PATCH: Check GLIBC_IFUNC to enable/disable ifunc features

[Siddhesh Poyarekar]

On Wed, Jun 29, 2016 at 11:15:22AM -0700, H.J. Lu wrote:
> The current IFUNC selection is based on microbenchmarks in glibc.  It
> should give the best performance for most workloads.  But other choices
> may have better performance for a particular workload or the hardware
> is too new to make the best choice.  We provide an environment variable,
> GLIBC_IFUNC=xxx=0:yyy=1:zzz=0...., to enable CPU/ARCH feature yyy,
> disable CPU/ARCH feature yyy and zzz, where the feature name is

I'm afraid this scheme of values is going to hard to parse under
tunables.  Tunables themselves have the same kind of syntax when used
with a single environment variable.  How about having a single
environment variable for each feature, such as GLIBC_IFUNC_X86_AVX and
so on instead?

Siddhesh