On 11/4/21 9:14 AM, Alexei Starovoitov wrote:

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

quoted

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));

asm volatile("" : "=r"(ret) : "0"(ret));

   return ret;
}

Right. We should prevent compilers from "inlining" return values.