• RazvanN (18/18) Jun 13 2019 import std.stdio : writeln;
• Daniel Kozak (6/24) Jun 13 2019 ldc will print
• Dennis (8/10) Jun 13 2019 What happens is that b lives in a register.
• Radu (3/21) Jun 13 2019 Check out gotbolt output:
• Patrick Schluter (3/21) Jun 13 2019 It's definitely not a codegen bug. It's a programmers assumption
• John Colvin (3/21) Jun 13 2019 Why would you think this should work? Locals are not the same as
• RazvanN (3/31) Jun 13 2019 I think it's odd that b changes its value for no apparent reason.
• John Colvin (2/34) Jun 13 2019 Undefined behaviour is often surprising.
• Walter Bright (5/31) Jun 14 2019 It's undefined behavior to access memory beyond the boundaries of the me...

RazvanN <razvan.nitu1305 gmail.com> writes:

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

Daniel Kozak <kozzi11 gmail.com> writes:

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

Dennis <dkorpel gmail.com> writes:

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)

Radu <void null.pt> writes:

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

Patrick Schluter <Patrick.Schluter bbox.fr> writes:

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 :-)

John Colvin <john.loughran.colvin gmail.com> writes:

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.

RazvanN <razvan.nitu1305 gmail.com> writes:

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.

John Colvin <john.loughran.colvin gmail.com> writes:

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.

Walter Bright <newshound2 digitalmars.com> writes:

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 :-(