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    SubjectRe: [PATCH v3 14/30] perf clang: Support compile IR to BPF object and add testcase
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    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@huawei.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.

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