[Bug c/104805] [12.0] x86_64 Extended asm may use rbp register to input/output even thougth "rbp" is in the clobber list when "rsp" and "rbp" are both in the in the clobber list

["570070308 at qq dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104805

--- Comment #5 from 。 <570070308 at qq dot com> ---
(In reply to Jakub Jelinek from comment #4)
> rbp is hard frame pointer, so depending on whether the function needs a
> frame pointer (at -O0 I think all functions do), the register isn't
> available for use (and therefore for clobbering) in inline asm.
> Only in functions where it isn't needed, it is not fixed then and can be
> used for other purposes.

So you explained why "rbp" cann't be in the clobber list with -O0, and may be
can be in the clobber list with -O1,-O2 or -O3(when the function don't needs a
frame pointer), I understand this now.

But when the "rbp" is in the clobber list successfully, it should not be used
to represent any input/output operands according to the doc because user may
change the %rbp and make the input/output operands wrong.

for example:
```
void kkk()
{
    char a;
    __asm__ volatile (
            "writing %%rbp\n\t"
            // %0 may point to error memory because %rbp change
            // for example -9(%rbp) represent char a
            "reading %0\n\t"
            "writing %0"
            :"+m"(a)
            :
            : "rsp","rbp"
            );
}
```

I have done a lot of experiments, if a register is list in the clobber list, it
will never appear to represent in the input/output operands, and the doc say so
too. Only "rbp" is an exception.

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Clobbers-and-Scratch-Registers
```
When the compiler selects which registers to use to represent input and output
operands, it does not use any of the clobbered registers. As a result,
clobbered registers are available for any use in the assembler code.
```