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

[Wangnan (F)]

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@xxxxxxxxxx>
> ...
> > +	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.