[Bug tree-optimization/64718] New: Bad 16-bit bswap replacement

[Bug tree-optimization/64718] New: Bad 16-bit bswap replacement

[aaz@q-fu.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64718

            Bug ID: 64718
           Summary: Bad 16-bit bswap replacement
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aaz@q-fu.com
            Target: i686-pc-linux-gnu

This code is reduced from htons((int)port) on FreeBSD.

The expression gets replaced with a rotation in a 32-bit register
instead of a 16-bit register, so the upper byte is lost.

int swap(int x) {
    return (unsigned short)((unsigned short)x << 8 | (unsigned short)x >> 8);
}

gcc -O2 -S swap.c

swap:
        movl    4(%esp), %eax
        roll    $8, %eax       # should be %ax
        movzwl  %ax, %eax
        ret


Complete program:

#include <stdio.h>

int swap(int x) {
    return (unsigned short)((unsigned short)x << 8 | (unsigned short)x >> 8);
}

int a = 0x1234;
int main() {
    int b = 0x1234;
    printf("%x -> %x\n", a, swap(a));
    printf("%x -> %x\n", b, swap(b));
}

gcc -O2 swap.c && ./a.out

1234 -> 3400
1234 -> 3412

[Bug tree-optimization/64718] Bad 16-bit bswap replacement

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64718

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-01-21
   Target Milestone|---                         |5.0
     Ever confirmed|0                           |1

[Bug tree-optimization/64718] Bad 16-bit bswap replacement

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64718

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |thomas.preudhomme at arm dot com

--- Comment #1 from H.J. Lu <hjl.tools at gmail dot com> ---
It is caused by r219256.

[Bug tree-optimization/64718] Bad 16-bit bswap replacement

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64718

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
A run-time testcase:

[hjl@gnu-mic-2 gcc-regression]$ cat pr64718.c
int
__attribute__ ((noinline, noclone))
swap(int x)
{
  return (unsigned short)((unsigned short)x << 8 | (unsigned short)x >> 8);
}

int a = 0x1234;

int
main()
{
  int b = 0x1234;
  if (swap (a) != 0x3412)
    __builtin_abort ();
  if (swap (b) != 0x3412)
    __builtin_abort ();
  return 0;
}
[hjl@gnu-mic-2 gcc-regression]$

[Bug tree-optimization/64718] Bad 16-bit bswap replacement

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64718

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|i686-pc-linux-gnu           |x86
           Priority|P3                          |P1

--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> ---
Also happens on x86-64.

[Bug tree-optimization/64718] Bad 16-bit bswap replacement

[thopre01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64718

thopre01 at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED

--- Comment #4 from thopre01 at gcc dot gnu.org ---
The problem comes from:

bswap_type = TREE_TYPE (src); in bswap_replace when dealing with 16-bit bswap.
I got a fix for that and testing is underway. Thanks for the testcase.

Best regards.

[Bug tree-optimization/64718] Bad 16-bit bswap replacement

[thopre01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64718

--- Comment #5 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> ---
Author: thopre01
Date: Wed Jan 28 10:20:19 2015
New Revision: 220203

URL: https://gcc.gnu.org/viewcvs?rev=220203&root=gcc&view=rev
Log:
2015-01-28  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    PR tree-optimization/64718
    * tree-ssa-math-opts.c (pass_optimize_bswap::execute): Make bswap_type
    be a 16bit unsigned integer when n->range is 16.
    (bswap_replace): Convert src to that type if necessary for all bswap
    sizes.  Fix rotation right notation in nearby comment.  Use bswap_type
    set in pass_optimize_bswap::execute ().

    gcc/testsuite/
    PR tree-optimization/64718
    * gcc.c-torture/execute/pr64718.c: New test.

Added:
    trunk/gcc/testsuite/gcc.c-torture/execute/pr64718.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/tree-ssa-math-opts.c

[Bug tree-optimization/64718] [5 Regression] Bad 16-bit bswap replacement

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64718

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
                 CC|                            |jakub at gcc dot gnu.org
         Resolution|---                         |FIXED
            Summary|Bad 16-bit bswap            |[5 Regression] Bad 16-bit
                   |replacement                 |bswap replacement

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.