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

On 10 Aug 2015 23:12, OndÅej BÃlka wrote:
> On Sun, Aug 09, 2015 at 11:09:20PM -0400, Mike Frysinger wrote:
> > On 09 Aug 2015 14:24, Zack Weinberg wrote:
> > > Is an IFUNC's variant-selecting function called only once per process,
> > > or every time?
> > 
> > it's once-per-process.  if it were every time, it'd defeat the point of the
> > optimization.
> No, its once per each shared library.

it depends on how you're counting.  i'm talking about each ifunc resolver -- it 
only executes once per process.  yes, the overall lookup of ifunc relocs happens
on a per-object basis, but it doesn't mean each resolver runs more than once.

> > > If we sent calls to 'memset' through the PLT (as is
> > > currently done for 'malloc') would that mean they were subject to IFUNC
> > > dispatch?
> > 
> > it's a double edge sword.  we specifically want to avoid the PLT for two 
> > reasons:
> > (1) speed (PLT is slow)
> > (2) interposition (we don't want someone exporting a memset symbol and then 
> > internal glibc code calling that instead of our own version)
> No, as I wrote in 
> [PATCH] x86-64: Remove plt bypassing of ifuncs. 
> thats completely flawed analysis. In best case you could save few
> cycles. As I looked on functions you would for most functions lose
> at leat twenty cycles as differences between implementations are that
> big.

it isn't a flawed analysis as i covered this explicitly in the part of my
e-mail that you snipped

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