Re: [RFC PATCH v3 0/3] Introduce BPF map tracing capability

[Yonghong Song]

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

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

>    return ret;
> }

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