[PATCH][x86][1/3]: Add -mzero-caller-saved-regs=[skip|used-gpr|all-gpr|used|all]

[Qing Zhao]

Hi, 

This is a PING for an old  patch proposed by  H. J. Lu on Oct, 2018:  

https://gcc.gnu.org/legacy-ml/gcc-patches/2018-10/msg02079.html

This is the first patch of the total 3 patches set, which provides the following new feature:

-mzero-caller-saved-regs=[skip|used-gpr|all-gpr|used|all] command-line
option and zero_caller_saved_regs("skip|used|all") function attribue:

1. -mzero-caller-saved-regs=skip and zero_caller_saved_regs("skip")

Don't zero caller-saved registers upon function return.

2. -mzero-caller-saved-regs=used-gpr and zero_caller_saved_regs("used-gpr")

Zero used caller-saved integer registers upon function return.

3. -mzero-caller-saved-regs=all-gpr and zero_caller_saved_regs("all-gpr”)

Zero all caller-saved integer registers upon function return.

4. -mzero-caller-saved-regs=used and zero_caller_saved_regs("used")

Zero used caller-saved integer and vector registers upon function return.

5. -mzero-caller-saved-regs=all and zero_caller_saved_regs("all")

Zero all caller-saved integer and vector registers upon function return.

This feature is needed by Linux kernel security improvement. Please refer to Kees Cook’s talk on Linux Plumber Conference 2019:
https://outflux.net/slides/2019/lpc/gcc-and-clang.pdf <https://outflux.net/slides/2019/lpc/gcc-and-clang.pdf>
Tested on  x86-64 with bootstrapping GCC trunk, regression tests exposed several new regressions, these new regressions are
fixed by 2 following patches I will send in next two emails.

Any comment?

thanks.

Qing