| Issue |
173800
|
| Summary |
Miscompile: Return value of no_caller_saved_registers function on x86-64 gets destroyed
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Vogtinator
|
With a simple function such as this:
```
__attribute__((no_caller_saved_registers)) __attribute__((target("general-regs-only")))
int getPerCPU()
{
int ret;
asm volatile("movl $0x1337, %[ret]" : [ret] "=r" (ret));
return ret;
}
```
The return value is not actually returned, but the return value register has its original value.
```
> clang -c test.c && objdump -d test.o
test.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <getPerCPU>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 50 push %rax <--------- %rax saved on stack
5: b8 37 13 00 00 mov $0x1337,%eax
a: 89 45 f4 mov %eax,-0xc(%rbp)
d: 8b 45 f4 mov -0xc(%rbp),%eax
10: 58 pop %rax <--------- previous %rax restored from stack
11: 5d pop %rbp
12: c3 ret
```
The `__attribute__((target("general-regs-only")))` part is to make it compile with gcc as well, which emits correct code:
```
> gcc -c test.c && objdump -d test.o
test.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <getPerCPU>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: b8 37 13 00 00 mov $0x1337,%eax
9: 89 45 fc mov %eax,-0x4(%rbp)
c: 8b 45 fc mov -0x4(%rbp),%eax
f: 5d pop %rbp
10: c3 ret
```
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