Is implicit string literal concatenation a good thing?

[Sergey Gromov]

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.