From: Georg-Johann Lay <avr@gjlay.de>
To: Vladimir Makarov <vmakarov@redhat.com>, gcc@gcc.gnu.org
Subject: Re: [IRA] Code bloat due to register spills in v9, v10, v11, v12 and master
Date: Sat, 10 Dec 2022 13:15:02 +0100 [thread overview]
Message-ID: <a9d6479c-947e-2636-167f-1022dbc73816@gjlay.de> (raw)
In-Reply-To: <dd4f3b80-129f-308e-1a00-1c134a93f9bd@redhat.com>
Am 09.12.22 um 22:14 schrieb Vladimir Makarov:
>
> On 2022-12-09 14:23, Georg-Johann Lay wrote:
>> There is the following code size regression, filed as
>>
>> https://gcc.gnu.org/PR90706
>>
>
> I am sorry, I feel your frustration. I was not aware of this PR.
> Unfortunately, the PR was marked as P4 and I have too many open PRs and
> should prioritize them.
>
> I've just started to work on this issue. It is hard for me to say when
> it will be fixed. I'll give an update on the next week.
Hi Vladimir,
Thank you so much.
As far as I understand, avr is a ternary target and hence PRs for avr
will have priority P4 or P5?
Johann
>> Simple test cases are, for example
>>
>> #define PORT (*((unsigned char volatile*) 0x24))
>>
>> unsigned short var16;
>>
>> void func (void)
>> {
>> if (2048000ul * var16 > 1200000ul)
>> PORT |= 1;
>> }
>>
>> When I compile it with
>>
>> $ avr-gcc -Os bloat1.c -c && avr-size bloat1.o
>>
>> the code size increases from 36 bytes (v8) to 88 bytes (v13).
>>
>> Apart from that, register pressure is much higher because a frame
>> pointer is set up for no reason, and the values go through stack slots
>> for no reason.
>>
>> Even test cases which don't require any code like
>>
>> long func2 (void)
>> {
>> long var32;
>> __asm ("; some code %0" : "=r" (var32));
>> return var32;
>> }
>>
>> increase in register pressure (x2), stack usage (from 0 to 6 bytes)
>> and code size from 2 bytes (v8) to 34 bytes (v13).
>>
>> Some projects like QMK "solved" the problem by declaring GCC > v8 to
>> be "incompatible" with their project, see
>> https://github.com/qmk/qmk_firmware/issues/6719
>>
>> In own projects I observed the problem, too, and the only solution is
>> to use v8 or older. Options like -fcaller-saves or -fira-algorithm=
>> have no effect.
>>
>> To configure, I used --target=avr --disable-nls --with-dwarf2
>> --enable-languages=c,c++ --with-gnu-as --with-gnu-ld --disable-shared,
>> so nothing special.
>>
>> The problem is present in v9, v10, v11, v12 and master (future v13),
>> so sitting around for quite a while, so maybe it's not fixed because
>> RA maintainers are not aware of the problem.
next prev parent reply other threads:[~2022-12-10 12:15 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-09 19:23 Georg-Johann Lay
2022-12-09 21:14 ` Vladimir Makarov
2022-12-10 12:15 ` Georg-Johann Lay [this message]
2022-12-10 14:10 ` Richard Biener
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