Re: [PATCH] powerpc: New feature - HWCAP/HWCAP2 bits in the TCB

[Richard Henderson <rth at twiddle dot net>]

On 06/09/2015 04:06 PM, Steven Munroe wrote:
> On Tue, 2015-06-09 at 15:47 +0100, Szabolcs Nagy wrote:
> > On 08/06/15 22:03, Carlos Eduardo Seo wrote:
> > > The proposed patch adds a new feature for powerpc. In order to get
> > > faster access to the HWCAP/HWCAP2 bits, we now store them in the TCB.
> > > This enables users to write versioned code based on the HWCAP bits
> > > without going through the overhead of reading them from the auxiliary
> > > vector.
>
> > i assume this is for multi-versioning.
>
> The intent is for the compiler to implement the equivalent of
> __builtin_cpu_supports("feature"). X86 has the cpuid instruction, POWER
> is RISC so we use the HWCAP. The trick to access the HWCAP[2]
> efficiently as getauxv and scanning the auxv is too slow for inline
> optimizations.

There is getauxval(), which doesn't scan auxv for HWCAP[2], but rather reads 
the variables private to glibc that already contain this information.  That 
ought to be fast enough for the builtin, rather than consuming space in the TCB.



r~