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

[Georg-Johann Lay <avr@gjlay.de>]

There is the following code size regression, filed as

https://gcc.gnu.org/PR90706

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.

Thanks for any help,

Johann