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

[Russell Wallace]

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.

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?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150202/31449423/attachment.html>