[Bug c/48090] New: gcc 4.5.2 miscompilation when building on arm

[Bug c/48090] New: gcc 4.5.2 miscompilation when building on arm

[arnaud.patard@rtp-net.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

           Summary: gcc 4.5.2 miscompilation when building on arm
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: arnaud.patard@rtp-net.org


With gcc-4.5.2, even with gcc-4.5-20110310, the following line of code gets
miscompiled (See attachment) :

size -= nblk * 512;

size and nblk are 64 bits.
gcc tries to compute -nblk. nblk is stored in r0,r1. gcc produces the following
asm code :

        rsbs    r1, r0, #0
        rsc     r2, r1, #0

So r1 gets corrupted by the rsbs insn, thus getting wrong value in r2.

I'm building the asm code with :
gcc -O2  -march=armv5t list2.i -o - -S

When I use :
gcc -O2 -fno-cse-follow-jumps  -march=armv5t list2.i -o - -S

I get :

        rsbs    r0, r0, #0
        rsc     r1, r1, #0

which is fine.

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[arnaud.patard@rtp-net.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #1 from arnaud patard <arnaud.patard@rtp-net.org> 2011-03-12 09:02:30 UTC ---
Created attachment 23636
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23636
test file

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

Mikael Pettersson <mikpe at it dot uu.se> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mikpe at it dot uu.se

--- Comment #2 from Mikael Pettersson <mikpe at it dot uu.se> 2011-03-12 12:03:18 UTC ---
I get correct-looking code on armv5tel-linux with vanilla gcc-4.6-20110305 and
gcc-4.5-20110310, with 4.4 the code looks different but not obviously broken.

Can you make the test case standalone and include a runtime check for the
possible miscompilation?

How was your gcc configured?

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[arnaud.patard@rtp-net.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #3 from arnaud patard <arnaud.patard@rtp-net.org> 2011-03-12 12:16:12 UTC ---
(In reply to comment #2)
> I get correct-looking code on armv5tel-linux with vanilla gcc-4.6-20110305 and
> gcc-4.5-20110310, with 4.4 the code looks different but not obviously broken.

I said -march=armv5t (or -march=armv4t I guess). It's important as
-march=armv5te won't trigger the bug.

>
> Can you make the test case standalone and include a runtime check for the
> possible miscompilation?

My test case was to build tar and run the delete01.at check but it's not really
a standalone test.
So far, I've not managed to have a test case small than compiling the
preprocessed file.

> 
> How was your gcc configured?

I reproduce it with my gcc and debian gcc 4.5.2 so I believe this rules out
misconfiguration.

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #4 from Mikael Pettersson <mikpe at it dot uu.se> 2011-03-12 15:42:13 UTC ---
I have a runtime test case (given test file + separate file with main and
missing support functions) that works with gcc-4.5-20110310 -march=armv5te but
fails with -march=armv5t.  gcc-4.4.5 and 4.6-20110305 work with both armv5te
and armv5t.

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #5 from Mikael Pettersson <mikpe at it dot uu.se> 2011-03-12 17:38:12 UTC ---
Created attachment 23637
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23637
standalone test case

Reduced standalone test case.  Fails with gcc-4.5-20110310 -march=armv5t, works
with -march=armv5te.  Also works with gcc-4.4 and gcc-4.6.

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #6 from Mikael Pettersson <mikpe at it dot uu.se> 2011-03-12 21:19:40 UTC ---
The test case was fixed for 4.6 by r159644.  However, that patch
<http://gcc.gnu.org/ml/gcc-patches/2010-05/msg01528.html> was described as a
minor improvement to double-word register allocation and not a correctness fix,
so the underlying bug may be latent on trunk.

I'll investigate some more tomorrow.

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

Mikael Pettersson <mikpe at it dot uu.se> changed:

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

--- Comment #7 from Mikael Pettersson <mikpe at it dot uu.se> 2011-03-13 15:09:36 UTC ---
The miscompilation on 4.5 branch started with r154181, the PR42031 fix; see
<http://gcc.gnu.org/ml/gcc-patches/2009-11/msg00725.html>.

On the standalone test case, r154181 changed the -march=armv5t -O2 code as
follows:

--- pr48090.s-r154180   2011-03-13 15:39:16.000000000 +0100
+++ pr48090.s-r154181   2011-03-13 15:43:21.000000000 +0100
@@ -81,11 +81,9 @@
        mov     r6, r1
        bl      seek_archive
        cmp     r1, #0
-       mov     r3, r0
-       mov     r4, r1
        blt     .L14
-       rsbs    r1, r3, #0
-       rsc     r2, r4, #0
+       rsbs    r1, r0, #0
+       rsc     r2, r1, #0
        mov     r4, r2, asl #9
        mov     r3, r1, asl #9
        orr     r4, r4, r1, lsr #23

which breaks the second 'rsc' insn since it now reads the updated r1 value
instead of the original one.

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

Mikael Pettersson <mikpe at it dot uu.se> changed:

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

--- Comment #8 from Mikael Pettersson <mikpe at it dot uu.se> 2011-03-13 19:32:46 UTC ---
The test case broke due to the combination of r154181 and r147087.  r147087 is
Kazu's "expmed.c: Improve multiplication by more constants" patch, see
<http://gcc.gnu.org/ml/gcc-patches/2009-04/msg00915.html>.

Backporting either of these to 4.4.5 doesn't break the test case, but when both
are backported at the same time it breaks.  And reverting r147087 from current
4.5 branch unbreaks the test case.

[Bug c/48090] gcc 4.5.2 miscompilation when building on arm

[arnaud.patard@rtp-net.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #9 from arnaud patard <arnaud.patard@rtp-net.org> 2011-03-13 20:40:27 UTC ---
I confirm that backporting r159644 and r159683 make things work. From comment
8, I guess that the bug is still there and that one can still hit it sooner or
later, right  ? (btw, amazing job)

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
          Component|c                           |target
      Known to work|                            |4.4.5, 4.6.0
   Target Milestone|---                         |4.5.3
            Summary|gcc 4.5.2 miscompilation    |[4.5 Regression] gcc 4.5.2
                   |when building on arm        |miscompilation when
                   |                            |building on arm

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.04.10 10:41:58
            Version|unknown                     |4.5.2
     Ever Confirmed|0                           |1

--- Comment #10 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-04-10 10:41:58 UTC ---
(In reply to comment #9)
> I confirm that backporting r159644 and r159683 make things work. From comment
> 8, I guess that the bug is still there and that one can still hit it sooner or
> later, right  ? (btw, amazing job)

It probably papers over it as you guessed.

This bug lacks proper analysis.

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:

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

--- Comment #11 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-04-11 10:24:09 UTC ---
(In reply to comment #10)
> (In reply to comment #9)
> > I confirm that backporting r159644 and r159683 make things work. From comment
> > 8, I guess that the bug is still there and that one can still hit it sooner or
> > later, right  ? (btw, amazing job)
> 
> It probably papers over it as you guessed.
> 
> This bug lacks proper analysis.

The problem is latent in all versions of the compiler and is to do with the
pattern *arm_negdi2 and has nothing to do with the afore mentioned revisions.
Will submit this after a round of testing.



Index: gcc/config/arm/arm.md
===================================================================
--- gcc/config/arm/arm.md    (revision 172252)
+++ gcc/config/arm/arm.md    (working copy)
@@ -3554,7 +3554,7 @@
 ;; The constraints here are to prevent a *partial* overlap (where %Q0 == %R1).
 ;; The first alternative allows the common case of a *full* overlap.
 (define_insn "*arm_negdi2"
-  [(set (match_operand:DI         0 "s_register_operand" "=&r,r")
+  [(set (match_operand:DI         0 "s_register_operand" "=r,&r")
     (neg:DI (match_operand:DI 1 "s_register_operand"  "0,r")))
    (clobber (reg:CC CC_REGNUM))]
   "TARGET_ARM"

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #12 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-04-12 13:42:52 UTC ---
Author: ramana
Date: Tue Apr 12 13:42:48 2011
New Revision: 172318

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=172318
Log:

Fix PR target/48090

2011-04-12  Ramana Radhakrishnan  <ramana.radhakrishnan@linaro.org>

       PR target/48090
       * config/arm/arm.md (*arm_negdi2): Fix early clobber constraints.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/arm/arm.md

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #13 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-04-12 13:52:49 UTC ---
Author: ramana
Date: Tue Apr 12 13:52:46 2011
New Revision: 172320

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=172320
Log:

Fix PR target/48090


Modified:
    branches/gcc-4_5-branch/gcc/ChangeLog
    branches/gcc-4_5-branch/gcc/config/arm/arm.md

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #14 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-04-12 13:53:39 UTC ---
Still need to backport and test on the 4.6 branch. That is next.

Ramana

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[froydnj at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #15 from froydnj at codesourcery dot com <froydnj at codesourcery dot com> 2011-04-12 13:55:57 UTC ---
On Tue, Apr 12, 2011 at 01:53:48PM +0000, ramana at gcc dot gnu.org wrote:
> Still need to backport and test on the 4.6 branch. That is next.

Small procedural note: it is preferred to go trunk -> 4.6 -> 4.5, rather
than trunk -> 4.5 -> 4.6, even if the bug is reported against 4.5.

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #16 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-04-13 07:41:13 UTC ---
Author: ramana
Date: Wed Apr 13 07:41:08 2011
New Revision: 172363

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=172363
Log:

Fix PR target/48090


Modified:
    branches/gcc-4_6-branch/gcc/ChangeLog
    branches/gcc-4_6-branch/gcc/config/arm/arm.md

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

--- Comment #17 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-04-13 07:56:00 UTC ---
(In reply to comment #15)
> On Tue, Apr 12, 2011 at 01:53:48PM +0000, ramana at gcc dot gnu.org wrote:
> > Still need to backport and test on the 4.6 branch. That is next.
> 
> Small procedural note: it is preferred to go trunk -> 4.6 -> 4.5, rather
> than trunk -> 4.5 -> 4.6, even if the bug is reported against 4.5.

Yes I know but it was just a function of what tree I had ready to test and
finish at that point of time. Will do it in the right order from next time.

I don't have a 4.4 tree handy and this bug should be latent there as well - so
if someone wants to backport it please do so but I don't have the time to do it
in the next couple of weeks. 

cheers
Ramana

[Bug target/48090] [4.5 Regression] gcc 4.5.2 miscompilation when building on arm

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48090

Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:

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

--- Comment #18 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-04-27 09:44:05 UTC ---
Fixed.