[PATCH] Pack MCSymbol flags and common alignment

[Rafael Espíndola]

LGTM

On 26 June 2015 at 16:02, Pete Cooper <peter_cooper at apple.com> wrote:
>
>> On Jun 22, 2015, at 10:24 PM, Pete Cooper <peter_cooper at apple.com> wrote:
>>
>>
>>> On Jun 22, 2015, at 9:35 PM, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
>>>
>>> Or use one fewer bit for ELF. Nothing is going to align a common to 4 GB :-)
>> If only that were true, but i’m sure someone will be crazy enough to write an asm test doing just that :)
>>
>> ...
>>>
>>> On 23 June 2015 at 00:23, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
>>>> Another option would be to use one of the low bits it SectionOrFragment.
>> I like this solution.  Will get a patch ready tomorrow.
> Sorry, tomorrow ended up being a few days.
>
> How does this look for the packing a bit in to SectionOrFragment?
>
>
>
>>
>> Pete
>>>>
>>>> With that you could keep 5 bits in the base for common.
>>>>
>>>>
>>>>
>>>> On 22 June 2015 at 19:19, Pete Cooper <peter_cooper at apple.com> wrote:
>>>>> I should say, i forgot to clang-format this.  I’ll of course do that before commit as there’s a bunch of 80-cols violations right now.
>>>>>
>>>>> Pete
>>>>>> On Jun 22, 2015, at 4:05 PM, Pete Cooper <peter_cooper at apple.com> wrote:
>>>>>>
>>>>>> Hi Rafael
>>>>>>
>>>>>> MCSymbol has 32-bits of flags and 32-bits of common alignment.
>>>>>>
>>>>>> However, we only need to represent power of 2 alignments, and no subclasses need more than 16 bits of flags.  We also have 20 bits of bitfields remaining unused.
>>>>>>
>>>>>> This patch takes those 20-bits and exposes them as SubclassData.
>>>>>>
>>>>>> On COFF, we used 16-bits for flags and the remainder are unused.
>>>>>> On ELF, we used 5 bits to store the log2(align) which allows alignments up to 2^31, i.e., any power of 2 which fits in an unsigned.  The remaining 15 are enough for the ELF flags.
>>>>>> On MachO, we need 16-bits for flags, and the remaining 4-bits are enough for 2^15 which MCSymbolMachO::getEncodedFlags() already asserted we didn’t exceed.
>>>>>>
>>>>>> This ultimately gets MCSymbol down to 24-bits for the base class.  I don’t think its possible to pack the data in any better at this point.
>>>>>>
>>>>>> Cheers,
>>>>>> Pete
>>>>>>
>>>>>> <mcsymbol-packing.patch>
>>>>>
>>
>>
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