commit ddf099ff4e19e7cdfbef2a4f6f02f99793c40868
author Stuart Cook <Zalathar@users.noreply.github.com> Mon Apr 07 22:29:20 2025 +1000
committer GitHub <noreply@github.com> Mon Apr 07 22:29:20 2025 +1000
tree 785ba05c651fea4c0bdc0462059b6204647af841
parent f4c429fde57a4a59cb8537205e85688741aeffc6
parent 4322b6e97d6ad438c6c9ef1ea92e8ba00c056d9c

Rollup merge of #139397 - Zalathar:virtual, r=jieyouxu

coverage: Build the CGU's global file table as late as possible

Embedding coverage metadata in the output binary is a delicate dance, because per-function records need to embed references to the per-CGU filename table, but we only want to include files in that table if they are successfully used by at least one function.

The way that we build the file tables has changed a few times over the last few years. This particular change is motivated by experimental work on properly supporting macro-expansion regions, which adds some additional constraints that our previous implementation wasn't equipped to deal with.

LLVM is very strict about not allowing unused entries in local file tables. Currently that's not much of an issue, because we assume one source file per function, but to support expansion regions we need the flexibility to avoid committing to the use of a file until we're completely sure that we are able and willing to produce at least one coverage mapping region for it. In particular, when preparing a function's covfun record, we need the flexibility to decide at a late stage that a particular file isn't needed/usable after all.

(It's OK for the *global* file table to contain unused entries, but we would still prefer to avoid that if possible, and this implementation also achieves that.)