[PATCH] D85062: [WebAssembly] GC constructor functions in otherwise unused archive objects

[Sam Clegg via Phabricator via llvm-commits]

sbc100 added inline comments.


================
Comment at: lld/wasm/InputFiles.h:103
+    if (archiveName.empty())
+      markLive();
   }
----------------
sunfish wrote:
> sbc100 wrote:
> > My understanding is that any file that is created as an ObjFile is by definition live. and that all files in `symtab->objectFiles` are also by definition live.
> > 
> > Archive files don't create any `ObjFile`s until they are pulled into the link for some reason (i.e. they are live).
> > 
> > What am I missing here?
> There are effectively two GC algorithms in wasm-ld today. The first selects the objects that aren't in archives, plus the objects in archives they (transitively) reference. The second one is MarkLive.cpp, which selects exported functions, plus functions they (transitively) reference.
> 
> What this patch is saying is, if a constructor function gets pulled in because its object is selected in the first phase, but MarkLive.cpp's GC determines that no functions in that object are transitively called from an export in the second phase, the constructor doesn't need to be called.
I see .. so something like this:

1. Transitive dependency pulls object out of archive.
2. Source of dependency turns out to not be live in the final link due to `--gc-sections`.
3. Object file should no longer be considered part of the link after all, reversing the decision made in (1).

If I'm understanding correctly, one possible problem with this is that is makes `--gc-sections` observable.    I could get a static ctor that runs with `--no-gc-sections` but then is not run with `--gc-sections`.... maybe this is so subtle as not to matter?   But I would normally expect those two builds with be identical in their behaviour.  




Repository:
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  https://reviews.llvm.org/D85062/new/

https://reviews.llvm.org/D85062