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 14:08:00 -0000 [thread overview]
Message-ID: <20170619140802.GK16550@gate.crashing.org> (raw)
In-Reply-To: <66275bc9-7d97-b990-4c86-2de1f4a6a2fa@arm.com>
Hi!
On Mon, Jun 19, 2017 at 02:46:59PM +0100, Richard Earnshaw (lists) wrote:
> Many parallel set insns are of the form of a single set that also sets
> the condition code flags. In this case the cost of such an insn is
> normally the cost of the part that doesn't set the flags, since updating
> the condition flags is simply a side effect.
>
> 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.
> This patch addresses this problem by allowing insn_rtx_cost to ignore
> the condition setting part of a PARALLEL iff there is exactly one
> comparison set and one non-comparison set. If the only set operation is
> a comparison we still use that as the basis of the insn cost.
I'll test this on a zillion archs, see what the effect is.
Have you considered costing general parallels as well?
Segher
next prev parent reply other threads:[~2017-06-19 14:08 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 [this message]
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
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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