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digitalmars.D.bugs - [Issue 5100] New: -O Degrades performance of loop statements

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5100

           Summary: -O Degrades performance of loop statements
           Product: D
           Version: D1 & D2
          Platform: All
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: ibuclaw ubuntu.com



Two example cases:

loop1.d
---------
void main()
{
    for (int i = 0; i < int.max; i++)
    {
    }
}


loop2.d
---------
void main()
{
    int i = 0;
    while(i < int.max)
    {
        i++;
    }
}


$ dmd loop1.d
$ time ./loop1 
real    0m2.914s
user    0m2.884s
sys    0m0.012s

$ ./dmd loop1.d -O
$ time ./loop1 
real    0m5.695s
user    0m5.684s
sys    0m0.004s

$ ./dmd loop2.d 
$ time ./loop2
real    0m2.912s
user    0m2.892s
sys    0m0.004s

$ ./dmd loop2.d -O
$ time ./loop2
real    0m5.703s
user    0m5.688s
sys    0m0.004s


The speed of execution slows by almost double when optimisations are turned on.
Something isn't right here...

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5100


Stephan Dilly <spam extrawurst.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |spam extrawurst.org



---


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---






related.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au



Cannot reproduce. On Windows, for both test cases, without -O it's about 5
seconds (does an INC and CMP of a stack variable). With -O it is about 1 second
(just does INC and CMP of EAX).

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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objdump without -O on Linux:

push   %ebp
mov    %esp,%ebp
sub    $0x4,%esp
movl   $0x0,-0x4(%ebp)
cmpl   $0x7fffffff,-0x4(%ebp)
jge    1c <_Dmain+0x1c>
addl   $0x1,-0x4(%ebp)
jmp    d <_Dmain+0xd>
xor    %eax,%eax
leave  
ret    


objdump with -O on Linux

push   %ebp
mov    %esp,%ebp
xor    %eax,%eax
add    $0x1,%eax
cmp    $0x7fffffff,%eax
jb     5 <_Dmain+0x5>
pop    %ebp
xor    %eax,%eax
ret    


Looks to be same as what Don said that was on his Windows box.


Wonder why Linux is slower... (must be a quirk, that or my Intel Atom CPU is to
blame).

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Been playing about with GCC, this seems to be a better performant:

Objdump:

push   %ebp
mov    %esp,%ebp
and    $0xfffffff0,%esp
push   %eax
sub    $0xc,%esp
lea    0x0(%esi),%esi
add    $0x1,%eax
cmp    $0x7fffffff,%eax
jne    30 <_Dmain+0x10>
add    $0xc,%esp
mov    %ebp,%esp
pop    %ebp
ret    



GCC assembly:

.globl _Dmain
        .type   _Dmain,  function
_Dmain:
.LFB0:
        .cfi_startproc
        .cfi_personality 0x0,__gdc_personality_v0
        pushl   %ebp
        .cfi_def_cfa_offset 8
        movl    %esp, %ebp
        .cfi_offset 5, -8
        .cfi_def_cfa_register 5
        andl    $-16, %esp
        pushl   %eax
        .cfi_escape 0x10,0x3,0x7,0x55,0x9,0xf0,0x1a,0x9,0xfc,0x22
        subl    $12, %esp
        .p2align 4,,7
        .p2align 3
.L4:
        addl    $1, %eax
        cmpl    $2147483647, %eax
        jne     .L4
        addl    $12, %esp
        movl    %ebp, %esp
        popl    %ebp
        ret
        .cfi_endproc
.LFE0:
        .size   _Dmain, .-_Dmain


Can attach the full .s file if needed.

Regards

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Dec 07 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla digitalmars.com



14:08:23 PST ---
Perhaps it's because gcc is doing:

    ADD EAX,1

instead of:

    INC EAX

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Maybe not...

I actually get the reverse on my new laptop with 2.057,

$ dmd loop2.d
$ objdump loop2.o -d
push   %ebp
mov    %esp,%ebp
sub    $0x4,%esp
movl   $0x0,-0x4(%ebp)
cmpl   $0x7fffffff,-0x4(%ebp)
jge    1b <_Dmain+0x1b>
incl   -0x4(%ebp)
jmp    d <_Dmain+0xd>
xor    %eax,%eax
leave  
ret    

$ time ./loop2 
real    0m11.780s
user    0m11.769s
sys    0m0.004s

$ dmd loop2.d -O
$ objdump loop2.o -d
push   %ebp
mov    %esp,%ebp
xor    %eax,%eax
inc    %eax
cmp    $0x7fffffff,%eax
jb     5 <_Dmain+0x5>
pop    %ebp
xor    %eax,%eax
ret    

$ time ./loop2 
real    0m3.936s
user    0m3.924s
sys    0m0.008s

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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And on my netbook:

$ dmd loop2.d
$ time ./loop2
real    0m2.948s
user    0m2.924s
sys    0m0.012s

$ dmd loop2.d -O
$ time ./loop2
real    0m5.725s
user    0m5.688s
sys    0m0.012s


Specs of Netbook:
processor    : 0
vendor_id    : GenuineIntel
cpu family    : 6
model        : 28
model name    : Intel(R) Atom(TM) CPU N270     1.60GHz
stepping    : 2
cpu MHz        : 800.000
cache size    : 512 KB
cpu cores    : 1


Specs of Laptop:
processor    : 0
vendor_id    : AuthenticAMD
cpu family    : 20
model        : 2
model name    : AMD E-450 APU with Radeon(tm) HD Graphics
stepping    : 0
cpu MHz        : 825.000
cache size    : 512 KB
cpu cores    : 2


Regards

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prev sibling parent d-bugmail puremagic.com writes:
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My gut feeling is that the main source of it slowing down is the needless push
and pop of the frame pointer.

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