Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list

[Willy Tarreau <w@1wt.eu>]

Hello Ammar,

First, thanks for your patch. I have a few questions below.

On Mon, Oct 11, 2021 at 11:03:44AM +0700, Ammar Faizi wrote:
> Linux x86-64 syscall only clobbers rax, rcx and r11 (and "memory").
> 
>   - rax for the return value.
>   - rcx to save the return address.
>   - r11 to save the rflags.
> 
> Other registers are preserved.
> 
> Having r8, r9 and r10 in the syscall clobber list is harmless, but this
> results in a missed-optimization.
> 
> As the syscall doesn't clobber r8-r10, GCC should be allowed to reuse
> their value after the syscall returns to userspace. But since they are
> in the clobber list, GCC will always miss this opportunity.

I agree with your conclusion about this but need to be perfectly sure
about the exact clobber list since I got a different impression when
implementing the calls. I got the impression that these ones were
clobbered by reading entry_64.S below:

 * Registers on entry:
 * rax  system call number
 * rcx  return address
 * r11  saved rflags (note: r11 is callee-clobbered register in C ABI)
 * rdi  arg0
 * rsi  arg1
 * rdx  arg2
 * r10  arg3 (needs to be moved to rcx to conform to C ABI)
 * r8   arg4
 * r9   arg5
 * (note: r12-r15, rbp, rbx are callee-preserved in C ABI)

See this last note ? Indicating that r12-15, rbp and rbx are preserved
made me think it's not the case for the other ones, but that might of
course be a misunderstanding.

And calling.h says this:

 x86 function call convention, 64-bit:
 -------------------------------------
  arguments           |  callee-saved      | extra caller-saved | return
 [callee-clobbered]   |                    | [callee-clobbered] |
 ---------------------------------------------------------------------------
 rdi rsi rdx rcx r8-9 | rbx rbp [*] r12-15 | r10-11             | rax, rdx [**]

Note that it's indicated "function call convention", not "syscall",
leaving it open to have extra restrictions on syscalls. Later by
reading the POP_REGS macro called with pop_rdi=0 and skipr11rcx=1
on syscall leave, it seems to restore everything but r11, rcx, rax
and rdi (which is restored last with rsp).

As such, could you please add in your commit message a link to the
location where you found that authoritative information above it there
is a better place (i.e. one that doesn't require to read all the macros)?
This would significantly help anyone facing the same doubts about this
in the future.

Thank you!
Willy