[LLVMdev] Current state-of-the-art in whole program optimization

[David Blaikie]

On Mon, Feb 2, 2015 at 2:01 PM, Russell Wallace <russell.wallace at gmail.com>
wrote:

> As I understand it, clang currently has the -flto flag which activates
> some degree of whole program optimization. However, this requires clang to
> be presented with all the source files at the same time. This is easy
> enough to arrange in a greenfield project, but may be trickier with an
> existing program.
>
> On Unix, C and C++ programs are usually compiled with makefiles (whether
> handwritten or auto generated), and these typically call the compiler on
> one source file at a time and then call the system linker only after
> compilation has proceeded to machine code object files. Thus, even with
> clang as a drop-in replacement for GCC, there is the challenge of obtaining
> exact compilation instructions from the makefile and then modifying these
> to allow clang to see all the files at once, no later than bitcode stage.
>

This is generally already addressed by just making 'object' files that are
actually LLVM bitcode. I forget which flags are needed, exactly, but you'll
also need a linker that's aware of this trick (there's an LLVM plugin for
the gold linker, for example).

I forget which flags you have to pass to clang to make it do this (maybe it
is just -flto) but once you pass that flag and have an LLVM-aware linker,
it should 'just work' at least as far as the build system issues are
concerned.

- David


>
> On Windows, it's common to have a Visual Studio project file that msbuild
> can use. This sometimes does call the compiler on multiple files at the
> same time; then there's the question of how to get clang-cl invoked with
> suitable options.
>
> At least that's my thinking on this aspect of the problem so far; there
> are doubtless things I'm missing. What's currently being done in this area?
>
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