[Bug rtl-optimization/22563] New: performance regression for gcc newer than 2.95

[Bug rtl-optimization/22563] New: performance regression for gcc newer than 2.95

[danalis at cis dot udel dot edu]

We ran bench++ to look for c++ samples that ran slower at -O3 with
gcc-[34].x than with gcc-2.95.  We're attaching one such case,
minimized as far as we can (so it might not be testing the same
thing as the original code).  It consists of a simple function that
accesses bitfields, called in a loop from main. 
gcc-3.4.3/gcc-4.0.0/gcc-4.1-20050627 all produce binaries that seem
to be ten times slower on this than those produced by gcc-2.95.3.
All the compilers happily inlined
the function, which is fine.

Here's the code from the older compiler:

.L12:
        movb $86,%dl
        movb %dl,b_rec
        movb %dl,%al
        andb $7,%al
        cmpb $6,%al
        je .L14
        call abort
        .align 4
.L14:
        andb $240,%dl
        cmpb $80,%dl
        je .L11
        call abort
        .align 4
.L11:
        decl %ecx
        testl %ecx,%ecx
        jg .L12

And here's code from gcc-4.1-20050625:
        jmp     .L16
        .p2align 4,,7
.L27:
        andb    $-16, %dl
        cmpb    $80, %dl
        jne     .L25
        decl    %ebx
        je      .L26
.L16:
        movl    %ecx, %eax
        andl    $-8, %eax
        orl     $6, %eax
        movl    %eax, b_rec
        andb    $-9, b_rec
        movl    b_rec, %eax
        andl    $-241, %eax
        orl     $80, %eax
        movl    %eax, b_rec
        movl    %eax, %ecx
        movzbl  b_rec, %edx
        movb    %dl, %al
        andb    $7, %al
        cmpb    $6, %al
        je      .L27

We'll attach the preprocessed source.

-- 
           Summary: performance regression for gcc newer than 2.95
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: danalis at cis dot udel dot edu
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i686-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] performance regression for gcc newer than 2.95

[danalis at cis dot udel dot edu]

------- Additional Comments From danalis at cis dot udel dot edu  2005-07-19 19:20 -------
Created an attachment (id=9307)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=9307&action=view)
reduced test

Is it bitchy to complain about few nanoseconds slowdown (per iteration) :)

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] [3.4/4.0/4.1 Regression] performance regression for gcc newer than 2.95

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-07-19 19:53 -------
There are a couple problems here, first we don't move the store to b_rec out side of the loop.  Doing 
that on the mainline, we remove the loop as it is now unswitchable and really just empty.

In fact that will not really be what you wantted but hey fast empty loops :).

The other issue is that we don't constant prop the constants as we have a BIT_FIELD_REF which is most 
likely the cause of the orginal regression in the first place though BIT_FIELD_REF was there in 2.95.3.

We can reduce your testcase down to stores really but that might not help the orginal code (except for 
the fact this is just a benchmark which is really useless).

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |missed-optimization
      Known to fail|                            |3.3.3 3.0.4 3.2.3 3.4.0
      Known to work|                            |2.95.3
   Last reconfirmed|0000-00-00 00:00:00         |2005-07-19 19:53:16
               date|                            |
            Summary|performance regression for  |[3.4/4.0/4.1 Regression]
                   |gcc newer than 2.95         |performance regression for
                   |                            |gcc newer than 2.95
   Target Milestone|---                         |3.4.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] [3.4/4.0/4.1 Regression] performance regression for gcc newer than 2.95

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-07-22 21:12 -------
Moving to 4.0.2 pre Mark.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|3.4.5                       |4.0.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] [3.4/4.0/4.1 Regression] performance regression for gcc newer than 2.95

[danalis at cis dot udel dot edu]

------- Additional Comments From danalis at cis dot udel dot edu  2005-08-04 19:16 -------
For the record the reduced test case was derived from h000007.cpp of bench++

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] [3.4/4.0/4.1 Regression] performance regression for gcc newer than 2.95

[dann at godzilla dot ics dot uci dot edu]

------- Additional Comments From dann at godzilla dot ics dot uci dot edu  2005-08-25 02:49 -------
This message:
http://gcc.gnu.org/ml/gcc/2005-08/msg00208.html

was asking for the reason for the slowdown for S000005e

AFAICT the inner loop for the benchmark (in s000005e_test) gets compiled to: 

.L153:
        fstl    (%edx)
        leal    8(%edx), %eax
        fstl    (%eax)
        fstl    8(%eax)
        fstl    16(%eax)
        fstl    24(%eax)
        fstl    32(%eax)
        fstl    40(%eax)
        fstl    48(%eax)
        leal    56(%eax), %edx
        cmpl    %edx, %ecx
        jne     .L153

and to:

.L9:
        movl    $0, (%edx)
        movl    $1074266112, 4(%edx)
        movl    $0, 8(%edx)
        movl    $1074266112, 12(%edx)
        movl    $0, 16(%edx)
        movl    $1074266112, 20(%edx)
        movl    $0, 24(%edx)
        movl    $1074266112, 28(%edx)
        movl    $0, 32(%edx)
        movl    $1074266112, 36(%edx)
        movl    $0, 40(%edx)
        movl    $1074266112, 44(%edx)
        movl    $0, 48(%edx)
        movl    $1074266112, 52(%edx)
        movl    $0, 56(%edx)
        movl    $1074266112, 60(%edx)
        addl    $64, %edx
        cmpl    %edx, %ebx
        jne     .L9

by 4.1

The 4.1 code looks much worse...

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] [3.4/4.0/4.1 Regression] performance regression for gcc newer than 2.95

[mmitchel at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.0.2                       |4.0.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] [3.4/4.0/4.1 Regression] performance regression for gcc newer than 2.95

[hubicka at gcc dot gnu dot org]

------- Comment #8 from hubicka at gcc dot gnu dot org  2005-11-03 22:54 -------
Actually the code 4.1 in comment #5 should execute faster on true i686. It is
longer and will trigger partial memory stalls on later chips.

Honza


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] [3.4/4.0/4.1 Regression] performance regression for gcc newer than 2.95

[mmitchel at gcc dot gnu dot org]

------- Comment #7 from mmitchel at gcc dot gnu dot org  2005-10-31 04:12 -------
Leaving as P2.

I've seen reports of similar bitfield problems on a variety of problems.  This
kind of code doesn't show up much in scientific computing, but it does show up
in network applications, operating-system kernels, etc.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563

[Bug rtl-optimization/22563] [3.4/4.0/4.1 Regression] performance regression for gcc newer than 2.95

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2005-10-27 00:20 -------
Hmm, this is truely all bit-field issues.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |19466
              nThis|                            |
           Severity|normal                      |minor


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22563