From: Jeff Law <law@redhat.com>
To: Richard Guenther <richard.guenther@gmail.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: RFA: Avoiding unprofitable speculation
Date: Wed, 17 Aug 2011 22:50:00 -0000 [thread overview]
Message-ID: <4E4C15BF.10403@redhat.com> (raw)
In-Reply-To: <CAFiYyc3OOBwkEUDC8F9OcbWmrYFgMoC-DbwuphgyD7110-jm_A@mail.gmail.com>
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On 08/17/11 01:21, Richard Guenther wrote:
>
> We don't have a way to distinguish branch-taken vs. branch-not-take
> costs, right?
Not that I'm aware of. However, BRANCH_COST does allow the backend to
change the cost of a branch based on its predictability.
> I would expect that for non-pipelined archs the branch does have the
> same cost all the time, so ifcvt would be correct, no?
The branch we're able to eliminate is typically (always?) an
unconditional branch. The unconditional branch we eliminate is only
executed some percentage of the time based on the conditional branch
that remains in the stream.
So regardless of the underlying cost of the branch we eliminate, we
still want to compare the cost of the speculated insns against the
scaled cost of the branch we're able to eliminate.
> Do you happen to know how valgrind counts branches here?
I don't know all the low level details -- I'd have to sit down with the
annotation code in the cachegrind plugin. Presumably since valgrind
also attempts to model branch prediction it carries a number of counters
associated with each branch which get bumped as needed by the
instrumented code.
Jeff
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next prev parent reply other threads:[~2011-08-17 19:26 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-08-16 22:00 Jeff Law
2011-08-17 9:34 ` Richard Guenther
2011-08-17 22:50 ` Jeff Law [this message]
2011-08-18 22:59 ` Richard Henderson
2011-08-19 16:49 ` Jeff Law
2011-09-27 0:11 ` Jeff Law
2011-09-27 13:51 ` Richard Guenther
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