[cfe-dev] Getting LLVM IR from clang

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Fri Nov 30 09:17:53 PST 2018


-O0 -flto would still run the always-inliner, for example (& would skip any
frontend (clang IR generation-level) optimizations you might be interested
in), and would add the "optnone" attribute to the functions which may not
be what you want if you're trying to investigate the -O3 pipeline

On Fri, Nov 30, 2018 at 12:12 AM Alex Denisov <1101.debian at gmail.com> wrote:

> > Collecting IR from LTO probably isn't what's desired - since that'll be
> already optimized in the individual compilation stage (to then be further
> optimized in the LTO stage).
>
> Hm, I'm a bit surprised now. So even if I call "clang -flto -O0 -c main.c"
> the actual bitcode will be optimized? Then which optimizations will be
> applied?
> Does it also affect -fembed-bitcode?
>
> > On 29. Nov 2018, at 20:33, David Blaikie <dblaikie at gmail.com> wrote:
> >
> > Collecting IR from LTO probably isn't what's desired - since that'll be
> already optimized in the individual compilation stage (to then be further
> optimized in the LTO stage).
> >
> > As others mentioned, -Xclang -disable-llvm-optzns. Also note that
> changing optimization flags (-O0 versus -O3, etc) will change the IR that's
> generated - most notably, at -O0, all the functions in the IR will be
> attributed with 'nodebug', so running this IR through opt -O3 will be
> basically a no-op. This catches folks by surprise sometimes. So use the -O
> flag you want, but pass disable-llvm-optzns to get that 'optimized' IR (as
> far as Clang's codegen is concerned) prior to the LLVM optimizations
> running on it.
> >
> > On Thu, Nov 29, 2018 at 12:49 AM Alex Denisov via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
> > Hi David,
> >
> > There are at least two robust ways to get the Bitcode/IR:
> >
> > 1. LTO. You can enable link-time optimization by adding -flto compiler
> flags, in the end all object files (*.o) will in fact be bitcode files.
> I.e.:
> >
> >     > file gmock.dir/gmock-gtest-all.cc.o
> >     gmock.dir/gmock-gtest-all.cc.o: LLVM bitcode, wrapper x86_6
> >
> > 2. Embedded bitcode. Clang 3.9 and higher has an option -fembed-bitcode.
> When enabled, the resulting executable will contain additional section
> containing bitcode.
> > You can use this great tool[1] to extract the bitcode from an executable.
> >
> > Both approaches produce bitcode (binary format), if you need IR
> (human-readable format), then you can post-process the bitcode by running
> llvm-dis against each bitcode file.
> >
> > I hope it helps.
> >
> > [1] https://github.com/JDevlieghere/LibEBC
> >
> > > On 28. Nov 2018, at 23:22, David Greene via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
> > >
> > > How do I get the LLVM IR produced by clang?  I know that -emit-llvm
> > > produces IR but it seems to be after processing by LLVM (things are
> > > optimized out, for example).  Is there an "official" way to get the IR
> > > coming right out of clang's codegen?  I can do a hack with -mllvm
> > > -print-before-all but that's icky.
> > >
> > > Thanks!
> > >
> > >                          -David
> > > _______________________________________________
> > > cfe-dev mailing list
> > > cfe-dev at lists.llvm.org
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20181130/bc70915a/attachment.html>


More information about the cfe-dev mailing list