On Wed, Nov 3, 2021 at 9:23 PM Yonghong Song <yhs@xxxxxx> wrote:
>
> asm("") indeed helped preserve the call.
>
> [$ ~/tmp2] cat t.c
> int __attribute__((noinline)) foo() { asm(""); return 1; }
> int bar() { return foo() + foo(); }
> [$ ~/tmp2] clang -O2 -c t.c
> [$ ~/tmp2] llvm-objdump -d t.o
>
> t.o: file format elf64-x86-64
>
> Disassembly of section .text:
>
> 0000000000000000 <foo>:
> 0: b8 01 00 00 00 movl $1, %eax
> 5: c3 retq
> 6: 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:(%rax,%rax)
>
> 0000000000000010 <bar>:
> 10: 50 pushq %rax
> 11: e8 00 00 00 00 callq 0x16 <bar+0x6>
> 16: e8 00 00 00 00 callq 0x1b <bar+0xb>
> 1b: b8 02 00 00 00 movl $2, %eax
> 20: 59 popq %rcx
> 21: c3 retq
> [$ ~/tmp2]
>
> Note with asm(""), foo() is called twice, but the compiler optimization
> knows foo()'s return value is 1 so it did calculation at compiler time,
> assign the 2 to %eax and returns.
Missed %eax=2 part...
That means that asm("") is not enough.
Maybe something like:
int __attribute__((noinline)) foo()
{
int ret = 0;
asm volatile("" : "=r"(var) : "0"(var));
return ret;
}
