negative performance speedup with -fno-plt

[Farid Zakaria]

Thank you for the advice; I will look into that with perf if it has statistics.

I noticed also that I had only built python + libpython without a PLT
but glibc itself continued to have a PLT
(I'm using NixOS to make changes).

Looks like trying to override CFLAGS to build glibc without PLT is
challenging; it didn't like me just setting CFLAGS :)

I wonder if removing PLT from glibc will exacerbate the problem I'm
noticing but also have me see more speedups.
For those curious here is my python benchmark thus far:
https://pasteboard.co/RdcGYZH4m0kN.png

On Sun, Sep 15, 2024 at 11:42 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Farid Zakaria via Libc-help:
>
> > I'm comparing it against a Python built with BIND_NOW (-z,now) to
> > account for no-lazy binding in both. I'm a bit stumped at what could
> > be causing some negative speedups. Anyone got leads that might cause a
> > difference?
>
> Look at mispredicted indirect branches.  With the PLT, all the different
> calls to external functions share one indirect branch, but without it,
> you get an indirect branch for each call site.  It must be predicted
> indepedently, and your CPU might not be able to track so man indirect
> branches.
>
> Thanks,
> Florian
>