import std.stdio : writeln;

void main()
{
    int a = 7;
    int b = 9;

    /* some code later */

    *(&a + 1) = 2;
    writeln(b);
    writeln(b);
}

This code prints:

9
2

with latest version of dmd. Is this a bug or am I missing something? For the record,
gdc prints (as expected):

2
2

ldc will print
9
9
and sigsegv :D

On Thu, Jun 13, 2019 at 1:30 PM RazvanN via Digitalmars-d < digitalmars-d@puremagic.com> wrote:

> import std.stdio : writeln;
>
> void main()
> {
>      int a = 7;
>      int b = 9;
>
>      /* some code later */
>
>      *(&a + 1) = 2;
>      writeln(b);
>      writeln(b);
> }
>
> This code prints:
>
> 9
> 2
>
> with latest version of dmd. Is this a bug or am I missing
> something? For the record,
> gdc prints (as expected):
>
> 2
> 2
>
>

On Thursday, 13 June 2019 at 11:28:25 UTC, RazvanN wrote:
> Is this a bug or am I missing something? For the record,
> gdc prints (as expected):

What happens is that b lives in a register.
After the first call, the register is invalidated and it has to be reloaded with the value on the stack (modified by your pointer).

I'm pretty sure *(&a + 1) = 2; is undefined behavior, so both dmd and gdc are correct.

(Assembly can be seen here: https://run.dlang.io/is/bjxVXg)

On Thursday, 13 June 2019 at 11:28:25 UTC, RazvanN wrote:
> import std.stdio : writeln;
>
> void main()
> {
>     int a = 7;
>     int b = 9;
>
>     /* some code later */
>
>     *(&a + 1) = 2;
>     writeln(b);
>     writeln(b);
> }
>
> This code prints:
>
> 9
> 2
>
> with latest version of dmd. Is this a bug or am I missing something? For the record,
> gdc prints (as expected):
>
> 2
> 2

Check out gotbolt output:

https://godbolt.org/z/sxo_8l

On Thursday, 13 June 2019 at 11:28:25 UTC, RazvanN wrote:
> import std.stdio : writeln;
>
> void main()
> {
>     int a = 7;
>     int b = 9;
>
>     /* some code later */
>
>     *(&a + 1) = 2;
>     writeln(b);
>     writeln(b);
> }
>
> This code prints:
>
> 9
> 2
>
> with latest version of dmd. Is this a bug or am I missing something? For the record,
> gdc prints (as expected):
>
> 2
> 2

It's definitely not a codegen bug. It's a programmers assumption bug :-)

On Thursday, 13 June 2019 at 11:28:25 UTC, RazvanN wrote:
> import std.stdio : writeln;
>
> void main()
> {
>     int a = 7;
>     int b = 9;
>
>     /* some code later */
>
>     *(&a + 1) = 2;
>     writeln(b);
>     writeln(b);
> }
>
> This code prints:
>
> 9
> 2
>
> with latest version of dmd. Is this a bug or am I missing something? For the record,
> gdc prints (as expected):
>
> 2
> 2

Why would you think this should work? Locals are not the same as struct members, you can't just walk a pointer around.

On Thursday, 13 June 2019 at 13:45:04 UTC, John Colvin wrote:
> On Thursday, 13 June 2019 at 11:28:25 UTC, RazvanN wrote:
>> import std.stdio : writeln;
>>
>> void main()
>> {
>>     int a = 7;
>>     int b = 9;
>>
>>     /* some code later */
>>
>>     *(&a + 1) = 2;
>>     writeln(b);
>>     writeln(b);
>> }
>>
>> This code prints:
>>
>> 9
>> 2
>>
>> with latest version of dmd. Is this a bug or am I missing something? For the record,
>> gdc prints (as expected):
>>
>> 2
>> 2
>
> Why would you think this should work? Locals are not the same as struct members, you can't just walk a pointer around.

I think it's odd that b changes its value for no apparent reason. I understand why it happens, still it is surprising.

On Thursday, 13 June 2019 at 15:42:38 UTC, RazvanN wrote:
> On Thursday, 13 June 2019 at 13:45:04 UTC, John Colvin wrote:
>> On Thursday, 13 June 2019 at 11:28:25 UTC, RazvanN wrote:
>>> import std.stdio : writeln;
>>>
>>> void main()
>>> {
>>>     int a = 7;
>>>     int b = 9;
>>>
>>>     /* some code later */
>>>
>>>     *(&a + 1) = 2;
>>>     writeln(b);
>>>     writeln(b);
>>> }
>>>
>>> This code prints:
>>>
>>> 9
>>> 2
>>>
>>> with latest version of dmd. Is this a bug or am I missing something? For the record,
>>> gdc prints (as expected):
>>>
>>> 2
>>> 2
>>
>> Why would you think this should work? Locals are not the same as struct members, you can't just walk a pointer around.
>
> I think it's odd that b changes its value for no apparent reason. I understand why it happens, still it is surprising.

Undefined behaviour is often surprising.

On 6/13/2019 4:28 AM, RazvanN wrote:
> import std.stdio : writeln;
> 
> void main()
> {
>      int a = 7;
>      int b = 9;
> 
>      /* some code later */
> 
>      *(&a + 1) = 2;
>      writeln(b);
>      writeln(b);
> }
> 
> This code prints:
> 
> 9
> 2
> 
> with latest version of dmd. Is this a bug or am I missing something? For the record,
> gdc prints (as expected):
> 
> 2
> 2
> 

It's undefined behavior to access memory beyond the boundaries of the memory object pointed to:

https://github.com/dlang/dlang.org/pull/2649/files#diff-2bcc1b9af1e2da4f63bfda4b26f114a2R90

in an update to the spec that nobody sees fit to pull :-(