linux-kernel - Re: [PATCH v3 14/30] perf clang: Support compile IR to BPF object and add testcase

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Date:   Mon, 28 Nov 2016 14:32:51 +0800
From:   "Wangnan (F)" <wangnan0@...wei.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     <acme@...hat.com>, <ast@...com>, <lizefan@...wei.com>,
        <hekuang@...wei.com>, <linux-kernel@...r.kernel.org>,
        <pi3orama@....com>, <joe@....org>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v3 14/30] perf clang: Support compile IR to BPF object and
 add testcase



On 2016/11/27 1:25, Alexei Starovoitov wrote:
> On Sat, Nov 26, 2016 at 07:03:38AM +0000, Wang Nan wrote:
>> getBPFObjectFromModule() is introduced to compile LLVM IR(Module)
>> to BPF object. Add new testcase for it.
>>
>> Test result:
>>    $ ./buildperf/perf test -v clang
>>    51: Test builtin clang support                               :
>>    51.1: Test builtin clang compile C source to IR              :
>>    --- start ---
>>    test child forked, pid 21822
>>    test child finished with 0
>>    ---- end ----
>>    Test builtin clang support subtest 0: Ok
>>    51.2: Test builtin clang compile C source to ELF object      :
>>    --- start ---
>>    test child forked, pid 21823
>>    test child finished with 0
>>    ---- end ----
>>    Test builtin clang support subtest 1: Ok
>>
>> Signed-off-by: Wang Nan <wangnan0@...wei.com>
> ...
>> +	legacy::PassManager PM;
>> +	if (TargetMachine->addPassesToEmitFile(PM, ostream,
>> +					       TargetMachine::CGFT_ObjectFile)) {
>> +		llvm::errs() << "TargetMachine can't emit a file of this type\n";
>> +		return std::unique_ptr<llvm::SmallVectorImpl<char>>(nullptr);;
>> +	}
>> +	PM.run(*Module);
> I'm pretty sure you want to add FunctionInlingPass as well otherwise
> I think llvm won't be doing much inlining and only very very simple
> programs will compile fine. See what we did on bcc side.

Thank you for your information. I though inlining should be done during
C to IR phase, and we have use -O2 for it. Let me check it.

> Also did you consider skipping elf generation and using in memory
> instead ? That will improve compile/run time.
>

Maybe in future work? Current design reuse libelf and bpf-loader for
loading BPF code. Skipping ELF generation make compiling faster, but
require another loader to be coexist with bpf-loader.c, or we need
a new libbpf. In my current experience, outputting ELF is not so slow
for both server and smartphone.

Thank you.