PING [Patch][Middle-end]Add -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]

[Kees Cook]

On Thu, Sep 03, 2020 at 09:29:54AM -0500, Qing Zhao wrote:
> On average, all the options starting with “used_…”  (i.e, only the registers that are used in the routine will be zeroed) have very low runtime overheads, at most 1.72% for integer benchmarks, and 1.17% for FP benchmarks. 
> If all the registers will be zeroed, the runtime overhead is bigger, all_arg is 5.7%, all_gpr is 3.5%, and all is 17.56% for integer benchmarks on average. 
> Looks like the overhead of zeroing vector registers is much bigger. 
> 
> For ROP mitigation, -fzero-call-used-regs=used-gpr-arg should be enough, the runtime overhead with this is very small.

That looks great; thanks for doing those tests!

(And it seems like these benchmarks are kind of a "worst case" scenario
with regard to performance, yes? As in it's mostly tight call loops?)

-- 
Kees Cook