This reverts commit 7a242387c950c7060143da6da0e6fb91f36bb458. Even after 175f8a44, the Modules/fmodules-validate-once-per-build-session.c test is not fixed on the clang-armv8-quick build bot. (Failure occurs on line 114.)
This reverts commit 18b885f66babff3a10451bc811ffc077d61ed8ee, effectively reapplying #139987. This commit fixes unit tests (for example ASTUnitTest.SaveLoadPreservesLangOptionsInPrintingPolicy) where the `ASTUnit::ModCache` pointer dereferenced within `ASTUnit::serialize()` was null. This commit makes sure each factory function does initialize `ASTUnit::ModCache`.
Timestamps are an implementation detail of the cross-process module
cache implementation. This PR hides it from the `ModuleCache` API, which
simplifies the in-process implementation.
In the past, timestamps used for
`-fmodules-validate-once-per-build-session` were found to be a source of
contention in the dependency scanner
([D149802](https://reviews.llvm.org/D149802),
https://github.com/llvm/llvm-project/pull/112452). This PR is yet
another attempt to optimize these. We now make use of the new
`ModuleCache` interface to implement the in-process version in terms of
atomic `std::time_t` variables rather the mtime attribute on
`.timestamp` files.
This PR adds new `ModuleCache` interface to Clang's implicitly-built
modules machinery. The main motivation for this change is to create a
second implementation that uses a more efficient kind of
`llvm::AdvisoryLock` during dependency scanning.
In addition to the lock abstraction, the `ModuleCache` interface also
manages the existing `InMemoryModuleCache` instance. I found that
compared to keeping these separate/independent, the code is a bit
simpler now, since these are two tightly coupled concepts. I can
envision a more efficient implementation of the `InMemoryModuleCache`
for the single-process case too, which will be much easier to implement
with the current setup.
This is not intended to be a functional change.
