[Issue 4488] New: Faster fixed-size array initialization from literal

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=4488

           Summary: Faster fixed-size array initialization from literal
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: performance
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2010-07-19 14:43:10 PDT ---
This D2 program initializes a small fixed-size array allocated on the stack:


import std.c.stdio: printf;
import std.c.stdlib: atof;
void main() {
    double x = atof("1.0");
    double y = atof("2.0");
    double[2] arr = [x, y];
    printf("%f\n", arr[1]);
}


The asm generated by dmd in an optimized build shows two calls to initialize
the array (one to build it on the heap and one to copy from heap to stack).
Even LDC leaves the first call.

Inizialization of small stack-allocated arrays like this are often used in
high-performance code, such two calls can reduce performance if they are inside
a function called in an inner loop.

So can such two calls be removed in this simple situation? The compiler can
recognize that there is no need for heap allocations in this case.

------------

DMD v.2.047 asm, optimized build:

__Dmain comdat
        sub ESP,04Ch
        mov EAX,offset FLAT:_DATA
        push    EBX
        push    ESI
        push    EAX
        call    near ptr _atof
        mov ECX,offset FLAT:_DATA[4]
        fstp    qword ptr 010h[ESP]
        push    010h
        push    ECX
        call    near ptr _atof
        add ESP,0FFFFFFFCh
        mov EDX,offset FLAT:_D12TypeInfo_xAd6__initZ
        fstp    qword ptr [ESP]
        push    dword ptr 020h[ESP]
        push    dword ptr 020h[ESP]
        push    2
        push    EDX
>       call    near ptr __d_arrayliteralT
        add ESP,018h
        push    EAX
        lea EBX,028h[ESP]
        push    EBX
>       call    near ptr _memcpy
        mov ESI,offset FLAT:_DATA[8]
        push    dword ptr 038h[ESP]
        push    dword ptr 038h[ESP]
        push    ESI
        call    near ptr _printf
        add ESP,01Ch
        xor EAX,EAX
        pop ESI
        pop EBX
        add ESP,04Ch
        ret

------------

LDC asm, optimized build:

_Dmain:
    subl    $52, %esp
    movl    $.str, (%esp)
    call    atof
    fstpt   28(%esp)
    movl    $.str1, (%esp)
    call    atof
    fstpt   16(%esp)
    movl    $2, 4(%esp)
    movl    $_D11TypeInfo_Ad6__initZ, (%esp)
>   call    _d_newarrayvT
    fldt    28(%esp)
    fstpl   (%eax)
    fldt    16(%esp)
    fstpl   40(%esp)
    movsd   40(%esp), %xmm0
    movsd   %xmm0, 8(%eax)
    movsd   %xmm0, 4(%esp)
    movl    $.str2, (%esp)
    call    printf
    xorl    %eax, %eax
    addl    $52, %esp
    ret $8

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