Strict aliasing in D

[Peter Alexander]

On Sunday, 29 January 2012 at 14:05:25 UTC, Daniel Murphy wrote:
> "Denis Shelomovskij" <verylonglogin.reg at gmail.com> wrote in 
> message news:jg3f21$1jqa$1 at digitalmars.com...
>> It was originally posted to D.learn but there was no reply. So:
>>
>> Is there a strict aliasing rule in D?
>>
>> I just saw 
>> https://bitbucket.org/goshawk/gdc/changeset/b44331053062
>
> Struct aliasing is required when doing array operations.
>
> eg.
> int[] a, b, c;
> a[] = b[] + c[];
>
> The arrays used must not overlap.
> I'm pretty sure that's what that commit was about.

That's not strict aliasing, that's just a language rule about 
aliasing for vector ops.

Strict aliasing is the assumption that no two pointers of 
different types point to the same location (char is an 
exception). For example, with -fstrict-aliasing in gcc, the 
following could generate unexpected code:

int* i = new int;
float* f = (float*)i;
*i = 0;
*f = 1.0f;
printf("%d\n", *i); // could still write 0

Because of strict aliasing, the compiler may assume that the 
assignment *f = 1.0f can't affect the int, so it could just print 
out 0 instead of whatever 1.0f is as an int. To get around it, 
you would have to either use a union (which is still 
implementation defined, but works around aliasing), or make i 
volatile.

As for D, I can't see anything in the standard that prevents two 
pointers of different types from pointing to the same location, 
but I suspect it is an assumption that is being made.