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Re: [PATCH roland/arm] ARM: Define PI_STATIC_AND_HIDDEN.


On 04/14/2015 06:39 PM, Roland McGrath wrote:
I suspect this is now safe.  (It caused no apparent mischief in my
arm-linux-gnueabihf build using GCC 4.8.2.)  But I don't actually know
what GCC version is implied by "added by the GCC TLS patches".  I'm
guessing that 4.6 (our minimum) is new enough that this is always true
now.  If that's true, it would be nice to know the appropriate GCC
version number for sure and mention that in the comment.  If it is
really still the case that some supported compiler versions or
configurations do not reliably do pure PI access to static/hidden,
then I'd like to have enough information about that to write a proper
configure test.

Did you do any benchmarking?  I enabled this on tilegx out of curiosity,
and I found it was both larger (ld.so size increased 0.3%) and slower
(average of 0.1% slower when running fork/exec of a no-op program
in a loop with 10,000 iterations).  The timing results do vary enough
that I'd want to do more extensive testing to be able to say anything
definitive, but it's not very encouraging.

I admit I'm not sure why this might be, but it seems like the right
things were happening, e.g. _dl_start_final is missing and _dl_start
is bigger in the version with PI_STATIC_AND_HIDDEN defined.

All that said, if it's the more standard way, or is desirable for some
other reason, I'm happy to enable it for tilegx, but...

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com


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