https://issues.dlang.org/show_bug.cgi?id=16098

Ketmar Dark <ketmar@ketmar.no-ip.org> changed:

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                 CC|                            |ketmar@ketmar.no-ip.org

--- Comment #1 from Ketmar Dark <ketmar@ketmar.no-ip.org> ---
afair, *any* `align` is simply ignored for stack vars in DMD.

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https://issues.dlang.org/show_bug.cgi?id=16098

Walter Bright <bugzilla@digitalmars.com> changed:

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                 CC|                            |bugzilla@digitalmars.com

--- Comment #2 from Walter Bright <bugzilla@digitalmars.com> ---
(In reply to Ketmar Dark from comment #1)
> afair, *any* `align` is simply ignored for stack vars in DMD.

No, this is not true, if the align <= the stack alignment.

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https://issues.dlang.org/show_bug.cgi?id=16098

--- Comment #3 from Vladimir Panteleev <dlang-bugzilla@thecybershadow.net> ---
Well, this is annoying.

Broken code:

    align(16) uint[128] state;
    asm { fxsave state; }

"Fixed" code:

    uint[128 + 4] buf;
    auto state = cast(uint*)((cast(size_t)buf.ptr + 0xF) & ~size_t(0xF));
    version (X86_64)
        asm { mov RAX, state; fxsave 0[RAX]; }
    else
        asm { mov EAX, state; fxsave 0[EAX]; }

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https://issues.dlang.org/show_bug.cgi?id=16098

Vladimir Panteleev <dlang-bugzilla@thecybershadow.net> changed:

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                 CC|                            |dlang-bugzilla@thecybershad
                   |                            |ow.net

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https://issues.dlang.org/show_bug.cgi?id=16098

Imperatorn <johan_forsberg_86@hotmail.com> changed:

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                 CC|                            |johan_forsberg_86@hotmail.c
                   |                            |om

--- Comment #4 from Imperatorn <johan_forsberg_86@hotmail.com> ---
Can we fix this?

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https://issues.dlang.org/show_bug.cgi?id=16098

kinke <kinke@gmx.net> changed:

           What    |Removed                     |Added
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           Keywords|C++                         |backend
                 CC|                            |kinke@gmx.net

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https://issues.dlang.org/show_bug.cgi?id=16098

kinke <kinke@gmx.net> changed:

           What    |Removed                     |Added
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            Summary|align(32) not respected for |align(N) not respected for
                   |stack variables             |stack variables if N >
                   |                            |platform stack alignment

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https://issues.dlang.org/show_bug.cgi?id=16098

kinke <kinke@gmx.net> changed:

           What    |Removed                     |Added
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           Hardware|x86_64                      |All
                 OS|Linux                       |All
           Severity|normal                      |blocker

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https://issues.dlang.org/show_bug.cgi?id=16098

--- Comment #5 from kinke <kinke@gmx.net> ---
This issue blocks https://github.com/dlang/phobos/pull/8460. On Win32 with its 4-bytes stack alignment, the following assertion fails most of the time:

void main() {
    byte a;
    align(8) byte b;
    assert((cast(size_t) &b) % 8 == 0);
}

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https://issues.dlang.org/show_bug.cgi?id=16098

--- Comment #6 from Walter Bright <bugzilla@digitalmars.com> ---
One way to fix this is to:

1. gather all the locals with larger alignment and put them as fields in a struct variable

2. the first field in the struct is enough alignment padding to cover the worst case alignment bytes needed. For example, if the stack is aligned at 4, and the alignment of the fields is 16, then this field needs to be 12 bytes in size.

3. create a special pointer variable P that points to the aligned start of the struct fields, computed at function entry:

    P = (&struct + 15) & ~0xF;  // for 16 byte alignment

4. rewrite all references to the variables in the struct as P->field

5. be careful that it works with closures allocated with the gc

This should be doable all in the glue code, no need to modify the backend.

Of course, this can be done explicitly for the moment, to unblock code.

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