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coverage: Emit the filenames section before encoding per-function mappings #117042

Merged
merged 6 commits into from
Oct 23, 2023
Merged

coverage: Emit the filenames section before encoding per-function mappings #117042

merged 6 commits into from
Oct 23, 2023

Commits on Oct 22, 2023

  1. coverage: Split FunctionCoverage into distinct collector/finished p… 

    …hases

This gives us a clearly-defined place to run code after the instance's MIR has
been traversed by codegen, but before we emit its `__llvm_covfun` record.
    @Zalathar
    Zalathar committed Oct 22, 2023
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  2. coverage: Fetch expressions and mappings separately 

    The combined `get_expressions_and_counter_regions` method was an artifact of
having to prepare the expressions and mappings at the same time, to avoid
ownership/lifetime problems with temporary data used by both.

Now that we have an explicit transition from `FunctionCoverageCollector` to the
final `FunctionCoverage`, we can prepare any shared data during that step and
store it in the final struct.
    @Zalathar
    Zalathar committed Oct 22, 2023
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  3. coverage: Build the global file table ahead of time

    @Zalathar
    Zalathar committed Oct 22, 2023
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  4. coverage: Encapsulate local-to-global file mappings

    @Zalathar
    Zalathar committed Oct 22, 2023
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  5. coverage: Encode function mappings without re-sorting them 

    The main change here is that `VirtualFileMapping` now uses an internal hashmap
to de-duplicate incoming global file IDs. That removes the need for
`encode_mappings_for_function` to re-sort its mappings by filename in order to
de-duplicate them.

(We still de-duplicate runs of identical filenames to save work, but this is
not load-bearing for correctness, so a sort is not necessary.)
    @Zalathar
    Zalathar committed Oct 22, 2023
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  6. coverage: Emit the filenames section before encoding per-function map… 

    …pings

Most coverage metadata is encoded into two sections in the final executable.
The `__llvm_covmap` section mostly just contains a list of filenames, while the
`__llvm_covfun` section contains encoded coverage maps for each instrumented
function.

The catch is that each per-function record also needs to contain a hash of the
filenames list that it refers to. Historically this was handled by assembling
most of the per-function data into a temporary list, then assembling the
filenames buffer, then using the filenames hash to emit the per-function data,
and then finally emitting the filenames table itself.

However, now that we build the filenames table up-front (via a separate
traversal of the per-function data), we can hash and emit that part first, and
then emit each of the per-function records immediately after building. This
removes the awkwardness of having to temporarily store nearly-complete
per-function records.
    @Zalathar
    Zalathar committed Oct 22, 2023
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