Hello,

the code below behaves differently when compiled with or without the -O flag (both DMD and LDC2).

Two questions:

(1) does the D language explicitly specifies what the following expression should do? If yes, where?

 '<ulong> |= <ubyte> << <ubyte>'

In the example below, there seems to be a cast to 32 bits first, then a shift, then a cast to 64 bits.

(2) why the different result when putting the -O flag?

Best regards,
Guillaume Lathoud
.

import std.stdio;

void main()
{
  ubyte b = 84;
  ulong l = 0;
  ubyte shift = 50;

  l |= b << shift;

  writefln( "%064b", l );

  /*
    DMD64 D Compiler v2.080.1

    dmd cast_question.d ; cast_question
    0000000000000000000000000000000000000001010100000000000000000000

    dmd -O cast_question.d ; cast_question
    0000000000000000000000000000000000000000000000000000000000000000
  */

  /*
    LDC - the LLVM D compiler (1.10.0):
    based on DMD v2.080.1 and LLVM 6.0.0

    ldmd2 cast_question.d ; cast_question
    0000000000000000000000000000000000000001010100000000000000000000

    ldmd2 -O cast_question.d ; cast_question
    0000000000000000000000000000000000000000000000000000000000000000
   */
}

Guillaume Lathoud wrote:

this is UB. by the specs, values are promoted to int, and shifting int by 50 is UB. so both results are nonsense.

On Thursday, 20 September 2018 at 11:08:32 UTC, ketmar wrote:
> Guillaume Lathoud wrote:
>
> this is UB. by the specs, values are promoted to int, and shifting int by 50 is UB. so both results are nonsense.

Thanks!

On Thursday, 20 September 2018 at 11:14:05 UTC, Guillaume Lathoud wrote:
> Thanks!

FYI, it's undefined in D mainly because the behavior of the actual Intel CPU instruction is undefined in such cases:

https://c9x.me/x86/html/file_module_x86_id_285.html

"it is undefined for SHL and SHR instructions where the count is greater than or equal to the size (in bits) of the destination operand".

On Friday, 21 September 2018 at 01:44:33 UTC, Vladimir Panteleev wrote:
> On Thursday, 20 September 2018 at 11:14:05 UTC, Guillaume FYI, it's undefined in D mainly because the behavior of the actual Intel CPU instruction is undefined in such cases:
>
> https://c9x.me/x86/html/file_module_x86_id_285.html
>
> "it is undefined for SHL and SHR instructions where the count is greater than or equal to the size (in bits) of the destination operand".

Thanks, good to know.