[Issue 12751] New: Avoid heap allocations in some cases of array concatenation

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12751

          Issue ID: 12751
           Summary: Avoid heap allocations in some cases of array
                    concatenation
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: DMD
          Assignee: nobody at puremagic.com
          Reporter: bearophile_hugs at eml.cc

void main() {
    int[10] a;
    int[20] b;
    int[30] c = a ~ b;
}



Compiled with -O -release -inline -noboundscheck (dmd 2.066alpha) gives:


__Dmain comdat
L0:     sub ESP,0F8h
        mov ECX,0Ah
        xor EAX,EAX
        push    EBX
        push    ESI
        push    EDI
        lea EDI,0Ch[ESP]
        rep
        stosd
        mov ECX,014h
        lea EDI,03Ch[ESP]
        rep
        stosd
        mov EAX,0Ah
        mov EBX,offset FLAT:_D11TypeInfo_Ai6__initZ
        push    078h
        lea ECX,040h[ESP]
        push    ECX
        push    014h
        lea EDX,018h[ESP]
        push    EDX
        push    EAX
        push    EBX
        call    near ptr __d_arraycatT
        add ESP,014h
        push    EDX
        lea ESI,094h[ESP]
        push    ESI
        call    near ptr _memcpy
        add ESP,0Ch
        xor EAX,EAX
        pop EDI
        pop ESI
        pop EBX
        add ESP,0F8h
        ret


But the compiler should perform that operation without heap allocations. So
that should also be writeable:

void main() @nogc {
    int[10] a;
    int[20] b;
    int[30] c = a ~ b;
}

--