tg at gmplib.org
Fri Apr 12 20:10:13 CEST 2013
David Miller <davem at davemloft.net> writes:
From: Torbjorn Granlund <tg at gmplib.org>
Date: Fri, 12 Apr 2013 19:30:42 +0200
> David Miller <davem at davemloft.net> writes:
> It isn't really conditional execution on sparc, the resources and
> timing required for the "move" instruction are constant whether the
> condition matches or not.
> That's not enough.
> It needs to have the same data-dependency behaviour too.
> And it needs to be a documented feature, since it is easy to imagine an
> implementation which does create different data-dependency behaviour,
> even if all current ones do not.
> Using carry seems safer. Of course, it is conceivable that a
> carry-dependent instruction could run diffferently depending on the
> value of the carry flag, but that'd be quite strange.
Even on a pipeline that dynamically renames the condition codes?
You know, something we've never seen before :-)
I am not worrying mainly about WAR and WAW dependencies, but about plain
RAW dependencies. Does the movXX read the source register or not, for a
false condition? Does it read the destnation, or does it only write it
(for a true condition)?
If we could trust movXX to be silent, we should of course not bother to
create that mask, but replace that and the logops with 4 moveq for a
> (For generic sparc64 code, we should perhaps avoid movXX for other
> reasons too, since it interacts poorly with loads on US1-2, IIRC.)
Now that's a more solid reason to avoid movcc.
Keeping cnd functions and tabselect side-channel silent is crucial.
(And while not specificaly designed for side-channel silence, also
mul_k, addmul_k and submul_k should be silent.)
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