[Bug target/103882] New: Register corruption in ASM only functions when optization is -O2/-Os/-O3

["krystalgamer at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103882

            Bug ID: 103882
           Summary: Register corruption in ASM only functions when
                    optization is -O2/-Os/-O3
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krystalgamer at protonmail dot com
  Target Milestone: ---
              Host: x86_64-linux-gnu
            Target: mips-linux-gnu
             Build: x86_64-linux-gnu

When a function is only composed of an `asm` statement if the optimizations are
at least -O2 it will make it assume none of the registers will be tainted.


Example code:

```
int is_first_char_a(const char *f){
        return f[0] == 'a';
}

void fail(){
        asm __volatile__ (
                "li $a0, 0x60606060\n"
                );
}

int example(char *c){
        fail();
        return is_first_char_a(c);
}

void __start(){}
```


Compiled with `-nostdlib -O1`, generates the following code:

```
00400180 <example>:
  400180:       27bdffe0        addiu   sp,sp,-32
  400184:       afbf001c        sw      ra,28(sp)
  400188:       afb00018        sw      s0,24(sp)
  40018c:       00808025        move    s0,a0
  400190:       0c10005d        jal     400174 <fail>
  400194:       00000000        nop
  400198:       82020000        lb      v0,0(s0)
  40019c:       38420061        xori    v0,v0,0x61
  4001a0:       2c420001        sltiu   v0,v0,1
  4001a4:       8fbf001c        lw      ra,28(sp)
  4001a8:       8fb00018        lw      s0,24(sp)
  4001ac:       03e00008        jr      ra
  4001b0:       27bd0020        addiu   sp,sp,32
```


Compiled with `-nostdlib -O2`, generates the following code:

```
00400180 <example>:
  400180:       3c046060        lui     a0,0x6060
  400184:       34846060        ori     a0,a0,0x6060
  400188:       80820000        lb      v0,0(a0)
  40018c:       38420061        xori    v0,v0,0x61
  400190:       03e00008        jr      ra
  400194:       2c420001        sltiu   v0,v0,1
```


As can be seen before -O1 levels of optimization the compiler saves $a0 in $s0
and then restores it. -O2 level and beyond causes the compiler to assume that
a0 is preserved.



>From what I can tell this might be because the optimizer doesn't take into
consideration ASM statements(which is good), but it makes wrong assumptions. A
possible solution would be to save the caller-saved registers whenever a
function with an ASM statement is called.