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The LLVM API that we use to encode coverage mappings already has its own code
for removing unused coverage expressions and renumbering the rest.
This lets us get rid of our own complex renumbering code, making it easier to
change our coverage code in other ways.
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This splits off `BcbCounter` from MIR's `CoverageKind`, allowing the two types
to evolve in different directions as necessary.
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Operand types are now tracked explicitly, so there is no need to reserve ID 0
for the special always-zero counter.
As part of the renumbering, this change fixes an off-by-one error in the way
counters were counted by the `coverageinfo` query. As a result, functions
should now have exactly the number of counters they actually need, instead of
always having an extra counter that is never used.
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Operand types are now tracked explicitly, so there is no need for expression
IDs to avoid counter IDs by descending from `u32::MAX`. Instead they can just
count up from 0, and can be used directly as indices when necessary.
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Because the three kinds of operand are now distinguished explicitly, we no
longer need fiddly code to disambiguate counter IDs and expression IDs based on
the total number of counters/expressions in a function.
This does increase the size of operands from 4 bytes to 8 bytes, but that
shouldn't be a big deal since they are mostly stored inside boxed structures,
and the current coverage code is not particularly size-optimized anyway.
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The actual motivation here is to prevent `rustfmt` from suddenly reformatting
these enum variants onto a single line, when they become slightly shorter in
the future.
But there's no harm in adding some helpful documentation at the same time.
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Remove the `..` from the body, only a few invocations used it and it's
inconsistent with rust syntax.
Use `;` instead of `,` between consts. As the Rust syntax gods inteded.
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This removes the `custom` format functionality as its only user was
trivially migrated to using a normal format.
If a new use case for a custom formatting impl pops up, you can add it
back.
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Version 5 adds Branch Regions which are a prerequisite for branch coverage.
Version 6 can use the zeroth filename as prefix for other relative files.
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A colleague contacted me and asked why Rust's counters start at 1, when
Clangs appear to start at 0. There is a reason why Rust's internal
counters start at 1 (see the docs), and I tried to keep them consistent
when codegenned to LLVM's coverage mapping format. LLVM should be
tolerant of missing counters, but as my colleague pointed out,
`llvm-cov` will silently fail to generate a coverage report for a
function based on LLVM's assumption that the counters are 0-based.
See:
https://github.com/llvm/llvm-project/blob/main/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp#L170
Apparently, if, for example, a function has no branches, it would have
exactly 1 counter. `CounterValues.size()` would be 1, and (with the
1-based index), the counter ID would be 1. This would fail the check
and abort reporting coverage for the function.
It turns out that by correcting for this during coverage map generation,
by subtracting 1 from the Rust Counter ID (both when generating the
counter increment intrinsic call, and when adding counters to the map),
some uncovered functions (including in tests) now appear covered! This
corrects the coverage for a few tests!
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Found with https://github.com/est31/warnalyzer.
Dubious changes:
- Is anyone else using rustc_apfloat? I feel weird completely deleting
x87 support.
- Maybe some of the dead code in rustc_data_structures, in case someone
wants to use it in the future?
- Don't change rustc_serialize
I plan to scrap most of the json module in the near future (see
https://github.com/rust-lang/compiler-team/issues/418) and fixing the
tests needed more work than I expected.
TODO: check if any of the comments on the deleted code should be kept.
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The angle brackets were confusing my IDE and I thought they were unnecessary. I was wrong.
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Working expression optimization, and some improvements to branch-level source coverage
This replaces PR #78040 after reorganizing the original commits (by request) into a more logical sequence of major changes.
Most of the work is in the MIR `transform/coverage/` directory (originally, `transform/instrument_coverage.rs`).
Note this PR includes some significant additional debugging capabilities, to help myself and any future developer working on coverage improvements or issues.
In particular, there's a new Graphviz (.dot file) output for the coverage graph (the `BasicCoverageBlock` control flow graph) that provides ways to get some very good insight into the relationships between the MIR, the coverage graph BCBs, coverage spans, and counters. (There are also some cool debugging options, available via environment variable, to alter how some data in the graph appears.)
And the code for this Graphviz view is actually generic... it can be used by any implementation of the Rust `Graph` traits.
Finally (for now), I also now output information from `llvm-cov` that shows the actual counters and spans it found in the coverage map, and their counts (from the `--debug` flag). I found this to be enormously helpful in debugging some coverage issues, so I kept it in the test results as well for additional context.
`@tmandry` `@wesleywiser`
r? `@tmandry`
Here's an example of the new coverage graph:
* Within each `BasicCoverageBlock` (BCB), you can see each `CoverageSpan` and its contributing statements (MIR `Statement`s and/or `Terminator`s)
* Each `CoverageSpan` has a `Counter` or and `Expression`, and `Expression`s show their Add/Subtract operation with nested operations. (This can be changed to show the Counter and Expression IDs instead, or in addition to, the BCB.)
* The terminators of all MIR `BasicBlock`s in the BCB, including one final `Terminator`
* If an "edge counter" is required (because we need to count an edge between blocks, in some cases) the edge's Counter or Expression is shown next to its label. (Not shown in the example below.) (FYI, Edge Counters are converted into a new MIR `BasicBlock` with `Goto`)
<img width="1116" alt="Screen Shot 2020-10-17 at 12 23 29 AM" src="https://user-images.githubusercontent.com/3827298/96331095-616cb480-100f-11eb-8212-60f2d433e2d8.png">
r? `@tmandry`
FYI: `@wesleywiser`
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