[cfe-dev] Getting LLVM IR from clang

[Alex Denisov via cfe-dev]

> 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
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