case was actually _good_ for cmov, because there was just the one conditional (which was 100% ALU) thing that was serialized. In real life, the two data sources also come from memory, and _any_ of them being delayed ends up delaying the cmov, and screwing up your out-of-order pipeline because you now introduced a serialization point that was very possibly not necessary at all.
In contrast, a conditional branch-around serializes absolutely NOTHING, because branches get predicted.
> I think you will see benefit of cmov if you can manage to put some > instructions which does NOT modify eflags between testl and cmov.
A lot of the time, the conditional _is_ the critical path.
The whole point of this discussion was that cmov isn't really all that great. It has fundamental problems that a conditional branch that gets predicted simply does not have.
That's qiute apart from the fact that cmov has rather limited semantics, and that in 99% of all cases you have to use a conditional branch anyway.
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