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.
