[LLVMdev] How to write a custom LTO pass?

[John Criswell]

On 5/18/15 2:58 PM, Stephen Thomas wrote:
>
>
> On Mon, May 18, 2015 at 3:34 PM, John Criswell <jtcriswel at gmail.com 
> <mailto:jtcriswel at gmail.com>> wrote:
>
>     On 5/18/15 2:16 PM, Stephen Thomas wrote:
>>     I'd like to write an inter-modular analysis pass. My
>>     understanding is that this should take the form of an LTO pass.
>>     However, I have been unable to find any resources on the
>>     mechanics of writing and running a custom LTO pass. Does anyone
>>     have any pointers?
>
>     All you need to do is to write a ModulePass and modify the libLTO
>     plugin to run it.  Writing a ModulePass is described in the "How
>     to Write an LLVM Pass" document in the LLVM documentation.
>
>
> Great, that makes sense.
>
>     To modify libLTO, you'll need to read the documents on how to
>     install it, and you'll need to modify the plugin (in
>     llvm/tools/LTO, I think).
>
>     Alternatively, you can modify the LLVM library that defines which
>     passes get run when you use -O2 and -flto optimization, but the
>     result is the same: you'll need to install a new libLTO plugin.
>
>
> Just to check my understanding: I will have to change LLVM source code 
> (probably to some .cpp file in llvm/tools/LTO or in llvm/lib/LTO) and 
> recompile in order to run my custom LTO pass? I'm double checking 
> because for custom intra-modular analysis passes, it is not a 
> requirement to change LLVM source code. Instead, one just adds the 
> name of the .so to the opt invocation. 
> (http://llvm.org/docs/WritingAnLLVMPass.html#running-a-pass-with-opt).

The opt command (which is really a utility program and not part of the 
compiler toolchain proper) loads LLVM passes; I believe Clang (which is 
part of the compiler toolchain proper) can as well.

However, to the best of my knowledge, there is no way to tell the gold 
linker to load another .so file with an LLVM pass and run it during 
link-time optimization.  If there is a way, I'd like to know what it is.  :)

Regards,

John Criswell

>
> Thanks,
> Steve
>
>
>     Regards,
>
>     John Criswell
>
>>
>>     Thanks,
>>     Steve
>>
>>
>>     _______________________________________________
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>
>
>     -- 
>     John Criswell
>     Assistant Professor
>     Department of Computer Science, University of Rochester
>     http://www.cs.rochester.edu/u/criswell
>
>


-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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