• d-bugmail puremagic.com (141/141) Apr 21 2012 http://d.puremagic.com/issues/show_bug.cgi?id=7963

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=7963

           Summary: dmd doesn't inline std.algorithm.swap on a 'alias
                    this' struct
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            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 2012-04-21 16:49:12 PDT ---
I have seen that dmd doesn't inline swap() when it's used on a simple struct
that contains a fixed size array with alias this:

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This is a version that uses std.algorithm.swap, inlining of swap doesn't
happen:


import std.algorithm: swap;
struct Foo {
    int[2] xy;
    alias xy this;
}
int main() {
    auto f1 = Foo([1, 2]);
    auto f2 = Foo([1, 2]);
    swap(f1, f2);
    return f1[0];
}


__Dmain    comdat
L0:        sub    ESP,014h
        mov    ECX,1
        mov    EDX,offset FLAT:_D12TypeInfo_G2i6__initZ
        push    EBX
        push    ESI
        push    ECX
        sub    ESP,4
        push    2
        push    EDX
        call    near ptr __d_arrayliteralTX
        add    ESP,0Ch
        mov    EBX,EAX
        pop    ECX
        mov    [EBX],ECX
        mov    ECX,2
        mov    EAX,[EBX]
        mov    4[EBX],ECX
        mov    EDX,4[EBX]
        mov    0Ch[ESP],EAX
        mov    010h[ESP],EDX
        mov    EDX,offset FLAT:_D12TypeInfo_G2i6__initZ
        push    ECX
        push    EDX
        call    near ptr __d_arrayliteralTX
        mov    ESI,EAX
        lea    ECX,014h[ESP]
        mov    dword ptr [EAX],1
        mov    EAX,[ESI]
        mov    dword ptr 4[ESI],2
        mov    EDX,4[ESI]
        mov    01Ch[ESP],EAX
        mov    020h[ESP],EDX
        push    ECX
        lea    EAX,020h[ESP]
        call    near ptr
_D3std9algorithm21__T4swapTS5test33FooZ4swapFNaNbNeKS5test33FooKS5test33FooZv
        add    ESP,8
        mov    EAX,0Ch[ESP]
        pop    ESI
        pop    EBX
        add    ESP,014h
        ret

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This is a version that uses a shorter (and less correct) swapping function,
here inlining happens:


void swap2(T)(ref T lhs, ref T rhs) pure nothrow {
    auto tmp = lhs;
    lhs = rhs;
    rhs = tmp;
}
struct Foo {
    int[2] xy;
    alias xy this;
}
int main() {
    auto f1 = Foo([1, 2]);
    auto f2 = Foo([1, 2]);
    swap2(f1, f2);
    return f1[0];
}


__Dmain    comdat
L0:        push    EAX
        mov    ECX,offset FLAT:_D12TypeInfo_G2i6__initZ
        push    EAX
        mov    EAX,1
        push    EBX
        push    ESI
        push    EDI
        push    EAX
        sub    ESP,4
        push    2
        push    ECX
        call    near ptr __d_arrayliteralTX
        add    ESP,0Ch
        mov    EDX,EAX
        pop    EAX
        mov    ESI,2
        mov    EDI,offset FLAT:_D12TypeInfo_G2i6__initZ
        push    ESI
        mov    [EDX],EAX
        mov    4[EDX],ESI
        push    EDI
        call    near ptr __d_arrayliteralTX
        mov    EBX,EAX
        mov    dword ptr [EAX],1
        mov    EAX,[EBX]
        mov    4[EBX],ESI
        mov    EDX,4[EBX]
        mov    014h[ESP],EAX
        mov    018h[ESP],EDX
        add    ESP,8
        mov    EAX,0Ch[ESP]
        pop    EDI
        pop    ESI
        pop    EBX
        add    ESP,8
        ret

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swap() is a very basic function, it's used all the time in sorts. I have found
this performance problem because the missed inlining slows down the sort
routine I was using.

Test done with dmd 2.060alpha, with -O -release -inline.


Note: this problem doesn't happen if alias this is not used:

struct Foo2 {
    int[2] xy;
}

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