On 11/28/2014 03:36 PM, Ehsan Akhgari wrote:
On 2014-11-28 6:17 AM, Nicolas B. Pierron wrote:
On 11/28/2014 11:06 AM, Jonathan Kew wrote:
On 28/11/14 08:46, L. David Baron wrote:
On Friday 2014-11-28 10:12 +0900, Mike Hommey wrote:
The downside from doing so, though, is that non-unified build *will*
be broken, and code "purity" (right includes in the right sources,
mostly) won't be ensured. Do you think this is important enough to keep
non-unified builds around?
Another disadvantage here is that it will make adding or removing
source files harder, because you'll have to clean up the accumulated
"nonunified" bustage that shows up when files are shifted around
between unified files. (This might be somewhat harder to fix a year
or two later than it is when causing it.)
IMO, it seems worth maintaining a non-unified build, to minimize this
obscure fragility that will otherwise tend to accumulate over time. We
could
reduce the infrastructure load by doing the non-unified build on a more
occasional basis; perhaps once a day would be enough?
We already have builds that (normally) happen once a day: nightlies. How
about switching to a pattern where in addition to the nightly build,
we also
kick off a non-unified build for each platform on the same changeset? If
that fails, we file a bug, and the normal expectation should be that such
bugs can and will be fixed within a day (more or less), so the
non-unified
builds aren't left perma-broken.
I agree, we should keep non-unified builds as it keeps our individual
files valid from the C++ point-of-view. If this is taking too many
resources, then I think it is acceptable to do it less frequently.
The question is: what do we gain from doing that, technical purity aside?
Note that as Mike mentioned, even with doing both unified and non-unified
builds, you may still get build failures when adding/removing .cpp files, so
keeping support for non-unified builds will not fix that issue.
Indeed, both will report errors, but not at the same time.
What we gain is some kind of confidence that we might be using the right
symbols, and not another one which appear to have the same name (similar to
Bug 1105781)
Note that issues similar to Bug 1105781 might still compile correctly based
on the breadth first lookup strategy of namespaces. This problem is not
unique to unified builds, only emphasized.
What is identified by non-unified build is a problem of responsibility.
Finding missing symbols is the responsibility of the person who is
adding references without including headers. This is not at the charge
of the person who is adding/removing files from a moz.build.
That is not the only failure mode though. You may for example get into a
situation where Unified_foo0.cpp includes windows.h and that header #defines
CreateEvent to something else, and you remove a file from the unified
compilation causing a file from Unified_foo1.cpp to fall into
Unified_foo0.cpp and break because of the CreateEvent macro that is now in
effect in that translation unit.
Also, as I have mentioned upthread, we have never been in a situation where
each source file includes all of the headers that it requires, and unified
builds only barely make that problem worse.
I would be happy to drop the non-unified builds if we have a way to verify
that the symbol resolution are bound the same way in unified and non-unified
builds.
In fact, the only value provided by the non-unified build does not imply
that we have to link anything. Only that we resolve the symbols in a
translation unit.
Would there be a way to instrument a compiler such that we can produce such
reports in unified build and non-unified builds, and then compare them
against each others? This would also be useful for class definitions in
headers, to ensure that one class is always compiled the same way in all
translation units.
--
Nicolas B. Pierron
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