[cfe-dev] [RFC] Zeroing Caller Saved Regs

[Bill Wendling via cfe-dev]

[This feature addresses https://bugs.llvm.org/show_bug.cgi?id=37880
and https://github.com/KSPP/linux/issues/84.]

Clang has been ramping up its support of the Linux kernel. We recently
added "asm goto with outputs", a long requested feature. We want to
continue building our relationship with the Linux community.

KSPP is a project to improve security in the Linux kernel, through
both kernel changes and compiler features. One compiler feature they
want is the ability to zero out caller-saved registers on function
return as a defense against stale register contents being used as a
side-channel or speculation path.

The option will be "opt-in" for each target. Targets that don't
support the flag should probably emit a warning or error.

Our proposal for the feature is modeled off of H. J. Lu's
description[1] (copied with some modifications):

```
Add -mzero-caller-saved-regs=[skip|used-gpr|all-gpr|used|all]
command-line option and zero_caller_saved_regs function attributes:

* Don't zero caller-saved registers upon function return (default):

    -mzero-caller-saved-regs=skip
    zero_caller_saved_regs("skip")

* Zero used caller-saved integer registers upon function return:

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

* Zero all integer registers upon function return:

    -mzero-caller-saved-regs=all-gpr
    zero_caller_saved_regs("all-gpr")

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

    -mzero-caller-saved-regs=used
    zero_caller_saved_regs("used")

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

    -mzero-caller-saved-regs=all
    zero_caller_saved_regs("all")
```

-bw

[1] https://github.com/clearlinux-pkgs/gcc/blob/master/0001-x86-Add-mzero-caller.patch