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

[Mike Frysinger <vapier at gentoo dot org>]

On 11 Aug 2015 07:55, OndÅej BÃlka wrote:
> On Mon, Aug 10, 2015 at 10:48:03PM -0400, Mike Frysinger wrote:
> > 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.
>
> No, you are wrong.

i have no idea what your problem is, but you need to take it down a notch

 You should verify, not make guesses. Its clearly
> called twice in following program.

yes, once per PLT slot.  once the slot is resolved, it isn't executed again, on 
a per-process basis.

> > > > > If we sent libc.so-internal 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
> 
> No, as you snipped essential part its flawed.

i have no idea what you thought you read, but cleary it isn't what i wrote.
i'm not going to bother responding here anymore since it's completely pointless 
trying to talk to you.
-mike

Attachment: signature.asc
Description: Digital signature