Passes in DMD compiler, but breaks in GDC during assert tests.

B4S1L3 suggested it might be an old compiler bug. I need confirmation if this is an old (not yet caught up) or a new one (to which I'll have to write or have a work-around until the bug is fixed).

// http://dpaste.com/0M7M8NK
uint[] sub(uint[] lhs, const (uint)[] rhs) {
    long t;    //temporary, doubles as carry

    foreach(i, ref v; lhs) {
        t += v;
        t -= rhs[i];
        v = cast(uint) t;
        //reset carry
        t >>= uint.sizeof*8;
    }

    return lhs;
}

unittest {
    uint[3] lhs = [99, 201, 300],
            rhs = [-1, 0, 0];

    assert(sub(lhs, rhs) == [100, 200, 300]);
}

On Tuesday, 20 June 2017 at 01:58:22 UTC, Era Scarecrow wrote:
>    long t;    //temporary, doubles as carry
>    <snip>
>         t >>= uint.sizeof*8;


 To note, looking in compiler explorer (GDC 5.2.0), the following output is present (for the above line):

        mov     rax, QWORD PTR [rbp-24]
        shr     rax, 32
        mov     QWORD PTR [rbp-24], rax

 'shr' is unsigned right shift, when it should be 'sar' signed right shift. That is my guess where the bug is.

On Tuesday, 20 June 2017 at 02:14:06 UTC, Era Scarecrow wrote:
>  To note, looking in compiler explorer (GDC 5.2.0), the following output is present (for the above line):

 Apparently on GDC 6.3 this doesn't fail. So likely an old compiler bug.

 Adding the following after the shift more or less confirms the unsigned shift is the error.

    t >>= uint.sizeof*8;
    version(GNU) {
        if (t)
            t |= 0xffffffff_00000000L;
    }

On Tuesday, 20 June 2017 at 03:31:24 UTC, Era Scarecrow wrote:
>  Apparently on GDC 6.3 this doesn't fail. So likely an old compiler bug.

 Got a message not long ago, apparently the assertion still happens on GDC 6.3. So...

On Tuesday, 20 June 2017 at 18:49:24 UTC, Era Scarecrow wrote:
> On Tuesday, 20 June 2017 at 03:31:24 UTC, Era Scarecrow wrote:
>>  Apparently on GDC 6.3 this doesn't fail. So likely an old compiler bug.
>
>  Got a message not long ago, apparently the assertion still happens on GDC 6.3. So...

OK. Bugzilla? I'll have a look.

(By the way, you can inspect disassembly here: https://explore.dgnu.org/)

Regards,
Iain.

On Tuesday, 20 June 2017 at 19:06:22 UTC, Iain Buclaw wrote:
> On Tuesday, 20 June 2017 at 18:49:24 UTC, Era Scarecrow wrote:
>> On Tuesday, 20 June 2017 at 03:31:24 UTC, Era Scarecrow wrote:
>>>  Apparently on GDC 6.3 this doesn't fail. So likely an old compiler bug.
>>
>>  Got a message not long ago, apparently the assertion still happens on GDC 6.3. So...
>
> OK. Bugzilla? I'll have a look.
>

OK, no need.  It's close enough to a prior bug (171).

https://github.com/D-Programming-GDC/GDC/pull/501

Regards,
Iain.