[PATCH] Pack MCSymbol flags and common alignment

[Duncan P. N. Exon Smith]

> On 2015-Jul-01, at 12:06, Pete Cooper <peter_cooper at apple.com> wrote:
> 
>> 
>> 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.

This compiles cleanly for me.  Is it an MSVC issue?
--
$ clang++ -std=c++11 t.cpp  -c -Wall -Wextra
$ cat t.cpp 
struct T {
  static const int X = 4;
  int Bits : X;
};
--