On Wed, Nov 3, 2021 at 9:23 PM Yonghong Song [off-list ref] 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;
}