From: Paul Koning <paulkoning@comcast.net>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: GCC Development <gcc@gcc.gnu.org>
Subject: Re: How do I stop gcc from loading data into registers when that's not needed?
Date: Wed, 23 May 2018 00:50:00 -0000 [thread overview]
Message-ID: <50B057F6-F0AB-4213-91DE-2A988C72436F@comcast.net> (raw)
In-Reply-To: <20180522192609.GQ17342@gate.crashing.org>
> On May 22, 2018, at 3:26 PM, Segher Boessenkool <segher@kernel.crashing.org> wrote:
>
>
> -fdump-rtl-combine-all (or just -da or -dap), and then look at the dump
> file. Does combine try this combination? If so, it will tell you what
> the resulting costs are. If not, why does it not try it?
>
>> Sorry, I'm not very familiar with this area of GCC either. Did you confirm
>> that combine at least tries to merge the memory ops into the instruction?
>
> It should, it's a simple reg dependency. In many cases it will even do
> it if it is not single-use (via a 3->2 combination).
I examined what gcc does with two simple functions:
void c2(void)
{
if (x < y)
z = 1;
else if (x != y)
z = 42;
else
z = 9;
}
void c3(void)
{
if (x < y)
z = 1;
else
z = 9;
}
Two things popped out.
1. The original RTL (from the expand phase) has a memory->register move for x and y in c2, but it doesn't for c3 (it simply generates a memory/memory compare there). What triggers the different choice in that phase?
2. The reported costs for the various insns are
r22:HI=['x'] 6
cmp(r22:HI,r23:HI) 4
cmp(['x'],['y']) 16
so the added cost for the memory argument in the cmp is 6 -- the same as the whole cost for the mov. That certainly explains the behavior. It isn't what I want it to be. Which target hook(s) are involved in these numbers? I don't see them in my rtx_costs hook.
paul
next prev parent reply other threads:[~2018-05-23 0:50 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-18 18:03 Paul Koning
2018-05-18 18:07 ` Richard Biener
[not found] ` <018C29D6-6245-4D31-B43B-623E080A6F87@comcast.net>
2018-05-22 8:49 ` Richard Biener
2018-05-22 19:26 ` Segher Boessenkool
2018-05-23 0:50 ` Paul Koning [this message]
2018-05-23 9:47 ` Richard Biener
2018-05-24 0:33 ` Paul Koning
2018-05-24 18:25 ` Segher Boessenkool
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=50B057F6-F0AB-4213-91DE-2A988C72436F@comcast.net \
--to=paulkoning@comcast.net \
--cc=gcc@gcc.gnu.org \
--cc=segher@kernel.crashing.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).