[Bug c/115589] New: (aarch64) clobbered register variables

[Bug c/115589] New: (aarch64) clobbered register variables

[pietro.braione at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115589

            Bug ID: 115589
           Summary: (aarch64) clobbered register variables
           Product: gcc
           Version: 11.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pietro.braione at gmail dot com
  Target Milestone: ---

/*
 * compiled with gcc -O0, on my platform (Ubuntu 22.04 aarch64) prints:
 * 
 * 4
 * 2
 *
 * This is a very simplified example. In my production code I have
 * variables that are assigned register x21-x23, and gcc generates
 * code that uses these registers as temporaries without saving
 * elsewhere the content of the variables, that are lost.
 */
#include <stdio.h>

int a = 1, b = 1, c = 1, d = 1;

void main() {
  register int x asm("x0");
  x = 0;
  printf("%d\n", a + b + c + d);
  printf("%d\n", x);
}

[Bug target/115589] (aarch64) clobbered register variables

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115589

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |inline-asm

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
variables that are assigned register x21-x23, and gcc generates
 * code that uses these registers as temporaries without saving


Iirc this is not the use of local register variables.

[Bug target/115589] (aarch64) clobbered register variables

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115589

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gcc.gnu.org/onlinedocs/gcc-14.1.0/gcc/Local-Register-Variables.html

The only supported use for this feature is to specify registers for input and
output operands when calling Extended asm

[Bug target/115589] (aarch64) clobbered register variables

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115589

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Some developers use Local Register Variables in an attempt to improve gcc’s
allocation of registers, especially in large functions. In this case the
register name is essentially a hint to the register allocator. While in some
instances this can generate better code, improvements are subject to the whims
of the allocator/optimizers. Since there are no guarantees that your
improvements won’t be lost, this usage of Local Register Variables is
discouraged.

[Bug target/115589] (aarch64) clobbered register variables

[pietro.braione at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115589

--- Comment #4 from Pietro Braione <pietro.braione at gmail dot com> ---
I see. Sorry for the noise.