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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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