[cfe-dev] default alignment?

[Eric Niebler]

On 1/18/2011 1:32 AM, Chris Lattner wrote:
> On Jan 17, 2011, at 9:38 AM, Eric Niebler wrote:
>> On 1/18/2011 12:20 AM, Chris Lattner wrote:
>>> On Jan 17, 2011, at 1:28 AM, Eric Niebler wrote:
>>>> In Sema::ActOnPragmaPack, I see this:
>>>>
>>>> // FIXME: This should come from the target.
>>>> if (AlignmentVal == 0)
>>>>   AlignmentVal = 8;
>>>>
>>>> So, what's the right fix? Should this be
>>>> (Context.Target.getDoubleAlign() / Context.Target.getCharWidth())? (And
>>>> why 8? Where does that come from?)
>>>
>>> Hi Eric,
>>>
>>> I'm not familiar with #pragma pack show, but the 8 here is in bits.  The right fix is probably to call Context.Target.getCharWidth().
>>
>> Hi Chris,
>>
>> That's just can't be right. A default alignment of 1 byte would lead to
>> alignment faults all over the place. And consider the following:
>>
>> #pragma pack(show)
>> #pragma pack(1)
>> #pragma pack(show)
>>
>> Compiled with MSVC, I get:
>>
>> foo.c(1) : warning C4810: value of pragma pack(show) == 8
>> foo.c(3) : warning C4810: value of pragma pack(show) == 1
>>
>> Clearly, the "1" in the last line doesn't refer to a 1 *bit* alignment.
>> We're talking bytes here.
> 
> As I said, I don't know anything about pack(show).  The context of the code is:
> 
>   case Sema::PPK_Show: // pack(show)
>     // Show the current alignment, making sure to show the right value
>     // for the default.
>     AlignmentVal = Context->getAlignment();
>     // FIXME: This should come from the target.
>     if (AlignmentVal == 0)
>       AlignmentVal = 8;
>     if (AlignmentVal == PackStackEntry::kMac68kAlignmentSentinel)
>       Diag(PragmaLoc, diag::warn_pragma_pack_show) << "mac68k";
>     else
>       Diag(PragmaLoc, diag::warn_pragma_pack_show) << AlignmentVal;
>     break;
> 
> 
> Since "Context->getAlignment()" returns alignment in bits, it looks like show is just broken.  It should be something like:
> 
> ...
>       Diag(PragmaLoc, diag::warn_pragma_pack_show) << AlignmentVal/8;
>     break;


I think you're mistaken. Context->getAlignment() appears to be returning
alignment in bytes, not bits, and pragma pack(show) appears to be
working just fine. This is from test/Sema/pragma-pack.c:

#pragma pack(1)
#pragma pack(push, 8) // -> (eek, 2), (, 2), (, 1), 8
/* expected-warning {{value of #pragma pack(show) == 8}} */ #pragma
pack(show)
#pragma pack(pop) // -> (eek, 2), (,2), 1
/* expected-warning {{value of #pragma pack(show) == 1}} */ #pragma
pack(show)

Notice that we can pack(1) (1-byte alignment) and then pack(show)
displays 1.

Getting back to my original question:

> So, what's the right fix? Should this be
> (Context.Target.getDoubleAlign() / Context.Target.getCharWidth())?
> (And why 8? Where does that come from?)

Any help would be much appreciated.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com