zequanwu added inline comments.
================
Comment at: llvm/lib/ProfileData/Coverage/CoverageMapping.cpp:597
const auto &R = Segments[I];
- if (!(L.Line < R.Line) && !(L.Line == R.Line && L.Col < R.Col)) {
+ if (!(L.Line < R.Line) && !(L.Line == R.Line && L.Col <= R.Col)) {
LLVM_DEBUG(dbgs() << " ! Segment " << L.Line << ":" << L.Col
----------------
vsk wrote:
> Could you keep the original check and simply 'continue' if `L.Col == R.Col &&
> L.IsSkipped`?
`if (L.Line == R.Line && L.Col == R.Col && !L.HasCount)` is more specific.
================
Comment at: llvm/lib/ProfileData/Coverage/CoverageMapping.cpp:483
bool GapRegion = CR.value().Kind == CounterMappingRegion::GapRegion;
if (CR.index() + 1 == Regions.size() ||
----------------
vsk wrote:
> zequanwu wrote:
> > vsk wrote:
> > > zequanwu wrote:
> > > > vsk wrote:
> > > > > zequanwu wrote:
> > > > > > vsk wrote:
> > > > > > > Why is this deletion necessary? Do we need to introduce 0-length
> > > > > > > regions which alter the count? An example would help.
> > > > > > Because a single empty line will be a 0 length region. I don't know
> > > > > > why is this condition necessary before. Does zero-length region
> > > > > > exists before this change?
> > > > > >
> > > > > > example:
> > > > > > ```
> > > > > > int main() {
> > > > > >
> > > > > > return 0;
> > > > > > }
> > > > > > ```
> > > > > > Before, llvm-cov gives the following.
> > > > > > ```
> > > > > > Counter in file 0 1:12 -> 4:2, #0
> > > > > > Counter in file 0 2:1 -> 2:1, 0
> > > > > > Emitting segments for file: /tmp/a.c
> > > > > > Combined regions:
> > > > > > 1:12 -> 4:2 (count=1)
> > > > > > 2:1 -> 2:1 (count=0)
> > > > > > Segment at 1:12 (count = 1), RegionEntry
> > > > > > Segment at 2:1 (count = 0), RegionEntry, Skipped
> > > > > > Segment at 4:2 (count = 0), Skipped
> > > > > > 1| 1|int main() {
> > > > > > 2| |
> > > > > > 3| | return 0;
> > > > > > 4| |}
> > > > > > ```
> > > > > > After:
> > > > > > ```
> > > > > > Counter in file 0 1:12 -> 4:2, #0
> > > > > > Counter in file 0 2:1 -> 2:1, 0
> > > > > > Emitting segments for file: /tmp/a.c
> > > > > > Combined regions:
> > > > > > 1:12 -> 4:2 (count=1)
> > > > > > 2:1 -> 2:1 (count=0)
> > > > > > Segment at 1:12 (count = 1), RegionEntry
> > > > > > Segment at 2:1 (count = 0), RegionEntry, Skipped
> > > > > > Segment at 2:1 (count = 1)
> > > > > > Segment at 4:2 (count = 0), Skipped
> > > > > > 1| 1|int main() {
> > > > > > 2| |
> > > > > > 3| 1| return 0;
> > > > > > 4| 1|}
> > > > > > ```
> > > > > It looks like we do occasionally see 0-length regions, possibly due
> > > > > to bugs in the frontend
> > > > > (http://lab.llvm.org:8080/coverage/coverage-reports/coverage/Users/buildslave/jenkins/workspace/coverage/llvm-project/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp.html#L485).
> > > > >
> > > > > I don't expect the reporting tools to be able to handle duplicate
> > > > > segments in a robust way. Having duplicate segments was the source of
> > > > > "messed up" coverage bugs in the past, due to the contradiction
> > > > > inherent in having two different segments begin at the same source
> > > > > location.
> > > > >
> > > > > Do you see some other way to represent empty lines? Perhaps, if we
> > > > > emit a skipped region for an empty line, we can emit a follow-up
> > > > > segment that restores the previously-active region starting on the
> > > > > next line? So in this case:
> > > > >
> > > > > Segment at 1:12 (count = 1), RegionEntry
> > > > > Segment at 2:1 (count = 0), RegionEntry, Skipped
> > > > > Segment at 3:1 (count = 1)
> > > > > Segment at 4:2 (count = 0), Skipped
> > > > I think we should have the following, because the wrapped segment's
> > > > count will be used in next line (e.g. line 3).
> > > > ```
> > > > Segment at 1:12 (count = 1), RegionEntry
> > > > Segment at 2:1 (count = 0), RegionEntry, Skipped
> > > > Segment at 2:1 (count = 1)
> > > > Segment at 4:2 (count = 0), Skipped
> > > > ```
> > > I think one issue with that output is that it contains two segments that
> > > start at the same location (2:1). Historically, this sort of duplication
> > > was a source of bugs/inconsistencies (e.g two entry regions beginning at
> > > the same location with different counts), and I’m concerned that
> > > re-allowing multiple segments with the same start location could lead to
> > > regressions down the road.
> > >
> > > OTOH, your change is consistent with how non-zero length segments are
> > > handled, and it could be fragile to look for an alternative start
> > > location (like 3:1) that restores the count from before the skipped
> > > region.
> > >
> > > I’d be curious to know your thoughts on how to prevent regressions
> > > related to segments which share the same start location. Maybe they could
> > > only be allowed in this limited case?
> > Yes, they are two segments with the same location, but only the first is a
> > RegionEntry (might not cause the same bug you saw before). So the second
> > one is only used as wrapped segment, and will not conflict with first
> > segment RegionEntry.
> > We could only emit the second segment when the first segment is skipped
> > segment.
> That carveout sounds reasonable to me.
Since this broke a unit test, which has 5 zero-length regions points to same
location, I think it's reasonable only emitting the second segment when active
regions is not empty.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D84988/new/
https://reviews.llvm.org/D84988
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