[llvm-dev] intra vs inter module LTO (A. Ilchinger via llvm-dev)

[Jon Chesterfield via llvm-dev]

> Message: 2
> Date: Mon, 31 Dec 2018 18:55:13 +0100
> From: "A. Ilchinger via llvm-dev" <llvm-dev at lists.llvm.org>
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] intra vs inter module LTO
> Message-ID: <0MdFwl-1gvHrS3utf-00ISyE at mail.gmx.com>
> Content-Type: text/plain; charset=US-ASCII
>
> I'd like to know if LTO also works in the second step, when multiple .a
> libraries and potentially other .o files are linked together to the
> final binary. So can LTO also work successfully between modules (being
> inter-module).
>

LLVM mostly works in terms of an internal representation called bitcode. If
your .o file contains machine code (e.g. elf + x86) then the information
used by link time optimisation has almost all been discarded. Similarly if
you group some of these objects into a static library.

You can instead compile to bitcode and leave it as bitcode, e.g. using
-emit-llvm. If I understand correctly -flto essentially does this for you.
Bitcode can be joined to other bitcode with llvm-link, run through whatever
optimisation pipeline you like with opt, stored in a static archive with
llvm-ar, passed back into clang to make an object file etc.

I don't know if it's commonly done. I've seen it described internally as
'poor man's LTO', which seemed a bit sad to me as it's far more flexible.
The process I use for compiling projects is roughly:
- clang *.cpp -emit-llvm -o *.bc
- llvm-link directory/* -o directory.tmp.bc
- opt directory.tmp.bc -internalise -internalize-public-api-list=foo
-globaldce -O3 -o directory.bc
- recurse

Compile each source file to bitcode, each directory to a linked & optimised
bitcode file. The internalize stuff effectively renders some functions
static partway through the process and isn't necessary. This is recursive.
Eventually the top level is a bitcode file called src.bc, which gets turned
into a dynamic library or executable.

And if it is possible, do I have to pay attention to some compiler
> flags, linker options, etc., or does it work out-of-the-bos with -flto?
>

But I think all you're looking for so far is -emit-llvm + use llvm-ar. It's
possible -flto + use llvm-ar will be sufficient, but I haven't checked
exactly what combination of things -flto does (because I like working with
the pieces).

Cheers,

Jon
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