[IRA] Code bloat due to register spills in v9, v10, v11, v12 and master

[Georg-Johann Lay]

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.