[cfe-dev] default alignment?

[Eric Niebler]

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.

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