[Bug target/112478] riscv: asm clobbers not honored

[kito at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112478

Kito Cheng <kito at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |kito at gcc dot gnu.org
   Last reconfirmed|                            |2023-11-14
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |kito at gcc dot gnu.org

--- Comment #6 from Kito Cheng <kito at gcc dot gnu.org> ---
Oh, I guess I know what happened, I was confused by the commit you refer (but
it's the root cause as you pointed out!) since I thought it may related to far
jumps, but...actually not, the problem is something you describe in the title,
and can be demonstrate by following small program:

```c
void foo() {
    asm volatile("# " : ::"ra");
}

```

Before that commit:
```asm
foo:
        addi    sp,sp,-16
        sd      ra,8(sp)
 #APP
# 2 "x.c" 1
        # 
# 0 "" 2
 #NO_APP
        ld      ra,8(sp)
        addi    sp,sp,16
        jr      ra

```

After that commit:
```asm
foo:
.LFB0:
        .cfi_startproc
#APP
# 2 "x.c" 1
        # 
# 0 "" 2
#NO_APP
        ret
```

But why? because ra is accidentally become caller save register by following
change:

https://github.com/gcc-mirror/gcc/commit/71f906498ada9ec2780660b03bd6e27a93ad350c#diff-4083cffa971a940af1d435359a45dbfd4d5934384275b0ae5e0c71dece5fd866R331

So we no longer save it at prologue and epilogue longer...anyway I will take
this and send a patch to fix that soon.