[Bug rtl-optimization/90706] [10/11/12/13 Regression] Useless code generated for stack / register operations on AVR

["gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90706

--- Comment #13 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Created attachment 53812
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53812&action=edit
Test case with 32-bit integer.

This problem is still present in current master (future v13) and also occurs
with 32-bit integers.

> avr-gcc -S -Os -mul.c -fdump-rtl-ira

With v8, mul.s has 15 instructions.

With newer versions, mul.s has 26 additional instructions: 
* 12 silly, useless stores into / loads from frame.
* 12 instructions to setup the frame.
* More instructions due to sub-optimal register alloc.
* Uses 6 bytes stack frame where v8 needs no frame at all.

In the IRA dump, there is:

Pass 0 for finding pseudo/allocno costs
    a0 (r53,l0) best NO_REGS, allocno NO_REGS
    a2 (r49,l0) best GENERAL_REGS, allocno GENERAL_REGS
    a1 (r48,l0) best NO_REGS, allocno NO_REGS
...
Pass 1 for finding pseudo/allocno costs
    r53: preferred NO_REGS, alternative NO_REGS, allocno NO_REGS
    r49: preferred GENERAL_REGS, alternative NO_REGS, allocno GENERAL_REGS
    r48: preferred NO_REGS, alternative NO_REGS, allocno NO_REGS
...
      Spill a0(r53,l0)
      Spill a1(r48,l0)
      Allocno a2r49 of GENERAL_REGS(30) ...

So there are 2 register spills for no reason that lead to that code bloat.