https://d.puremagic.com/issues/show_bug.cgi?id=11910

           Summary: Writes to extern (C) struct have no effect
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: Windows
            Status: NEW
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: turkeyman@gmail.com


--- Comment #0 from Manu <turkeyman@gmail.com> 2014-01-12 19:08:24 PST ---
Is this bad codegen, or have I done something wrong?

extern (C) struct MFDefaults
{
    extern (C) struct InputDefaults
    {
        bool allowMultipleMice;
        bool mouseZeroIsSystemMouse;
        bool systemMouseUseWindowsCursor;
        bool useDirectInputKeyboard;
        bool useXInput;
    }

    InputDefaults input;
}

extern (C) __gshared MFDefaults gDefaults;

void f()
{
  gDefaults.input.useXInput = false; // assignment has no effect
}


This code builds and links successfully. In the debugger I can inspect the
values of the struct and they are correct, consistent with the struct in the C
code.
But when I assign to anything, it simply has no effect...

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David Nadlinger <code@klickverbot.at> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |code@klickverbot.at


--- Comment #1 from David Nadlinger <code@klickverbot.at> 2014-01-12 19:18:57 PST ---
I checked the generated code on Linux x86_64 and it seems fine (store immediate zero in gDefaults at offset 4). So, more details and instructions to reproduce, please. ;)

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--- Comment #2 from Manu <turkeyman@gmail.com> 2014-01-12 19:53:36 PST ---
I'm building with DMD-Win64.

Digging further, I see this instruction in the debugger: mov         byte ptr [gDefaults+9Ch (7F685FBF01Ch)],1

Which looks correct...
But note, the debugger helpfully shows the calculated store address:
7F685FBF01Ch
&gDefaults.input.useXInput == 7F685FEBC7Ch

It should write to ...FEBC7Ch, but instead it writes to ...FBF01Ch, which is WAY before where it should be writing.

If I inspect the memory surrounding the address that it actually writes to, it is filled with things like fully justified D symbols (with module name and everything), path strings, lots of zeroes and float 1.0's. Not the structure I expect to see.

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--- Comment #3 from Manu <turkeyman@gmail.com> 2014-01-12 20:00:13 PST ---
(In reply to comment #2)
> I'm building with DMD-Win64.
> 
> Digging further, I see this instruction in the debugger: mov         byte ptr [gDefaults+9Ch (7F685FBF01Ch)],1

Note: +9C in mine is because my structure is actually much larger, I trimmed it down to log the bug. The offset 9C looks right.

The debugger also illustrated 'gDefaults' as the store address, which I guess
means the address it writes to does match the symbol record in the debug info.
I guess the address emitted for gDefaults is wrong somehow?
But... that symbol should come from the external C library :/

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Jacob Carlborg <doob@me.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |doob@me.com


--- Comment #4 from Jacob Carlborg <doob@me.com> 2014-01-13 02:43:57 PST ---
(In reply to comment #3)

> Note: +9C in mine is because my structure is actually much larger, I trimmed it down to log the bug. The offset 9C looks right.
> 
> The debugger also illustrated 'gDefaults' as the store address, which I guess
> means the address it writes to does match the symbol record in the debug info.
> I guess the address emitted for gDefaults is wrong somehow?
> But... that symbol should come from the external C library :/

What is "gDefaults" supposed to be? A symbol from a C library? In that case you forgot "extern", the correct declaration should be:

extern (C) extern __gshared MFDefaults gDefaults;

Have a look at: http://dlang.org/interfaceToC.html#C%20Globals

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--- Comment #5 from Manu <turkeyman@gmail.com> 2014-01-13 03:31:27 PST ---
(In reply to comment #4)
> 
> What is "gDefaults" supposed to be? A symbol from a C library?

Yes, I thought that was clear from the 'extern (C)'.

> In that case you forgot "extern", the correct declaration should be:
> 
> extern (C) extern __gshared MFDefaults gDefaults;

Umm... I think you need to look at the OP again.

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--- Comment #6 from Manu <turkeyman@gmail.com> 2014-01-13 03:34:12 PST ---
(In reply to comment #4)
>
> In that case you forgot "extern", the correct declaration should be:
> 
> extern (C) extern __gshared MFDefaults gDefaults;
> 
> Have a look at: http://dlang.org/interfaceToC.html#C%20Globals

... oh
Crap, where's that edit button!

Wow, I have no words.
I never would have spotted that a mile off, even when you said it!

That seems extremely redundant :/

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--- Comment #7 from Jacob Carlborg <doob@me.com> 2014-01-13 04:20:35 PST ---
(In reply to comment #6)

> ... oh
> Crap, where's that edit button!
> 
> Wow, I have no words.
> I never would have spotted that a mile off, even when you said it!
> 
> That seems extremely redundant :/

If you don't specify "extern" you get the storage of the variable in the D code.

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Manu <turkeyman@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


--- Comment #8 from Manu <turkeyman@gmail.com> 2014-01-13 04:40:25 PST ---
(In reply to comment #7)
> If you don't specify "extern" you get the storage of the variable in the D code.

Right, but shouldn't I have gotten a multiply defined symbols link error in that case?

* Marked resolved, user error! >_<

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Manu <turkeyman@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|FIXED                       |INVALID


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