[Bug rtl-optimization/58726] New: wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[Bug rtl-optimization/58726] New: wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[su at cs dot ucdavis.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

            Bug ID: 58726
           Summary: wrong code at -Os on x86_64-linux-gnu (affecting
                    trunk/4.7/4.6, but not 4.8)
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk miscompiles the following code on x86_64-linux at -Os in
both 32-bit and 64-bit modes. 

This is a regression from 4.8.x. 

It also affects 4.6.x and 4.7.x. 

$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-unknown-linux-gnu/4.9.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure
--enable-languages=c,c++,objc,obj-c++,fortran,lto --disable-werror
--enable-checking=release --with-gmp=/usr/local/gcc-trunk
--with-mpfr=/usr/local/gcc-trunk --with-mpc=/usr/local/gcc-trunk
--with-cloog=/usr/local/gcc-trunk --prefix=/usr/local/gcc-trunk
Thread model: posix
gcc version 4.9.0 20131014 (experimental) [trunk revision 203511] (GCC) 
$ 
$ gcc-trunk -O1 small.c; a.out
-9162
$ gcc-trunk -Os small.c; a.out
-32768
$ 
$ gcc-4.8 -Os small.c; a.out
-9162
$ 
$ gcc-4.7 -Os small.c; a.out
-32768
$ gcc-4.6 -Os small.c; a.out
-32768
$ 

------------------------------------


int printf (const char *, ...);

int a, c;

union
{
  int f1;
  int f2:1;
} b;

short
foo (short p)
{
  return p < 0 ? p : a;
}

int
main ()
{
  b.f1 = 56374;
  unsigned short d;
  int e = b.f2;
  d = e == 0 ? b.f1 : 0;
  c = foo (d);
  printf ("%d\n", c);
  return 0;
}

[Bug rtl-optimization/58726] [4.7/4.8/4.9 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|wrong code at -Os on        |[4.7/4.8/4.9 Regression]
                   |x86_64-linux-gnu (affecting |wrong code at -Os on
                   |trunk/4.7/4.6, but not 4.8) |x86_64-linux-gnu (affecting
                   |                            |trunk/4.7/4.6, but not 4.8)
   Target Milestone|---                         |4.7.4
      Known to fail|                            |4.4.7, 4.5.4, 4.6.4, 4.7.3,
                   |                            |4.8.2, 4.9.0
             Status|UNCONFIRMED                 |NEW
      Known to work|                            |4.1.2, 4.3.4, 4.8.1
           Keywords|                            |wrong-code
   Last reconfirmed|                            |2013-10-15
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  Works with 4.8.1 but fails on the branch.  Seems to be a heisenbug.

[Bug rtl-optimization/58726] [4.7/4.8/4.9 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[mikpelinux at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

Mikael Pettersson <mikpelinux at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mikpelinux at gmail dot com

--- Comment #2 from Mikael Pettersson <mikpelinux at gmail dot com> ---
Started with r133452.

[Bug rtl-optimization/58726] [4.7/4.8/4.9 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
                 CC|                            |rsandifo at gcc dot gnu.org

[Bug rtl-optimization/58726] [4.7/4.8/4.9 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot gnu.org,
                   |                            |jakub at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Testcase suitable for gcc.c-torture/execute/:

int a, c;
union { int f1; int f2 : 1; } b;

short
foo (short p)
{
  return p < 0 ? p : a;
}

int
main ()
{
  if (sizeof (short) * __CHAR_BIT__ != 16
      || sizeof (int) * __CHAR_BIT__ != 32)
    return 0;
  b.f1 = 56374;
  unsigned short d;
  int e = b.f2;
  d = e == 0 ? b.f1 : 0;
  c = foo (d);
  if (c != (short) 56374)
    __builtin_abort ();
  return 0;
}

This seems to be a bug in the combiner, during combination of:
(gdb) p debug_rtx (i3)
(insn 22 21 23 2 (parallel [
            (set (reg:CCGOC 17 flags)
                (compare:CCGOC (and:HI (reg/v:HI 83 [ d ])
                        (const_int -9162 [0xffffffffffffdc36]))
                    (const_int 0 [0])))
            (set (reg:HI 85 [ D.1789 ])
                (and:HI (reg/v:HI 83 [ d ])
                    (const_int -9162 [0xffffffffffffdc36])))
        ]) pr58726.c:7 395 {*andhi_2}
     (expr_list:REG_DEAD (reg/v:HI 83 [ d ])
        (nil)))
(gdb) p debug_rtx (i2)
(insn 55 54 56 2 (parallel [
            (set (subreg:SI (reg/v:HI 83 [ d ]) 0)
                (if_then_else:SI (ltu:SI (reg:CC 17 flags)
                        (const_int 0 [0]))
                    (const_int -1 [0xffffffffffffffff])
                    (const_int 0 [0])))
            (clobber (reg:CC 17 flags))
        ]) pr58726.c:19 925 {*x86_movsicc_0_m1}
     (expr_list:REG_DEAD (reg:CC 17 flags)
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))
(gdb) p debug_rtx (i1)
(insn 54 15 55 2 (set (reg:CC 17 flags)
        (compare:CC (reg:QI 98 [ D.1788 ])
            (const_int 1 [0x1]))) pr58726.c:19 5 {*cmpqi_1}
     (expr_list:REG_DEAD (reg:QI 98 [ D.1788 ])
        (nil)))
The problem is that reg:HI 83 is substed multiple twice (with unique_copy == 0)
and apparently force_to_mode seems to destructively modify the expression,
which is shared, around line 8533.  On the second occurrence it is first
force_to_mode'd with mask 0xdc36, and on the first occurrence later on
simplify_comparison performs:
11187          /* If this is a sign bit comparison and we can do arithmetic in
11188         MODE, say that we will only be needing the sign bit of OP0.  */
11189          if (sign_bit_comparison_p && HWI_COMPUTABLE_MODE_P (mode))
11190        op0 = force_to_mode (op0, mode,
11191                     (unsigned HOST_WIDE_INT) 1
11192                     << (GET_MODE_PRECISION (mode) - 1),
11193                     0);
with mask 0x8000, which destructively modifies also the second occurrence.
So the question is, do we have to copy_rtx always just in case, or should some
routines (e.g. force_to_mode) be documented as not modifying the argument in
place?  I guess this issue can be fixed easily just by changing those two
SUBSTs at the end of force_to_mode in IF_THEN_ELSE handling (and there is
another one for ROTATE{,RT}), but the question is if we don't have tons of
similar latent issues.  The changes in place were done using SUBST, so if we
undo_all, they are properly reverted, the problem is when they aren't reverted,
as in this case.

[Bug rtl-optimization/58726] [4.7/4.8/4.9 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 31366
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31366&action=edit
gcc49-pr58726.patch

Untested patch that does that, fixes the testcase.

[Bug rtl-optimization/58726] [4.7/4.8/4.9 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Author: jakub
Date: Wed Dec  4 11:25:06 2013
New Revision: 205664

URL: http://gcc.gnu.org/viewcvs?rev=205664&root=gcc&view=rev
Log:
    PR rtl-optimization/58726
    * combine.c (force_to_mode): Fix comment typo.  Don't destructively
    modify x for ROTATE, ROTATERT and IF_THEN_ELSE.

    * gcc.c-torture/execute/pr58726.c: New test.

Added:
    trunk/gcc/testsuite/gcc.c-torture/execute/pr58726.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/combine.c
    trunk/gcc/testsuite/ChangeLog

[Bug rtl-optimization/58726] [4.7/4.8/4.9 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Author: jakub
Date: Wed Dec  4 15:51:02 2013
New Revision: 205672

URL: http://gcc.gnu.org/viewcvs?rev=205672&root=gcc&view=rev
Log:
    PR rtl-optimization/58726
    * combine.c (force_to_mode): Fix comment typo.  Don't destructively
    modify x for ROTATE, ROTATERT and IF_THEN_ELSE.

    * gcc.c-torture/execute/pr58726.c: New test.

Added:
    branches/gcc-4_8-branch/gcc/testsuite/gcc.c-torture/execute/pr58726.c
Modified:
    branches/gcc-4_8-branch/gcc/ChangeLog
    branches/gcc-4_8-branch/gcc/combine.c
    branches/gcc-4_8-branch/gcc/testsuite/ChangeLog

[Bug rtl-optimization/58726] [4.7 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
      Known to work|                            |4.8.3, 4.9.0
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org
            Summary|[4.7/4.8/4.9 Regression]    |[4.7 Regression] wrong code
                   |wrong code at -Os on        |at -Os on x86_64-linux-gnu
                   |x86_64-linux-gnu (affecting |(affecting trunk/4.7/4.6,
                   |trunk/4.7/4.6, but not 4.8) |but not 4.8)
      Known to fail|4.9.0                       |

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed for 4.8.3+ so far.

[Bug rtl-optimization/58726] [4.7 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

--- Comment #8 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
Let's close this, I don't think that we need the fix on the 4.7 branch.

[Bug rtl-optimization/58726] [4.7 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

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

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

--- Comment #9 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
.

[Bug rtl-optimization/58726] [4.7 Regression] wrong code at -Os on x86_64-linux-gnu (affecting trunk/4.7/4.6, but not 4.8)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58726

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.7.4                       |4.8.3