[PATCH] Pack MCSymbol flags and common alignment

[Pete Cooper]

> On Jul 1, 2015, at 12:00 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 
>> On 2015-Jun-30, at 14:30, Pete Cooper <peter_cooper at apple.com> wrote:
>> 
>> Here’s a new version of the patch for using 5-bits for alignment.  Please review when you get a chance.
>> 
>> Last one after this will just be to make flags 16-bits and move it to before Index.
>> 
>> Thanks,
>> Pete
>> 
>> <mcsymbol-packing.patch>
> 
>> diff --git a/include/llvm/MC/MCSymbol.h b/include/llvm/MC/MCSymbol.h
>> index 48b50d9..cb872be 100644
>> --- a/include/llvm/MC/MCSymbol.h
>> +++ b/include/llvm/MC/MCSymbol.h
>> @@ -109,6 +109,14 @@ protected:
>>   /// extension and achieve better bitpacking with MSVC.
>>   unsigned SymbolContents : 2;
>> 
>> +  /// The alignment of the symbol, if it is 'common', or -1.
>> +  ///
>> +  /// The alignment is stored as log2(align) + 1.  This allows all values from
>> +  /// 0 to 2^31 to be stored which is every power of 2 representable by an
>> +  /// unsigned.
>> +  const unsigned NumCommonAlignmentBits = 5;
> 
> I think you meant to make this static, right?  Otherwise it costs 4
> bytes.
Good point.  I did mean that.  I can make that change before I commit.
> 
>> +  unsigned CommonAlignLog2 : 5;
> 
> Can you use `NumCommonAlignmentBits` here?
Unfortunately that doesn’t work.  I would love to be able to use constant expressions for bitfield sizes.  Keeping the lines adjacent is the only thing I can think to do for now.

Cheers,
Pete
> 
>> +
>>   /// Index field, for use by the object file implementation.
>>   mutable uint32_t Index = 0;
>> 
>