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digitalmars.D.bugs - [Issue 4488] New: Faster fixed-size array initialization from literal

reply d-bugmail puremagic.com writes:
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 puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs 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 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Jul 19 2010
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4488


nfxjfg gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nfxjfg gmail.com


--- Comment #1 from nfxjfg gmail.com 2010-07-19 19:52:27 PDT ---
The problem is that array and struct initializers never worked with non-static
data (which is utterly retarded; even C can initialize any data with these;
what's even more retarded is that this was "fixed" by simply removing struct
initializers in D2 and replaced them by constructors, which are redundant to
opCall anyway... but I'm digressing.)

This means the above snippet is really:

double[2] arr;
arr = [x, y];

This makes it more obvious why it's allocating.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4488


Don <clugdbug yahoo.com.au> changed:

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


--- Comment #2 from Don <clugdbug yahoo.com.au> 2010-07-19 21:47:34 PDT ---
This looks like a duplicate of bug 2356.

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prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4488


bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |DUPLICATE


--- Comment #3 from bearophile_hugs eml.cc 2010-07-19 23:18:32 PDT ---
*** This issue has been marked as a duplicate of issue 2356 ***

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