[cfe-dev] Getting LLVM IR from clang

[David Blaikie via cfe-dev]

Yup - thanks for the correction!

On Thu, Nov 29, 2018 at 11:37 AM <paul.robinson at sony.com> wrote:

> most notably, at -O0, all the functions in the IR will be attributed with
> 'nodebug',
>
>
>
> Dave means 'optnone' here. This prevents (most) optimization.
>
> --paulr
>
>
>
> *From:* cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] *On Behalf Of *David
> Blaikie via cfe-dev
> *Sent:* Thursday, November 29, 2018 2:33 PM
> *To:* Alex Denisov
> *Cc:* David Greene; cfe-dev at lists.llvm.org
>
>
> *Subject:* Re: [cfe-dev] Getting LLVM IR from clang
>
>
>
> 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/20181129/5abbef82/attachment.html>