[PATCH] D48653: [XRay][compiler-rt] xray::Array Freelist and Iterator Updates

[Dean Michael Berris via Phabricator via llvm-commits]

dberris created this revision.
dberris added a reviewer: kpw.
Herald added a subscriber: mgorny.

We found a bug while working on a benchmark for the profiling mode which
manifests as a segmentation fault in the profiling handler's
implementation. This change adds unit tests which replicate the
issues in isolation.

We've tracked this down as a bug in the implementation of the Freelist
in the `xray::Array` type. This happens when we trim the array by a
number of elements, where we've been incorrectly assigning pointers for
the links in the freelist of chunk nodes. We've taken the chance to add
more debug-only assertions to the code path and allow us to verify these
assumptions in debug builds.

In the process, we also took the opportunity to use iterators to
implement both `front()` and `back()` which exposes a bug in the
iterator decrement operation.  In particular, when we decrement past a
chunk size boundary, we end up moving too far back and reaching the
`SentinelChunk` prematurely.

This change unblocks us to allow for contributing the non-crashing
version of the benchmarks in the test-suite as well.


https://reviews.llvm.org/D48653

Files:
  compiler-rt/lib/xray/tests/CMakeLists.txt
  compiler-rt/lib/xray/tests/unit/function_call_trie_test.cc
  compiler-rt/lib/xray/tests/unit/segmented_array_test.cc
  compiler-rt/lib/xray/xray_segmented_array.h

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