[PATCH] Pack MCSymbol flags and common alignment
Pete Cooper
peter_cooper at apple.com
Mon Jun 22 22:24:25 PDT 2015
> 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.
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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