Allow caching of configure results.

[AndreasPK]

Running configure takes a lot of time on windows.

Having a flag to use cached configure data for rebuilds would speed this up nicely.

I can run configure for GHC with -C myself. But adding the flag for the configure process of base is a different story.
Ideally libraries would also use the same configure cache as ghc, so it would be faster even on the first build:

Here are the times for base and ghc itself on my machine. (SSD, skylake i7)

name	./configure	./configure -C (rerun)	difference
ghc	61 s	18s	-43
base	1m 40s	31s	-1m 9s

[snowleopard]

@AndreasPK Good idea! Now that we got rid of ghc-cabal we should have a more fine-grain control over how packages are configured.

Let me tag @angerman @alpmestan @izgzhen -- I think they are more knowledgeable in how this corner of Hadrian could be improved.

[alpmestan]

@AndreasPK and I discussed this a little more on IRC. autoconf's caching mechanism handles a lot of the work. Andreas pointed out that we probably just need to run the "toplevel" configure script with -C and then pass --configure-option=-C to all the cabal packages when we're configuring them. This is expected to make a noticeable difference on Windows, but not that much on Linux.

[angerman]

Configure on macOS feels quite a bit slower than on Linux as well. For crosscompilation the slowness is just comical.

[angerman]

This used to be discussed on #ghc a few years ago, and I’m not sure about the exact details anymore, but I believe there was some fear around stale state. Maybe @bgamari remembers...

[AndreasPK]

I think it would be great to have the option. If it's a good default is indeed a different topic.

This used to be discussed on #ghc a few years ago, and I’m not sure about the exact details anymore, but I believe there was some fear around stale state. Maybe @bgamari remembers..

Personally I don't think this is a major issue if we delete the cache on make clean. There are already lot's of ways to break a working build tree with lingering build artefacts where make clean is required to fix it.

But I'm also perfectly happy if we make this available just as an optional setting.

[angerman]

I'm absolutely in favor of having this! I just wanted to add some history.