vsapsai marked an inline comment as done.
vsapsai added inline comments.
================
Comment at: clang/lib/Serialization/ModuleManager.cpp:183
// Get a buffer of the file and close the file descriptor when done.
- Buf = FileMgr.getBufferForFile(NewModule->File, /*isVolatile=*/false);
+ Buf = FileMgr.getBufferForFile(NewModule->File, /*isVolatile=*/true);
}
----------------
dblaikie wrote:
> vsapsai wrote:
> > dexonsmith wrote:
> > > rsmith wrote:
> > > > dexonsmith wrote:
> > > > > vsapsai wrote:
> > > > > > Made this change because if we don't have a valid module but opened
> > > > > > a corresponding .pcm file earlier, there is a high chance that .pcm
> > > > > > file was rebuilt.
> > > > > Please add a comment in the code explaining that.
> > > > This change is proving really bad for us. This prevents using `mmap`
> > > > for loading module files, and instead forces the entire file to be
> > > > loaded every time. Please revert.
> > > Can we limit the revert to explicit vs. implicit module builds? The
> > > scenario Volodymyr was defending against is implicit-only.
> > Richard, can you please tell what is your modules configuration and how you
> > are invoking clang? For implicit builds loading a module instead of `mmap`
> > is still better than building a module. But for explicit modules I see
> > there is no need to use `isVolatile` as modules aren't changing all the
> > time.
> Richard/Google use explicit modules, if that's the particular parameter
> you're asking about.
>
> So, yes, for Google's needs a solution that allows mmap to continue to be
> used for explicit modules would suffice, I believe.
>
> (not to in any way derail that from being addressed - I thought implicit
> modules weren't race-y - they're hashed, etc, so they shouldn't collide/be
> overwritten? Seems like a loss in performance to have to copy the whole thing
> into memory rather than using mmap, if that could be avoided by more precise
> hashing/collision avoidance?)
I need a specific command-line invocation to test with. If tests in
`clang/test/Modules/explicit*` are relevant to Google's setup, I can use those
as an example. It's just they were touched last time in 2016.
To discuss racy-ness, that is kinda inherent problem for incremental builds. If
a module becomes invalid, all of its users will try to get an up-to-date
version of it. And they can try to build that module themselves or check and
load already rebuilt module. Unfortunately, I don't see how different hashing
can help with it, after all multiple compiler invocations do want the same .pcm
file.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D72860/new/
https://reviews.llvm.org/D72860
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