On 18/09/12 21:04, Mike Frysinger wrote:
On Tuesday 18 September 2012 08:44:29 Andrew Stubbs wrote:
Clearly there are some technical challenges in doing this: we'd have to
hash all the object files and libraries (a la direct mode), but those
problems are surmountable, I think.

or just re-use build-id ...

Sorry, I'm probably being thick, but what do you mean?

The linker does not use any libraries not listed with "gcc '-###' whatever".

mmm different gcc flags can implicitly expand into -l### or different crt
objects, so you can't cache linking at the compiler driver level w/out re-
implementing much of the guts of gcc, and even then you'd break with
moderately patched gcc versions.

"-###" isn't meant to be a wildcard. That's an actual GCC option. I put quotes around it because most shells would interpret the hashes as the start of a comment.

"-###" causes gcc to print the commands that it would run, including the link line (well, collect2, but same difference). We can read that and bypass reimplementing all of gcc. As you say, without this feature we couldn't predict what gcc will do: the compiler wouldn't even need to be patched if customer specs files were used.

I'm also aware that it's not that interesting for many incremental
builds, where the final link will always be different, but my use case
is accelerating rebuilds of projects that my have many outputs, most of
which are likely to be unaffected by small code changes. It's also worth
noting that incremental builds are not the target use case for ccache in
general.

gold should already support incremental linking (ala build-id), so i don't
think that's already a fixed problem

As I said, the interesting use case is *not* incremental links. The interesting use case is accelerating "clean" builds. ccache can never help where genuinely new inputs are involved.

Andrew

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