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

pluto at agmk dot net gcc-bugzilla@gcc.gnu.org
Tue Sep 5 20:54:00 GMT 2006


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



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