[Bug c/57046] New: wrong code generated by gcc 4.8.0 on i686

[Bug c/57046] New: wrong code generated by gcc 4.8.0 on i686

[mattiase at acm dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

             Bug #: 57046
           Summary: wrong code generated by gcc 4.8.0 on i686
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mattiase@acm.org


Created attachment 29917
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29917
Test case including driver demonstrating the bug

Gcc 4.8.0 silently miscompiles the attached short test case (emac.c) on 32-bit
x86 with -O2. It appears that the value returned from get_value is thrown away.

gcc -V output:
Using built-in specs.
COLLECT_GCC=/usr/local/gcc/4.8.0/bin/gcc
COLLECT_LTO_WRAPPER=/home/local/linux/gcc/4.8.0/bin/../libexec/gcc/i686-pc-linux-gnu/4.8.0/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: ../gcc-4.8.0/configure --prefix=/usr/local/gcc/4.8.0
--with-mpc=/tmp/extra --with-gmp=/tmp/extra --with-mpfr=/tmp/extra
--with-isl=/tmp/extra --with-cloog=/tmp/extra
--with-as=/usr/local/binutils/2.23.2/bin/as
--with-ld=/usr/local/binutils/2.23.2/bin/ld.gold
--enable-languages=c,c++,objc,go
Thread model: posix
gcc version 4.8.0 (GCC)

[Bug c/57046] wrong code generated by gcc 4.8.0 on i686

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

--- Comment #1 from Mikael Pettersson <mikpe at it dot uu.se> 2013-04-23 11:31:06 UTC ---
Created attachment 29918
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29918
Single-file test case.

I can reproduce the wrong-code on x86_64-linux with gcc 4.9-20130421 and
4.8-20130418, using -m32 -O2 -Wall.  gcc 4.7 and 4.6 generate correct code.

[Bug c/57046] [4.8/4.9 Regression] wrong code generated by gcc 4.8.0 on i686

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |i?86-*-*
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |wrong-code
   Last reconfirmed|                            |2013-04-23
     Ever Confirmed|0                           |1
            Summary|wrong code generated by gcc |[4.8/4.9 Regression] wrong
                   |4.8.0 on i686               |code generated by gcc 4.8.0
                   |                            |on i686
   Target Milestone|---                         |4.8.1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> 2013-04-23 12:04:18 UTC ---
Confirmed.

[Bug rtl-optimization/57046] [4.8/4.9 Regression] wrong code generated by gcc 4.8.0 on i686

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P1
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |vmakarov at gcc dot gnu.org
          Component|c                           |rtl-optimization

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-04-23 12:09:58 UTC ---
Started with http://gcc.gnu.org/r192719 aka LRA merge, the problematic function
is emac_operation.

[Bug rtl-optimization/57046] [4.8/4.9 Regression] wrong code generated by gcc 4.8.0 on i686

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-04-23 12:26:35 UTC ---
We have after the get_value call:
(insn 73 30 32 6 (set (reg:SI 76 [ D.1441 ])
        (reg:SI 0 ax)) pr57046.c:42 85 {*movsi_internal}
     (expr_list:REG_DEAD (reg:SI 0 ax)
        (nil)))
(insn 32 73 33 6 (parallel [
            (set (reg:SI 73 [ D.1443 ])
                (ashift:SI (subreg:SI (reg:DI 60 [ D.1441 ]) 0)
                    (const_int 2 [0x2])))
            (clobber (reg:CC 17 flags))
        ]) 502 {*ashlsi3_1}
     (expr_list:REG_DEAD (reg:DI 60 [ D.1441 ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

and IRA decides to put pseudo 76 into %ebx and pseudo 60 into %ecx.  Either it
is an IRA bug, or IRA takes into account that we only really need the low
32-bits of pseudo 60 at that point.  In any case, reload loads SImode %ecx from
the stack and uses it in the shift, while LRA loads full DImode %ecx (i.e. %ecx
and %ebx) from the stack, then uses just the low bits from that (i.e. %ecx) in
the shift.  So the LRA generated code clobbers the value in %ebx, and get_value
call is even later on DCEd because of it.

[Bug rtl-optimization/57046] [4.8/4.9 Regression] wrong code generated by gcc 4.8.0 on i686

[vmakarov at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

Vladimir Makarov <vmakarov at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vmakarov at redhat dot com

--- Comment #5 from Vladimir Makarov <vmakarov at redhat dot com> 2013-04-23 15:34:40 UTC ---
(In reply to comment #4)
> We have after the get_value call:
> (insn 73 30 32 6 (set (reg:SI 76 [ D.1441 ])
>         (reg:SI 0 ax)) pr57046.c:42 85 {*movsi_internal}
>      (expr_list:REG_DEAD (reg:SI 0 ax)
>         (nil)))
> (insn 32 73 33 6 (parallel [
>             (set (reg:SI 73 [ D.1443 ])
>                 (ashift:SI (subreg:SI (reg:DI 60 [ D.1441 ]) 0)
>                     (const_int 2 [0x2])))
>             (clobber (reg:CC 17 flags))
>         ]) 502 {*ashlsi3_1}
>      (expr_list:REG_DEAD (reg:DI 60 [ D.1441 ])
>         (expr_list:REG_UNUSED (reg:CC 17 flags)
>             (nil))))
> 
> and IRA decides to put pseudo 76 into %ebx and pseudo 60 into %ecx.  Either it
> is an IRA bug, or IRA takes into account that we only really need the low
> 32-bits of pseudo 60 at that point.  In any case, reload loads SImode %ecx from
> the stack and uses it in the shift, while LRA loads full DImode %ecx (i.e. %ecx
> and %ebx) from the stack, then uses just the low bits from that (i.e. %ecx) in
> the shift.  So the LRA generated code clobbers the value in %ebx, and get_value
> call is even later on DCEd because of it.

It seems like a discrepancy in IRA which allocates in terms of subregisters and
LRA splitting (including call save/restore as in this case) in terms of
pseudos.  I guess fixing this might take about week.

[Bug rtl-optimization/57046] [4.8 Regression] wrong code generated by gcc 4.8.0 on i686

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |4.7.2, 4.9.0
            Summary|[4.8/4.9 Regression] wrong  |[4.8 Regression] wrong code
                   |code generated by gcc 4.8.0 |generated by gcc 4.8.0 on
                   |on i686                     |i686
      Known to fail|                            |4.8.0

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-04-26 18:03:02 UTC ---
Author: vmakarov
Date: Wed Apr 24 20:27:33 2013
New Revision: 198263

URL: http://gcc.gnu.org/viewcvs?rev=198263&root=gcc&view=rev
Log:
2013-04-24  Vladimir Makarov  <vmakarov@redhat.com>

    PR rtl-optimizations/57046
    * lra-constraints (split_reg): Set up lra_risky_transformations_p
    for multi-reg splits.

2013-04-24  Vladimir Makarov  <vmakarov@redhat.com>

    PR rtl-optimizations/57046
    * gcc.target/i386/pr57046.c: New test.


Added:
    trunk/gcc/testsuite/gcc.target/i386/pr57046.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/lra-constraints.c
    trunk/gcc/testsuite/ChangeLog

[Bug rtl-optimization/57046] [4.8 Regression] wrong code generated by gcc 4.8.0 on i686

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-05-02 19:29:52 UTC ---
Author: vmakarov
Date: Thu May  2 17:52:45 2013
New Revision: 198556

URL: http://gcc.gnu.org/viewcvs?rev=198556&root=gcc&view=rev
Log:
2013-05-02  Vladimir Makarov  <vmakarov@redhat.com>

    Backport from mainline
    2013-04-24  Vladimir Makarov  <vmakarov@redhat.com>

        PR rtl-optimizations/57046
        * lra-constraints (split_reg): Set up lra_risky_transformations_p
        for multi-reg splits.

2013-05-02  Vladimir Makarov  <vmakarov@redhat.com>

    Backport from mainline
    2013-04-24  Vladimir Makarov  <vmakarov@redhat.com>

        PR rtl-optimizations/57046
        * gcc.target/i386/pr57046.c: New test.


Added:
    branches/gcc-4_8-branch/gcc/testsuite/gcc.target/i386/pr57046.c
Modified:
    branches/gcc-4_8-branch/gcc/ChangeLog
    branches/gcc-4_8-branch/gcc/lra-constraints.c
    branches/gcc-4_8-branch/gcc/testsuite/ChangeLog

[Bug rtl-optimization/57046] [4.8 Regression] wrong code generated by gcc 4.8.0 on i686

[eraman at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

Easwaran Raman <eraman at google dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eraman at google dot com

--- Comment #8 from Easwaran Raman <eraman at google dot com> ---
*** Bug 57088 has been marked as a duplicate of this bug. ***