From: Segher Boessenkool <segher@kernel.crashing.org>
To: "Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [rtlanal] Do a better job of costing parallel sets containing flag-setting operations.
Date: Mon, 19 Jun 2017 15:09:00 -0000 [thread overview]
Message-ID: <20170619150940.GM16550@gate.crashing.org> (raw)
In-Reply-To: <c5199d5a-9002-c651-7684-61c1ac0331d0@arm.com>
On Mon, Jun 19, 2017 at 03:45:23PM +0100, Richard Earnshaw (lists) wrote:
> >> At present all such insns are treated as having unknown cost (ie 0) and
> >> combine assumes that such insns are infinitely more expensive than any
> >> other insn sequence with a non-zero cost.
> >
> > That's not what combine does: it optimistically assumes any combination
> > with unknown costs is an improvement.
>
> Actually the logic is
>
> int reject = old_cost > 0 && new_cost > old_cost;
>
> So reject will never be true if old cost is zero.
Yes, exactly; and neither if new_cost is zero. If any cost is unknown
combine just hopes for the best.
Segher
next prev parent reply other threads:[~2017-06-19 15:09 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-19 13:47 Richard Earnshaw (lists)
2017-06-19 14:08 ` Segher Boessenkool
2017-06-19 14:28 ` Richard Earnshaw (lists)
2017-06-19 15:06 ` Segher Boessenkool
2017-06-19 14:45 ` Richard Earnshaw (lists)
2017-06-19 15:09 ` Segher Boessenkool [this message]
2017-06-19 16:01 ` Richard Earnshaw (lists)
2017-06-19 17:41 ` Segher Boessenkool
2017-06-20 12:55 ` Segher Boessenkool
2017-06-30 9:03 ` Richard Earnshaw (lists)
2017-06-30 15:20 ` Jeff Law
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