[llvm] [llvm-cov] Add failing test for gap region line coverage bug (PR #203740)

[Maksim Levental via llvm-commits]

https://github.com/makslevental updated https://github.com/llvm/llvm-project/pull/203740

>From 49028574a85e2eab50db04cd24b5942c2e1c099b Mon Sep 17 00:00:00 2001
From: makslevental <maksim.levental at gmail.com>
Date: Sat, 13 Jun 2026 16:33:04 -0700
Subject: [PATCH] [llvm-cov] Add failing test for gap region line coverage bug
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

LineCoverageStats incorrectly reports lines as uncovered when a gap
region with count=0 wraps into a line that has non-entry segments
with count > 0.

The test uses a minimal reproducer: a scoped block with a never-taken
early return followed by a statement. The closing "}" produces a gap
region that wraps to the next line, suppressing its execution count.
The extra statement after the if-block is required — without it,
clang emits a region entry (MinRegionCount > 0) and the bug doesn't
trigger.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply at anthropic.com>
---
 .../Inputs/gap-region-quirk/gap-quirk-v2.cpp  |  23 ++++++++++++
 .../gap-region-quirk/gap-quirk.covmapping     | Bin 0 -> 309 bytes
 .../gap-region-quirk/gap-quirk.profdata       | Bin 0 -> 720 bytes
 .../llvm-cov/gap-region-line-coverage.test    |  34 ++++++++++++++++++
 4 files changed, 57 insertions(+)
 create mode 100644 llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk-v2.cpp
 create mode 100644 llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk.covmapping
 create mode 100644 llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk.profdata
 create mode 100644 llvm/test/tools/llvm-cov/gap-region-line-coverage.test

diff --git a/llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk-v2.cpp b/llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk-v2.cpp
new file mode 100644
index 0000000000000..f235382db64e8
--- /dev/null
+++ b/llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk-v2.cpp
@@ -0,0 +1,23 @@
+// A scoped block with a never-taken early return followed by at least one
+// statement is needed to produce the gap region pattern. Without the extra
+// statement after the if-block, clang emits a region entry on the line after
+// the closing "}" (MinRegionCount > 0) and the bug doesn't trigger. With the
+// extra statement, the closing "}" produces a gap region that wraps forward,
+// and the next line has only a non-entry segment (MinRegionCount == 0).
+bool process(bool err) {
+  {
+    if (err) {
+      return false;
+    }
+    int fd = 1;
+    (void)fd;
+  }
+  int result = 42;
+  (void)result;
+  return true;
+}
+
+int main() {
+  process(false);
+  return 0;
+}
diff --git a/llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk.covmapping b/llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk.covmapping
new file mode 100644
index 0000000000000000000000000000000000000000..993a744bcc67ae6e29cd152c9525a675d39fb7c5
GIT binary patch
literal 309
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literal 0
HcmV?d00001

diff --git a/llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk.profdata b/llvm/test/tools/llvm-cov/Inputs/gap-region-quirk/gap-quirk.profdata
new file mode 100644
index 0000000000000000000000000000000000000000..3acc509754fcf1cce00538e0b8d6f33f2a924602
GIT binary patch
literal 720
zcmeyLQ&5zjmf6V5fE}!0LKQ!N#y<h&vq6;!KxvpVW+<N#jfQd=VCn?aVIrvXf;N~=
zRB;Dhn8~Q(4<@sq3P5=cQZN^yN-Hd62MfS#fw|{^Iww}~gmqk4#TiU_u!=9(4GCFv
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L4j2Qb9!diMX9^?n

literal 0
HcmV?d00001

diff --git a/llvm/test/tools/llvm-cov/gap-region-line-coverage.test b/llvm/test/tools/llvm-cov/gap-region-line-coverage.test
new file mode 100644
index 0000000000000..8e93ef9609895
--- /dev/null
+++ b/llvm/test/tools/llvm-cov/gap-region-line-coverage.test
@@ -0,0 +1,34 @@
+// Tests that LineCoverageStats correctly handles lines where a gap region
+// with count=0 wraps into a line that has a non-entry segment with count > 0.
+//
+// In this test, line 15 ("int result = 42;") follows a scoped block containing
+// a never-taken early return. The gap region from the block's closing "}" wraps
+// to line 15, but line 15 has a non-entry segment with count=1 (the function
+// body continues). LineCoverageStats should report count=1 for line 15.
+//
+// Test inputs generated with:
+//   %clang -fprofile-instr-generate -fcoverage-mapping -O0 \
+//       -c -o gap-quirk.o Inputs/gap-region-quirk/gap-quirk-v2.cpp
+//   llvm-cov convert-for-testing gap-quirk.o -o gap-quirk.covmapping
+//   %clang -fprofile-instr-generate -fcoverage-mapping -O0 \
+//       -o gap-quirk Inputs/gap-region-quirk/gap-quirk-v2.cpp
+//   LLVM_PROFILE_FILE=gap-quirk.profraw ./gap-quirk
+//   llvm-profdata merge --sparse gap-quirk.profraw -o gap-quirk.profdata
+
+// RUN: llvm-cov show %S/Inputs/gap-region-quirk/gap-quirk.covmapping \
+// RUN:   -instr-profile %S/Inputs/gap-region-quirk/gap-quirk.profdata \
+// RUN:   -path-equivalence=/tmp,%S/Inputs/gap-region-quirk \
+// RUN:   2>&1 | FileCheck -check-prefix=SHOW %s
+
+// Line 15 ("int result = 42;") should show count 1, not 0.
+// SHOW:     15|      1|  int result = 42;
+
+// RUN: llvm-cov report %S/Inputs/gap-region-quirk/gap-quirk.covmapping \
+// RUN:   -instr-profile %S/Inputs/gap-region-quirk/gap-quirk.profdata \
+// RUN:   -path-equivalence=/tmp,%S/Inputs/gap-region-quirk \
+// RUN:   --show-branch-summary=false \
+// RUN:   2>&1 | FileCheck -check-prefix=REPORT %s
+
+// Line 15 should NOT be counted as missed.
+// REPORT: TOTAL
+// REPORT-SAME: 2