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Re: [whopr] Design/implementation alternatives for the driver and WPA
On Wed, Jun 4, 2008 at 12:33 PM, Chris Lattner <clattner@apple.com> wrote:
>
> Right, I understand that you design "stacks" on LTO. It just seems strange
> to work on the advanced stuff before the basic GCC LTO stuff is close to
> being useful.
To some degree, we're scratching our own itch here. Basic LTO doesn't
help us much. Obviously, though, we want to implement this in a way
which is generally useful to the external community. The scalability
techniques required to work with distcc are different from the
techniques which are useful on a single machine.
> I don't know anything about the implementation of the HP or Intel LTO
> implementation, but it sounds like there is much room for improvement there.
> With LLVM LTO, we see a compile-time slowdown on the order of 30-50% switch
> from O3 to O4, not an order of magnitude. There is also still much room for
> improvement in the LLVM implementation of course.
I think we're working from different baselines. We use distributed
techniques for compiling individual .o files. With a tool like
distcc, you can get something on the order of 20x speedup. Linking
becomes 20% or more of total execution time. LTO *is* an order of
magnitude increase compared to a basic link operation.
Ollie