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Message-ID: <71771c37-5e97-54e6-0d98-73878f3b74d5@fb.com>
Date: Thu, 4 Nov 2021 10:11:31 -0700
From: Yonghong Song <yhs@...com>
To: Alexei Starovoitov <alexei.starovoitov@...il.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 11/4/21 9:14 AM, Alexei Starovoitov wrote:
> 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));
asm volatile("" : "=r"(ret) : "0"(ret));
> return ret;
> }
Right. We should prevent compilers from "inlining" return values.
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