Re: Popcount optimization using AVX512

On Fri, Feb 09, 2024 at 10:24:32AM -0800, Andres Freund wrote:
> On 2024-01-26 07:42:33 +0100, Alvaro Herrera wrote:
> > This suggests that finding a way to make the ifunc stuff work (with good
> > performance) is critical to this work.
> 
> Ifuncs are effectively implemented as a function call via a pointer, they're
> not magic, unfortunately. The sole trick they provide is that you don't
> manually have to use the function pointer.

The IFUNC creators introduced it so glibc could use arch-specific memcpy with
the instruction sequence of a non-pointer, extern function call, not the
instruction sequence of a function pointer call.  I don't know why the
upthread ifunc_test.patch benchmark found ifunc performing worse than function
pointers.  However, it would be odd if toolchains have replaced the original
IFUNC with something equivalent to or slower than function pointers.