LDC, ARM (seems like x86 too) bug?

[Johan Engelen]

On Tuesday, 19 September 2017 at 09:52:12 UTC, Jack Applegame 
wrote:
> Code:
>
>> // values from linker script
>> extern(C) extern __gshared {
>> 	uint src_beg; // source begin 	uint trg_beg; // target begin
>> 	uint trg_end; // target end }
>> 
>> void copy(uint* ss, uint* ts, const uint* te) {
>>  	while(ts != te) *ts++ = *ss++;
>> }
>> 
>> void foo() {
>>  	copy(&src_beg, &trg_beg, &trg_end);
>> }

[snip]

> Apparently, LDC believes that addresses of `trg_beg` and 
> `trg_end` can not be initially equal and moves check to the 
> end. But this is not always the case.

This is a question of spec details. We are assuming C semantics 
for extern variables: just like normal variables, they cannot 
alias.
See also: 
https://stackoverflow.com/questions/5559281/c-externs-that-alias-the-same-address

LLVM optimizes these cases more aggressively than GCC: 
https://godbolt.org/g/yJdp9P

-Johan