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

[Mike Frysinger <vapier at gentoo dot org>]

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.

> 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)

we could create an internal symbol using the GLIBC_PRIVATE namespace which
would solve (2), but we'd still be left with (1).  and it's really hard to
quantify the speed tradeoffs between using the PLT everywhere and the choice
of two different mem/str funcs (e.g. SSE vs AVX).  at least on older cpus
where only SSE2 is available, we know there will be no speed increase, but
there will be a speed loss (but still hard to quantify).

> Is there any *other* way (that already exists - nothing that would
> require binutils changes) to cause libc.so-internal calls to 'memset' to
> be subject to IFUNC dispatch?  Compared to using the PLT, what are the
> costs and benefits of doing it that way?

since IFUNC only exists to handle the PLT slot, doing IFUNC w/out PLT 
fundamentally doesn't make sense.
-mike

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