[PATCH] Pack MCSymbol flags and common alignment

[Pete Cooper]

> On Jul 1, 2015, at 12:15 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
>> 
>> 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;
> };
You’re right.  Thats compiling for me too.  I don’t know what I was thinking, but i was sure i’d had a problem with this compiling with clang.  I did the same in Value.h for example:

  static const unsigned NumUserOperandsBits = 29;
  unsigned NumUserOperands : 29;

How does this patch look given the changes you found?
Pete

> --

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