[Bug rtl-optimization/54369] New: Delayed-branch pass in reorg.c removes too many instructions

[Bug rtl-optimization/54369] New: Delayed-branch pass in reorg.c removes too many instructions

[chaoyingfu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

             Bug #: 54369
           Summary: Delayed-branch pass in reorg.c removes too many
                    instructions
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: chaoyingfu@gcc.gnu.org


Created attachment 28079
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28079
A simple test for an issue in reorg.c

Please check this example when using GCC 4.6 for mips-linux-gnu.
(I think the issue is in GCC 4.7 and later as well.  But due to the basic block
ordering, I cannot reproduce this issue.)
Ex:
# /gcc46branch/build-mips/gcc/cc1plus -quiet Bug.i -o Bug.s -mips32r2 -O2
# cat Bug.s
_Z3bugP9ValueRTRK:
        .frame  $sp,0,$31               # vars= 0, regs= 0/0, args= 0, gp= 0
        .mask   0x00000000,0
        .fmask  0x00000000,0
        .set    noreorder
        .set    nomacro
        lw      $2,0($4) <---------
        lw      $2,0($2) <--------- (NO RETURN IN THIS FUNCTION.)
        .set    macro
        .set    reorder
        .end    _Z3bugP9ValueRTRK
        .cfi_endproc
$LFE13:
        .size   _Z3bugP9ValueRTRK, .-_Z3bugP9ValueRTRK
        .ident  "GCC: (GNU) 4.6.4 20120823 (prerelease) [gcc-4_6-branch
revision
 190613]"

  The delayed-branch pass in reorg.c is too aggressive, such that a return
instruction and others are removed.  From debugging code in reorg.c:
delete_jump() -> delete_computation() -> delete_related_insn(), I think we
should not use delete_related_insn() that removes branches and all the
following instructions.  A simple fix may be just using delete_insn() in
delete_computation().
Ex: # svn diff reorg.c
Index: reorg.c
===================================================================
--- reorg.c     (revision 190636)
+++ reorg.c     (working copy)
@@ -3298,7 +3298,7 @@
       delete_prior_computation (note, insn);
     }

-  delete_related_insns (insn);
+  delete_insn (insn);
 }

 /* If all INSN does is set the pc, delete it,

Ex: (Bug.s after this fix.)
# cat Bug.s
_Z3bugP9ValueRTRK:
        .frame  $sp,0,$31               # vars= 0, regs= 0/0, args= 0, gp= 0
        .mask   0x00000000,0
        .fmask  0x00000000,0
        .set    noreorder
        .set    nomacro
        lw      $2,0($4)
        lw      $2,0($2)
$L4:
        j       $31      <------------- OK
        srl     $2,$2,8  <------------- OK

        .set    macro
        .set    reorder
        .end    _Z3bugP9ValueRTRK
        .cfi_endproc
$LFE13:
        .size   _Z3bugP9ValueRTRK, .-_Z3bugP9ValueRTRK
        .ident  "GCC: (GNU) 4.6.4 20120823 (prerelease) [gcc-4_6-branch
revision
 190613]"

Thanks!

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2012-08-29
                 CC|                            |ebotcazou at gcc dot
                   |                            |gnu.org
     Ever Confirmed|0                           |1

--- Comment #1 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-08-29 22:42:54 UTC ---
delete_related_insns isn't supposed to do incorrect things though, so the issue
must really be analyzed first.

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[sje at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

Steve Ellcey <sje at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

--- Comment #2 from Steve Ellcey <sje at gcc dot gnu.org> 2012-08-31 00:30:01 UTC ---
I think I see where delete_related_insns is going wrong.  We call it
with a JUMP instruction that we want to remove because we are just
jumping to the next instruction (a label) which we would get to anyway
without the jump, then after delete_related_insns removes the jump it
checks the label it was jumping to, and if it finds no uses it calls
delete_related_insns with that label.  When delete_related_insns
is called with a label, it assumes it can remove all the code after
that label as unreachable.  But the code after the label can be reached
by the default fall-through code sequence.  So after removing a jump and
finding a label with no uses we should call delete_insn with that label,
not delete_related_insns.

So my proposed fix would be:

diff --git a/gcc/jump.c b/gcc/jump.c
index 9721fe1..fa3d65a 100644
--- a/gcc/jump.c
+++ b/gcc/jump.c
@@ -1207,7 +1207,7 @@ delete_related_insns (rtx insn)
        /* This can delete NEXT or PREV,
           either directly if NEXT is JUMP_LABEL (INSN),
           or indirectly through more levels of jumps.  */
-       delete_related_insns (lab);
+       delete_insn (lab);
       else if (tablejump_p (insn, NULL, &lab_next))
        {
          /* If we're deleting the tablejump, delete the dispatch table.

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[chaoyingfu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #3 from chaoyingfu at gcc dot gnu.org <chaoyingfu at gcc dot gnu.org> 2012-08-31 00:38:39 UTC ---
The fall-through path of this jump has a barrier (resulted from
__builtin_unreachable), and the target of this jump has a return instruction.

The algorithm in reorg.c may be more conservative to keep this jump for code
correctness.  Just another solution to this issue.  Thanks!

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #4 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-08-31 14:20:03 UTC ---
> I think I see where delete_related_insns is going wrong.  We call it
> with a JUMP instruction that we want to remove because we are just
> jumping to the next instruction (a label) which we would get to anyway
> without the jump, then after delete_related_insns removes the jump it
> checks the label it was jumping to, and if it finds no uses it calls
> delete_related_insns with that label.  When delete_related_insns
> is called with a label, it assumes it can remove all the code after
> that label as unreachable.

Only if there is a barrier just before the label though; now this barrier
should have been removed one level earlier.

The root cause of the problem is that MIPS runs its own version of the dbr pass
and doesn't make sure that barriers are correctly placed for it, unlike other
targets like SPARC.  So the fix is:

Index: config/mips/mips.c
===================================================================
--- config/mips/mips.c  (revision 184886)
+++ config/mips/mips.c  (working copy)
@@ -15083,7 +15083,10 @@ mips_reorg (void)
     }

   if (optimize > 0 && flag_delayed_branch)
-    dbr_schedule (get_insns ());
+    {
+      cleanup_barriers ();
+      dbr_schedule (get_insns ());
+    }
   mips_reorg_process_insns ();
   if (!TARGET_MIPS16
       && TARGET_EXPLICIT_RELOCS

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[sje at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #5 from Steve Ellcey <sje at gcc dot gnu.org> 2012-08-31 18:16:08 UTC ---
Thanks for digging into this Eric.  I tested your patch here against the
example and on the GCC testsuite and didn't see any problems.  Are you going to
check this in on the mainline?

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rsandifo at gcc dot gnu.org

--- Comment #6 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-08-31 18:55:11 UTC ---
> Thanks for digging into this Eric.  I tested your patch here against the
> example and on the GCC testsuite and didn't see any problems.

You're welcome.  Thanks for the testing.

> Are you going to check this in on the mainline?

Not without Richard's approval, so I've CCed him now.

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[rdsandiford at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #7 from rdsandiford at googlemail dot com <rdsandiford at googlemail dot com> 2012-09-01 08:16:23 UTC ---
"ebotcazou at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> writes:
>> Are you going to check this in on the mainline?
>
> Not without Richard's approval, so I've CCed him now.

Looks good, thanks.  Please go ahead.

Just out of interest, you said:

> The root cause of the problem is that MIPS runs its own version of the dbr pass
> and doesn't make sure that barriers are correctly placed for it, unlike other
> targets like SPARC.

But how does SPARC do it?  sparc_reorg starts:

  /* The only erratum we handle for now is that of the AT697F processor.  */
  if (!sparc_fix_at697f)
    return;

  /* We need to have the (essentially) final form of the insn stream in order
     to properly detect the various hazards.  Run delay slot scheduling.  */
  if (optimize > 0 && flag_delayed_branch)
    dbr_schedule (get_insns ());

without any explicit call to cleanup_barriers (which like you say usually
runs after machine_reorg but before delay_slots).

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #8 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-01 09:21:19 UTC ---
> Looks good, thanks.  Please go ahead.

Thanks.  On which branch(es)?

> But how does SPARC do it?  sparc_reorg starts:
> 
>   /* The only erratum we handle for now is that of the AT697F processor.  */
>   if (!sparc_fix_at697f)
>     return;
> 
>   /* We need to have the (essentially) final form of the insn stream in order
>      to properly detect the various hazards.  Run delay slot scheduling.  */
>   if (optimize > 0 && flag_delayed_branch)
>     dbr_schedule (get_insns ());
> 
> without any explicit call to cleanup_barriers (which like you say usually
> runs after machine_reorg but before delay_slots).

Gosh.  I didn't remember that sparc_reorg existed...  I was referring to the
regular pass pipeline, which is almost always used for SPARC.  So I found the
bug in the MIPS port and you found it in the SPARC port.  Time to swap? ;-)

[Bug rtl-optimization/54369] Delayed-branch pass in reorg.c removes too many instructions

[rdsandiford at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #9 from rdsandiford at googlemail dot com <rdsandiford at googlemail dot com> 2012-09-01 09:41:08 UTC ---
"ebotcazou at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> writes:
>> Looks good, thanks.  Please go ahead.
>
> Thanks.  On which branch(es)?

Hmm, looks like this goes all the way back to 4.5, so as any as
you've got the stamina for.  It should be a low-risk change.

Richard

[Bug rtl-optimization/54369] delayed-branch pass removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|ebotcazou at gcc dot        |
                   |gnu.org                     |
         AssignedTo|unassigned at gcc dot       |ebotcazou at gcc dot
                   |gnu.org                     |gnu.org
            Summary|Delayed-branch pass in      |delayed-branch pass removes
                   |reorg.c removes too many    |too many instructions
                   |instructions                |

[Bug rtl-optimization/54369] delayed-branch pass removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #10 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:36:31 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:36:27 2012
New Revision: 190858

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190858
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/mips/mips.c
    trunk/gcc/config/sparc/sparc.c

[Bug rtl-optimization/54369] delayed-branch pass removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #10 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:36:31 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:36:27 2012
New Revision: 190858

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190858
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/mips/mips.c
    trunk/gcc/config/sparc/sparc.c

--- Comment #11 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:37:00 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:36:54 2012
New Revision: 190859

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190859
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    branches/gcc-4_7-branch/gcc/ChangeLog
    branches/gcc-4_7-branch/gcc/config/mips/mips.c
    branches/gcc-4_7-branch/gcc/config/sparc/sparc.c

[Bug rtl-optimization/54369] delayed-branch pass removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #11 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:37:00 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:36:54 2012
New Revision: 190859

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190859
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    branches/gcc-4_7-branch/gcc/ChangeLog
    branches/gcc-4_7-branch/gcc/config/mips/mips.c
    branches/gcc-4_7-branch/gcc/config/sparc/sparc.c

--- Comment #12 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:37:52 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:37:49 2012
New Revision: 190860

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190860
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    branches/gcc-4_6-branch/gcc/ChangeLog
    branches/gcc-4_6-branch/gcc/config/mips/mips.c
    branches/gcc-4_6-branch/gcc/config/sparc/sparc.c

[Bug rtl-optimization/54369] delayed-branch pass removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #12 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:37:52 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:37:49 2012
New Revision: 190860

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190860
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    branches/gcc-4_6-branch/gcc/ChangeLog
    branches/gcc-4_6-branch/gcc/config/mips/mips.c
    branches/gcc-4_6-branch/gcc/config/sparc/sparc.c

[Bug rtl-optimization/54369] delayed-branch pass removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

--- Comment #10 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:36:31 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:36:27 2012
New Revision: 190858

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190858
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/mips/mips.c
    trunk/gcc/config/sparc/sparc.c

--- Comment #11 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:37:00 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:36:54 2012
New Revision: 190859

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190859
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    branches/gcc-4_7-branch/gcc/ChangeLog
    branches/gcc-4_7-branch/gcc/config/mips/mips.c
    branches/gcc-4_7-branch/gcc/config/sparc/sparc.c

--- Comment #12 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:37:52 UTC ---
Author: ebotcazou
Date: Sun Sep  2 10:37:49 2012
New Revision: 190860

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=190860
Log:
    PR rtl-optimization/54369
    * config/mips/mips.c (mips_reorg): Invoke cleanup_barriers before
    calling dbr_schedule.
    * config/sparc/sparc.c (sparc_reorg): Likewise.

Modified:
    branches/gcc-4_6-branch/gcc/ChangeLog
    branches/gcc-4_6-branch/gcc/config/mips/mips.c
    branches/gcc-4_6-branch/gcc/config/sparc/sparc.c

[Bug rtl-optimization/54369] delayed-branch pass removes too many instructions

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.6.4

--- Comment #13 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-09-02 10:47:46 UTC ---
On all active branches.

[Bug rtl-optimization/54369] delayed-branch pass removes too many instructions

[jackie.rosen at hushmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

Jackie Rosen <jackie.rosen at hushmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jackie.rosen at hushmail dot com

--- Comment #14 from Jackie Rosen <jackie.rosen at hushmail dot com> ---
*** Bug 260998 has been marked as a duplicate of this bug. ***
Seen from the domain http://volichat.com
Marked for reference. Resolved as fixed @bugzilla.