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

[Qing Zhao]

> On Sep 3, 2020, at 12:13 PM, Kees Cook <keescook@chromium.org> wrote:
> 
> 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?)

The top 3 benchmarks that have the most overhead from this option are: 531.deepsjeng_r, 541.leela_r, and 511.povray_r.
All of them are C++ benchmarks. 
I guess that the most important reason is  the smaller routine size in general (especially at the hot execution path or loops).
As a result, the overhead of these additional zeroing instructions in each routine will be relatively higher.  

Qing

> 
> -- 
> Kees Cook