Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.

[Zack Weinberg <zackw at panix dot com>]

On 08/09/2015 11:39 AM, H.J. Lu wrote:
> On Sun, Aug 9, 2015 at 8:20 AM, Zack Weinberg <zackw@panix.com> wrote:
>> On further investigation it appears not to -- specifically, internal
>> calls using __GI_foo appear to go straight to the default implementation
>> of 'foo'.
>>
>> If so, I am inclined to think that that is a bug -- there are a *lot* of
>> internal calls to memset and memcpy in libc, they should not miss out on
>> architectural tuning.  I don't particularly understand how IFUNC works,
>> but wouldn't it be sufficient to send internal calls to anything with an
>> IFUNC through the PLT?  (I suppose there would then be a question of
>> whether the architectural optimizations made up for the PLT overhead.)
> 
> Here is a description of IFUNC:
> 
> https://sites.google.com/site/x32abi/documents/ifunc.txt?attredirects=0&d=1

Thanks, that clarifies what IFUNC _does_, but it doesn't help me
understand how it interacts with the libc_hidden_* optimization.  I see
in the code that e.g. __GI_memset is pointed directly at __memset_sse2
(for amd64) but I do not understand whether that is a limitation of the
current implementation, a a deliberate choice to avoid indirection at
the cost of missing out on AVX2 tuning, or both.  And if it is a
limitation, I don't know what options we might have for lifting that
limitation.  I'm sure this was discussed when these patches originally
landed, but it was long enough ago that I am having trouble finding them
in the mailing list archive.

(I'm feeling mildly frustrated because I've had to change 66 files to
add one new function, and it would have been roughly a third fewer if
the memset implementation weren't so very complicated.)

zw

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