[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAADnVQK6kMbX379dy5XOo1s7DricX1z9WZ04PhUD6DoEPO+jsg@mail.gmail.com>
Date: Thu, 4 Nov 2021 09:14:49 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Yonghong Song <yhs@...com>
Cc: Joe Burton <jevburton.kernel@...il.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>,
bpf <bpf@...r.kernel.org>, Petar Penkov <ppenkov@...gle.com>,
Stanislav Fomichev <sdf@...gle.com>,
Joe Burton <jevburton@...gle.com>
Subject: Re: [RFC PATCH v3 0/3] Introduce BPF map tracing capability
On Wed, Nov 3, 2021 at 9:23 PM Yonghong Song <yhs@...com> 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;
}
Powered by blists - more mailing lists