Mon, 23 Feb 2009 03:48:17 +0000 (UTC), BCS wrote:
> Hello bearophile,
>
>> If there are guarantees that "abc" "def" are folded at compile time,
>> then the same guarantees can be specified for "abc" ~ "def". I can't
>> see a problem.
>
> While it is not part of the spec, I do see a problem. If it were added....
>
>>
>> I have also compiled this code with DMD:
>>
>> void main() {
>> string foo = "foo";
>> string bar = foo ~ "bar" ~ "baz";
>> }
>> Resulting asm, no optimizations:
>>
>> L0: push EBP
>> mov EBP,ESP
>> mov EDX,FLAT:_DATA[0Ch]
>> mov EAX,FLAT:_DATA[08h]
>> push dword ptr FLAT:_DATA[01Ch]
>> push dword ptr FLAT:_DATA[018h]
>> push dword ptr FLAT:_DATA[02Ch]
>> push dword ptr FLAT:_DATA[028h]
>
> note 6 things
>
>> push EDX
>> push EAX
>> push 3
>> mov ECX,offset FLAT:_D11TypeInfo_Aa6__initZ
>> push ECX
>> call near ptr __d_arraycatnT
>> xor EAX,EAX
>> add ESP,020h
>> pop EBP
>> ret
>> Resulting asm, with optimizations:
>>
>> L0: sub ESP,0Ch
>> mov EAX,offset FLAT:_D11TypeInfo_Aa6__initZ
>> push dword ptr FLAT:_DATA[01Ch]
>> push dword ptr FLAT:_DATA[018h]
>> push dword ptr FLAT:_DATA[02Ch]
>> push dword ptr FLAT:_DATA[028h]
>> push dword ptr FLAT:_DATA[0Ch]
>> push dword ptr FLAT:_DATA[08h]
>
> again 6 things
>
>> push 3
>
> I think that is a varargs call
>
>> push EAX
>> call near ptr __d_arraycatnT
>> add ESP,020h
>> add ESP,0Ch
>> xor EAX,EAX
>> ret
>> I can see just one arraycatn, so the two string literals are folded at
>> compile time, I think.
>>
>> Bye,
>> bearophile
>
> I think that DMD does some optimization for a~b~c etc. so that there is only
> one call for any number of chained ~ (array cat n). In this case I think
> it is doing that.
Surely enough, if you look into the compiled .obj you won't find
"barbaz" there. All sub-strings are separete, regardless of the
optimization options.
