[Bug rtl-optimization/114243] New: -fsplit-wide-types bloats code by more than 50%

[Bug rtl-optimization/114243] New: -fsplit-wide-types bloats code by more than 50%

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114243

            Bug ID: 114243
           Summary: -fsplit-wide-types bloats code by more than 50%
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gjl at gcc dot gnu.org
  Target Milestone: ---

Created attachment 57616
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57616&action=edit
pi-sigma.c: C99 test case

Compile the attached test case with:

$ avr-gcc pi-sigma.c -c -Os -mmcu=atmega8 -fstack-usage && avr-size pi-sigma.o

Then the code sizes are for respective versions of the compiler:

avr-gcc-v8:   624
avr-gcc-v14: 1008

which is an increase of code size of more than 60% !

The stack usage also increases by a lot. According to pi-sigma.su:

avr-gcc-v8:
-----------
pi-sigma.c:80:7:sigma   30      static
pi-sigma.c:86:7:pi_n    14      static

avr-gcc-v14:
------------
pi-sigma.c:80:7:sigma   86      static
pi-sigma.c:86:7:pi_n    36      static

That is for the 1st function the stack use almost triples!

With -fno-split-wide-types the performace of v14 code is similar to v8.

Target: avr
Configured with: ../../source/gcc-master/configure --target=avr --disable-nls
--with-dwarf2 --with-gnu-as --with-gnu-ld --disable-shared
--enable-languages=c,c++ 
Thread model: single
Supported LTO compression algorithms: zlib
gcc version 14.0.1 20240303 (experimental) (GCC)

[Bug rtl-optimization/114243] [avr] -fsplit-wide-types bloats code by more than 50%

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114243

--- Comment #1 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
May be related to PR110093.  As Vladimir noted in

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110093#c5

the problem is that data flow analysis cannot cope with the subregs generated
from lower-subregs, and register alloc chokes at it.

[Bug rtl-optimization/114243] [avr] -fsplit-wide-types bloats code by more than 50%

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114243

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Subreg improvements to ra is planned for gcc 15 as the riscv folks are running
into it for vector modes in some cases. Maybe that will improves the situation
here.