[cfe-dev] Getting LLVM IR from clang

[via cfe-dev]

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<mailto: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<mailto: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<mailto: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<mailto: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/20181129/80e6d205/attachment.html>