[Bug other/28957] New: unoptimal virtual method call.

[Bug other/28957] New: unoptimal virtual method call.

[pluto at agmk dot net]

struct A
{
        typedef void ( A::* pmf )();
        virtual ~A();
        virtual void foo() = 0;
};

void unoptimized_loop( A* a, A::pmf f )
{
        while ( 1 )
                ( a->*f )();
}

void optimized_loop( A* a, A::pmf f )
{
        typedef void (* pf)( A* );
        pf __f = (pf)( a->*f );
        while ( 1 )
                __f( a );
}

both loops do the same thing with different speed impact.

$ g++ pmf_opt.cpp -Wall -c -O2 -Wno-pmf-conversions --save-temps -fverbose-asm

_Z16unoptimized_loopP1AMS_FvvE:
        pushq   %r12    #
        movq    %rsi, %r12      # f, tmp68
        andl    $1, %r12d       #, tmp68
        pushq   %rbp    #
        leaq    (%rdi,%rdx), %rbp       #, tmp70
        pushq   %rbx    #
        movq    %rsi, %rbx      # f, f
        subq    $16, %rsp       #,
        movq    %rsi, (%rsp)    # f, f
        movq    %rdx, 8(%rsp)   # f, f
.L3:    testq   %r12, %r12      # tmp68
        movq    %rbx, %rax      # f, f$__pfn
        je      .L6     #,
        movq    (%rbp), %rax    #, tmp69
        movq    -1(%rax,%rbx), %rax     #, f$__pfn
.L6:    movq    %rbp, %rdi      # tmp70, prephitmp.36
        call    *%rax   # f$__pfn
        jmp     .L3

_Z14optimized_loopP1AMS_FvvE:
        pushq   %rbp    #
        movq    %rdi, %rbp      # a, a
        pushq   %rbx    #
        movq    %rsi, %rbx      # f, f$__pfn
        subq    $24, %rsp       #,
        testb   $1, %sil        #, f$__pfn
        movq    %rsi, 8(%rsp)   # f, f
        movq    %rdx, 16(%rsp)  # f, f
        je      .L14    #,
        movq    (%rdi,%rdx), %rax       #* f, tmp68
        movq    -1(%rax,%rsi), %rbx     #, f$__pfn
.L14:   movq    %rbp, %rdi      # a, a
        call    *%rbx   # f$__pfn
        jmp     .L14


-- 
           Summary: unoptimal virtual method call.
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: other
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pluto at agmk dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28957

[Bug other/28957] unoptimal virtual method call.

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-09-05 21:08 -------
-funswitch-loops fixes the loops.
For the first function, we get:
.L4:
        movl    %ebx, (%esp)
        call    *%esi
        jmp     .L4

and:
.L3:
        movl    (%ebx), %eax
        movl    %ebx, (%esp)
        call    *-1(%eax,%edi)
        .p2align 4,,4
        jmp     .L3


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28957

[Bug other/28957] unoptimal virtual method call.

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-09-05 21:11 -------
So this is either fixed with -funswitch-loops and/or -O3 which enables
-funswitch-loops.

And this has been fixed since 3.4.0 which added -funswitch-loops.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID
   Target Milestone|---                         |3.4.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28957

[Bug other/28957] unoptimal virtual method call.

[pluto at agmk dot net]

------- Comment #3 from pluto at agmk dot net  2006-09-05 21:35 -------
(In reply to comment #1)
> -funswitch-loops fixes the loops.

i don't think it is fixed. imho this is only a partial fix.

> For the first function, we get:
> .L4:
>         movl    %ebx, (%esp)
>         call    *%esi
>         jmp     .L4
> 
> and:
> .L3:
>         movl    (%ebx), %eax    <=== [1]
>         movl    %ebx, (%esp)
>         call    *-1(%eax,%edi)  <=== [1]
>         .p2align 4,,4
>         jmp     .L3

[1] is still less effective than optimized_loop.

        movl    12(%ebp), %eax  # f, f
        movl    8(%ebp), %esi   # a, a
        movl    16(%ebp), %edx  # f, f
        testb   $1, %al #, f$__pfn
        movl    %eax, %ebx      # f, f$__pfn
        je      .L13
        movl    (%esi,%edx), %eax       #* f$__delta, tmp65
        movl    -1(%eax,%ebx), %ebx     #, f$__pfn
.L13:
        movl    %esi, (%esp)    # a,
        call    *%ebx   # f$__pfn
        jmp     .L13


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28957

[Bug other/28957] unoptimal virtual method call.

[pluto at agmk dot net]

------- Comment #4 from pluto at agmk dot net  2006-09-12 12:58 -------
i dont't think it's invalid report.
-funswitch-loop produces only slightly better results.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28957