[Bug tree-optimization/31966] New: Miscompiles valid code with -ftree-vectorize and -march=nocona

[Bug tree-optimization/31966] New: Miscompiles valid code with -ftree-vectorize and -march=nocona

[lloyd at randombit dot net]

$ gcc-4.2.0 -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.2.0/configure --enable-languages=c,c++,objc,obj-c++
--program-suffix=-4.2.0
Thread model: posix
gcc version 4.2.0
$ gcc-4.2.0 divop.c -o divop
$ ./divop
          3CBA83
$ gcc-4.2.0 -ftree-vectorize -O2  divop.c -o divop
$ ./divop
          3CBA83
$ gcc-4.2.0 -ftree-vectorize -O2 -march=opteron divop.c -o divop
$ ./divop 
          3CBA83
$ gcc-4.2.0 -ftree-vectorize -O2 -march=nocona divop.c -o divop
$ ./divop
          2B672F

This also affects GCC 4.1.1, I have not checked 4.1.2 or the 4.3 snapshots. In
addition to -march=nocona it requires -O1/-O2/-O3 (-Os does not trigger it
under 4.2.0, though it _does_ in 4.1.1).


-- 
           Summary: Miscompiles valid code with -ftree-vectorize and -
                    march=nocona
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: lloyd at randombit dot net
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] Miscompiles valid code with -ftree-vectorize and -march=nocona

[lloyd at randombit dot net]

------- Comment #1 from lloyd at randombit dot net  2007-05-17 07:36 -------
Created an attachment (id=13567)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=13567&action=view)
A short testcase for bug 31966


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] Miscompiles valid code with -ftree-vectorize and -march=nocona

[lloyd at randombit dot net]

------- Comment #2 from lloyd at randombit dot net  2007-06-30 22:11 -------
The behavior still exists in the 4.3 20070622 snapshot. It does not occur using
-march=core2 (the actual CPU in question). The bad value results when using
-ftree-vectorize and -march or -mtune =nocona. -O, -O2, or -O3 is also
required; a binary compiled with -Os produces the correct value.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] Miscompiles valid code with -ftree-vectorize and -march=nocona

[dorit at gcc dot gnu dot org]

------- Comment #3 from dorit at gcc dot gnu dot org  2007-07-01 08:47 -------
I was able to reproduce the behavior on a Pentinum4 with a recent 4.3 snapshot.
It doesn't look like it's the vectorizer's fault - nothing gets vectorized.
However, if I disable the tree-level if-conversion (which is automatically
enabled by -ftree-vectorize) - the result of '-ftree-vectorize -O2
-march=nocona divop.c -o divop' becomes '3CBA83'. 


-- 

dorit at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] Miscompiles valid code with -ftree-vectorize and -march=nocona

[dorit at gcc dot gnu dot org]

------- Comment #4 from dorit at gcc dot gnu dot org  2007-07-01 08:52 -------
This may be related to PR32533 (also wrong-code due to tree-if-conversion).


-- 

dorit at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-07-01 08:52:43
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2/4.3 Regression] Miscompiles valid code with -ftree-vectorize

[ubizjak at gmail dot com]

------- Comment #5 from ubizjak at gmail dot com  2007-07-01 09:33 -------
Confirmed. This is the same bug as PR32533, but this one comes with the c
testcase. The problem is in ifcvt pass.

In -march=nocona case (-march=nocona -O2 -ftree-vectorize), we have following
code before ifcvt pass:

  if (high_top_bit_11 != 0)
    goto <bb 5>;
  else
    goto <bb 4>;

<bb 4>:
  if (d_7(D) <= high_19)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 5>:
  high_21 = high_19 - d_7(D);
  quotient_22 = quotient_20 | 1;

<bb 6>:
  # quotient_3 = PHI <quotient_20(4), quotient_22(5)>
  # high_1 = PHI <high_19(4), high_21(5)>
  j_23 = j_32 + 1;

This code is converted by ifcvt pass to:

  quotient_20 = quotient_31 << 1;          [+]
  D.2068_5 = high_top_bit_11 == 0;
  D.2069_4 = d_7(D) <= high_19;
  _ifc_.29_2 = D.2068_5 && D.2069_4;
  D.2071_29 = high_top_bit_11 == 0;
  D.2072_33 = d_7(D) > high_19;
  _ifc_.30_34 = D.2071_29 && D.2072_33;
  high_21 = high_19 - d_7(D);
  quotient_22 = quotient_20 | 1;           [++]
  quotient_3 = high_top_bit_11 == 0 ? quotient_20 : quotient_22;   <<< here!
  high_1 = high_top_bit_11 == 0 ? high_19 : high_21;               <<< here!
  j_23 = j_32 + 1;

The condition for quotient_3 [and high_1], produced by ifcvt pass is wrong, and
should be:

  quotient_3 = _ifc_.3034 ? quotient_20 : quotient_22;

This is evident from the inner loop of the testcase:

--cut here--
      {
      word high_top_bit = (high & MP_WORD_TOP_BIT);

      high <<= 1;
      high |= (n0 >> (MP_WORD_BITS-1-j)) & 1;
      quotient <<= 1;                                [+]

      if(high_top_bit || high >= d)                  <<<< _the_condition_
         {
         high -= d;
         quotient |= 1;                              [++]
         }
      }
--cut here--

Due to slighlty different gimple generation for -march=core2 (please look into
_.004t.gimple) where only if branch is created, ifcvt is able to create correct
code:

  quotient_20 = quotient_34 << 1;                           [+]
  D.2065_21 = high_top_bit_11 != 0;
  D.2066_22 = high_19 >= d_7(D);
  D.2067_23 = D.2065_21 || D.2066_22;
  high_24 = high_19 - d_7(D);
  quotient_25 = quotient_20 | 1;                            [++]
  quotient_3 = D.2067_23 ? quotient_25 : quotient_20;       <<<<  here


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ubizjak at gmail dot com
           Priority|P3                          |P1
            Summary|Miscompiles valid code with |[4.1/4.2/4.3 Regression]
                   |-ftree-vectorize and -      |Miscompiles valid code with
                   |march=nocona                |-ftree-vectorize
   Target Milestone|---                         |4.1.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2/4.3 Regression] Miscompiles valid code with -ftree-vectorize

[ubizjak at gmail dot com]

------- Comment #6 from ubizjak at gmail dot com  2007-07-01 14:20 -------
In the tree-if-conv.c, there is a comment with the reference to PR23115, where
similar problems were addressed:

     4) If  pred B is dominated by pred A then use pred B's condition.
        See PR23115.  */

For attached testcase, we have following CFG:

                 bb3
                /   \
               /     \
              /       \
             V         V
            bb5 <---- bb4
              \       /
               \     /
                \   /
                 V V
                 bb6

with the conditions:

bb3:  C3 = (high_top_bit_11 != 0) 
bb4:  C4 = (d_7(D) <= high_19)

predicates for blocks are then:

bb4: !C3
bb5: C3 || !C3 & C4       [ = C3 || C4 ]

In bb6, we have:

  # quotient_3 = PHI <quotient_20(4), quotient_22(5)>

For some reason domiated_by_p() failed to detect that bb5 is dominated by bb4
and if-conversion simply assigns bb4 predicate (!C3) as the phi node condition.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2/4.3 Regression] Miscompiles valid code with -ftree-vectorize

[ubizjak at gmail dot com]

------- Comment #7 from ubizjak at gmail dot com  2007-07-01 15:46 -------
Well, the problem is, that we "forgot" to take into account false branch from
bb4 to bb6. In notation form the Comment #6, this branch has the condition !C4,
so the total condition for branch from bb4 leading to bb6 would be: !C3 && !C4.

The conditions for both branches leading to phi node are then:

left: (C3 || C4) && 1  ----->    C3 || C4
right: !C3 && !C4  --------->  !(C3 || C4)

With above correction, phi

  # quotient_3 = PHI <quotient_20(4), quotient_22(5)>

would be substituted with:

  quotient_3 = !(C3 || C4) ? quotient_20 : quotient_22;

  or:

  quotient_3 = (C3 || C4) ? quotient_22 : quotient_20;

which just happens to be the correct condition.

QED.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2/4.3 Regression] Miscompiles valid code with -ftree-vectorize

[ubizjak at gmail dot com]

------- Comment #8 from ubizjak at gmail dot com  2007-07-02 14:02 -------
Patch at http://gcc.gnu.org/ml/gcc-patches/2007-07/msg00104.html


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |http://gcc.gnu.org/ml/gcc-
                   |                            |patches/2007-
                   |                            |07/msg00104.html


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2/4.3 Regression] Miscompiles valid code with -ftree-vectorize

[uros at gcc dot gnu dot org]

------- Comment #9 from uros at gcc dot gnu dot org  2007-07-02 14:26 -------
Subject: Bug 31966

Author: uros
Date: Mon Jul  2 14:26:11 2007
New Revision: 126206

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=126206
Log:
        PR tree-optimization/31966
        PR tree-optimization/32533
        * tree-if-conv.c (add_to_dst_predicate_list): Use "edge", not
        "basic_block" description as its third argument.  Update function
        calls to get destination bb from "edge" argument.  Save "cond" into
        aux field of the edge.  Update prototype for changed arguments.
        (find_phi_replacement_condition): Operate on incoming edges, not
        on predecessor blocks.  If there is a condition saved in the
        incoming edge aux field, AND it with incoming bb predicate.
        Return source bb of the first edge.
        (clean_predicate_lists): Clean aux field of outgoing node edges.
        (tree_if_conversion): Do not initialize cond variable. Move
        variable declaration into the loop.
        (replace_phi_with_cond_gimple_modify_stmt): Remove unneded
        initializations of new_stmt, arg0 and arg1 variables.

testsuite/ChangeLog:

        PR tree-optimization/31966
        PR tree-optimization/32533
        * gcc.dg/tree-ssa/pr31966.c: New runtime test.
        * gfortran.dg/pr32533.f90: Ditto.


Added:
    trunk/gcc/testsuite/gcc.dg/tree-ssa/pr31966.c
    trunk/gcc/testsuite/gfortran.dg/pr32533.f90
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/tree-if-conv.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2 Regression] Miscompiles valid code with -ftree-vectorize

[ubizjak at gmail dot com]

-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|ubizjak at gmail dot com    |
         AssignedTo|unassigned at gcc dot gnu   |ubizjak at gmail dot com
                   |dot org                     |
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2007-07-01 08:52:43         |2007-07-02 14:28:40
               date|                            |
            Summary|[4.1/4.2/4.3 Regression]    |[4.1/4.2 Regression]
                   |Miscompiles valid code with |Miscompiles valid code with
                   |-ftree-vectorize            |-ftree-vectorize


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2 Regression] Miscompiles valid code with -ftree-vectorize

[uros at gcc dot gnu dot org]

------- Comment #10 from uros at gcc dot gnu dot org  2007-07-04 05:41 -------
Subject: Bug 31966

Author: uros
Date: Wed Jul  4 05:40:58 2007
New Revision: 126301

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=126301
Log:
        PR tree-optimization/31966
        PR tree-optimization/32533
        * tree-if-conv.c (add_to_dst_predicate_list): Use "edge", not
        "basic_block" description as its third argument.  Update function
        calls to get destination bb from "edge" argument.  Save "cond" into
        aux field of the edge.  Update prototype for changed arguments.
        (if_convertible_loop_p): Clear aux field of incoming edges if bb
        contains phi node.
        (find_phi_replacement_condition): Operate on incoming edges, not
        on predecessor blocks.  If there is a condition saved in the
        incoming edge aux field, AND it with incoming bb predicate.
        Return source bb of the first edge.
        (clean_predicate_lists): Clean aux field of outgoing node edges.
        (tree_if_conversion): Do not initialize cond variable. Move
        variable declaration into the loop.
        (replace_phi_with_cond_gimple_modify_stmt): Remove unneded
        initializations of new_stmt, arg0 and arg1 variables.

testsuite/ChangeLog:

        PR tree-optimization/31966
        PR tree-optimization/32533
        * gcc.dg/tree-ssa/pr31966.c: New runtime test.
        * gfortran.dg/pr32533.f90: Ditto.


Added:
    branches/gcc-4_2-branch/gcc/testsuite/gcc.dg/tree-ssa/pr31966.c
    branches/gcc-4_2-branch/gcc/testsuite/gfortran.dg/pr32533.f90
Modified:
    branches/gcc-4_2-branch/gcc/ChangeLog
    branches/gcc-4_2-branch/gcc/testsuite/ChangeLog
    branches/gcc-4_2-branch/gcc/tree-if-conv.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2 Regression] Miscompiles valid code with -ftree-vectorize

[uros at gcc dot gnu dot org]

------- Comment #11 from uros at gcc dot gnu dot org  2007-07-04 05:49 -------
Subject: Bug 31966

Author: uros
Date: Wed Jul  4 05:49:31 2007
New Revision: 126302

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=126302
Log:
        PR tree-optimization/31966
        PR tree-optimization/32533
        * tree-if-conv.c (add_to_dst_predicate_list): Use "edge", not
        "basic_block" description as its third argument.  Update function
        calls to get destination bb from "edge" argument.  Save "cond" into
        aux field of the edge.  Update prototype for changed arguments.
        (if_convertible_loop_p): Clear aux field of incoming edges if bb
        contains phi node.
        (find_phi_replacement_condition): Operate on incoming edges, not
        on predecessor blocks.  If there is a condition saved in the
        incoming edge aux field, AND it with incoming bb predicate.
        Return source bb of the first edge.
        (clean_predicate_lists): Clean aux field of outgoing node edges.
        (tree_if_conversion): Do not initialize cond variable. Move
        variable declaration into the loop.
        (replace_phi_with_cond_gimple_modify_stmt): Remove unneded
        initializations of new_stmt, arg0 and arg1 variables.

testsuite/ChangeLog:

        PR tree-optimization/31966
        PR tree-optimization/32533
        * gcc.dg/tree-ssa/pr31966.c: New runtime test.
        * gfortran.dg/pr32533.f90: Ditto.


Added:
    branches/gcc-4_1-branch/gcc/testsuite/gcc.dg/tree-ssa/pr31966.c
    branches/gcc-4_1-branch/gcc/testsuite/gfortran.dg/pr32533.f90
Modified:
    branches/gcc-4_1-branch/gcc/ChangeLog
    branches/gcc-4_1-branch/gcc/testsuite/ChangeLog
    branches/gcc-4_1-branch/gcc/tree-if-conv.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966

[Bug tree-optimization/31966] [4.1/4.2 Regression] Miscompiles valid code with -ftree-vectorize

[ubizjak at gmail dot com]

------- Comment #12 from ubizjak at gmail dot com  2007-07-04 05:54 -------
Fixed on mainline and branches.


-- 

ubizjak at gmail dot com changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31966