[PATCH] Pack MCSymbol flags and common alignment

[Duncan P. N. Exon Smith]

> On 2015-Jul-01, at 12:18, Pete Cooper <peter_cooper at apple.com> wrote:
> 
>> 
>> 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
> 
> <mcsymbol-packing.patch>

> diff --git a/include/llvm/MC/MCSymbol.h b/include/llvm/MC/MCSymbol.h
> index 48b50d9..e52a346 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.
> +  static const unsigned NumCommonAlignmentBits = 5;

IIRC, you need something like this in MCSymbol.cpp for -O0 builds:

    const unsigned MCSymbol::NumCommonAlignmentBits;

to avoid linker errors.  However, maybe none of our supported
compilers require that anymore?

LGTM either way, just watch the bots if you leave out the explicit
definition.

> +  unsigned CommonAlignLog2 : NumCommonAlignmentBits;
> +
>    /// Index field, for use by the object file implementation.
>    mutable uint32_t Index = 0;
>