DIP45: fixing the dllimport/dllexport issue

[Martin Nowak]

On 08/29/2013 10:17 PM, Benjamin Thaut wrote:
>> How about LTO, when statically linking it should be possible to optimize
>> away the indirection.
>
> Rainer Schuetze stated that some linkers are capable of doing this
> optimizations. But I don't know aynthing further about this topic.
>
>>
>> module libA;
>> export int var;
>> int* _imp_var = &var; // created by compiler
>>
>> module foo;
>> import libA;
>>
>> void bar()
>> {
>>      auto val = var; // creates val = *_imp_var;
>> }
>>
>
> Yes that would work. Still there must be a reason why microsoft doesn't
> do stuff like that in their C++ toolchain. Its certanly going to cost
> performance.
I just tested it and it works.

lib.c

int var = 0xdeadbeaf;
int* _imp_var = &var;

main.c

#include <stdio.h>

extern int* _imp_var;
void main()
{
     printf("%d\n", *_imp_var);
}

cl /c /O2 /GL lib.c
cl /O2 /GL main.c lib.obj

get objconv from http://www.agner.org/optimize/

objconv -fasm main.exe

Search for deadbeaf in main.asm to get the symbol number (?_1176).
It directly loads the variable.

mov edx, dword ptr [?_1176]