Re: integrating make and git

From: Jeff King <peff@peff.net>
Date: Apr 16, 2009, 08:26 UTC
Message-ID: <20090416082615.GA27365@coredump.intra.peff.net>
In-Reply-To: <200904151930.32816.robin.rosenberg.lists@dewire.com>

On Wed, Apr 15, 2009 at 07:30:32PM +0200, Robin Rosenberg wrote:

> Take a look at ccache. It doesn't use Git, but it uses hashes of source, and
> compiler flags and associates that with the resulting object files, so it
> can avoid compiling. If you are building largs C/C++ (especially C++)
> projects you want it.

In theory, one could improve something like ccache by asking git the sha-1 of the file. Since git maintains a cache based on stat info, you can get away with not looking at the file contents at all (which saves CPU time in hashing, but also helps a lot when building from a cold cache).

In practice, this doesn't help because:

  1. ccache looks at more than just the file itself. I believe it
     actually runs it through cpp and hashes that.

  2. People combine ccache with make; if the stat data hasn't changed,
     in most cases, you will skip building before you even get to
     ccache.

But one could probably design a system to replace both ccache and make that relies on git's fast sha-1 reporting to avoid duplicate work. I suspect nobody has bothered because make+ccache is "fast enough" that the added complexity would not be worth it.

-Peff