[llvm-cov] Add gap region between binary operator '&& and ||' and RHS #149085

int-zjt · 2025-07-16T12:13:35Z

Issue Summary

We identified an inaccuracy in line coverage reporting when short-circuit evaluation occurs in multi-line conditional expressions. Specifically:

Un-executed conditions following line breaks may be incorrectly marked as covered
(e.g., conditionB in a non-executed && chain shows coverage)

    1|       |#include <iostream>
    2|       |
    3|      1|int main() {
    4|      1|    bool conditionA = false;
    5|      1|    bool conditionB = true;
    6|      1|    if (conditionA &&
    7|      1|        conditionB) {
    8|      0|        std::cout << "IF-THEN" << std::endl;
    9|      0|    }
   10|      1|    return 0;
   11|      1|}

Inconsistent coverage reporting across un-executed conditions
(adjacent un-executed conditions may show 1 vs 0 line coverage)

    1|       |#include <iostream>
    2|       |
    3|      1|int main() {
    4|      1|    bool conditionA = false;
    5|      1|    bool conditionB = true;
    6|      1|    bool conditionC = true;
    7|      1|    if (conditionA &&
    8|      1|        (conditionB ||
    9|      0|        conditionC)) {
   10|      0|        std::cout << "IF-THEN" << std::endl;
   11|      0|    }
   12|      1|    return 0;
   13|      1|}

Root Cause Analysis

The current WrappedSegment mechanism may propagates coverage data from the last segment of the LHS line to subsequent RHS lines. Because the LHS line's coverage data depends on count of its segments and WrappedSegment's count, it may causes false positive coverage for un-executed RHS conditions.

Proposed Solution

The current implementation correctly handles single-statement if bodies through GapRegion insertion:

if (cond)       // Line 1
    statement;  // Line 2

Processing workflow:

A GapRegion is created spanning from the end of the condition (Line 1) to the start of the body statement (Line 2)
This GapRegion inherits the execution counter from Line 2
During coverage calculation:

Line 2 uses this GapRegion as its WrappedSegment
Prevents propagation of Line 1's coverage data to Line 2

Generalize the GapRegion insertion strategy from if-statements to logical operators, placing RHS-synchronized segments at LHS-line-end to isolate coverage contexts during short-circuit evaluation.

llvmbot · 2025-07-16T12:14:05Z

@llvm/pr-subscribers-pgo
@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-codegen

Author: None (int-zjt)

Changes

Issue Summary

We identified an inaccuracy in line coverage reporting when short-circuit evaluation occurs in multi-line conditional expressions. Specifically:

Un-executed conditions following line breaks may be incorrectly marked as covered
(e.g., conditionB in a non-executed && chain shows coverage)

    1|       |#include &lt;iostream&gt;
    2|       |
    3|      1|int main() {
    4|      1|    bool conditionA = false;
    5|      1|    bool conditionB = true;
    6|      1|    if (conditionA &amp;&amp;
    7|      1|        conditionB) {
    8|      0|        std::cout &lt;&lt; "IF-THEN" &lt;&lt; std::endl;
    9|      0|    }
   10|      1|    return 0;
   11|      1|}

Inconsistent coverage reporting across un-executed conditions
(adjacent un-executed conditions may show 1 vs 0 line coverage)

    1|       |#include &lt;iostream&gt;
    2|       |
    3|      1|int main() {
    4|      1|    bool conditionA = false;
    5|      1|    bool conditionB = true;
    6|      1|    bool conditionC = true;
    7|      1|    if (conditionA &amp;&amp;
    8|      1|        (conditionB ||
    9|      0|        conditionC)) {
   10|      0|        std::cout &lt;&lt; "IF-THEN" &lt;&lt; std::endl;
   11|      0|    }
   12|      1|    return 0;
   13|      1|}

Root Cause Analysis

The current WrappedSegment mechanism may propagates coverage data from the last segment of the LHS line to subsequent RHS lines. Because the LHS line's coverage data depends on count of its segments and WrappedSegment's count, it may causes false positive coverage for un-executed RHS conditions.

Proposed Solution

The current implementation correctly handles single-statement if bodies through GapRegion insertion:

if (cond)       // Line 1
    statement;  // Line 2

Processing workflow:

A GapRegion is created spanning from the end of the condition (Line 1) to the start of the body statement (Line 2)
This GapRegion inherits the execution counter from Line 2
During coverage calculation:

Line 2 uses this GapRegion as its WrappedSegment
Prevents propagation of Line 1's coverage data to Line 2

Generalize the GapRegion insertion strategy from if-statements to logical operators, placing RHS-synchronized segments at LHS-line-end to isolate coverage contexts during short-circuit evaluation.

Full diff: https://github.com/llvm/llvm-project/pull/149085.diff

2 Files Affected:

(modified) clang/lib/CodeGen/CoverageMappingGen.cpp (+10)
(added) compiler-rt/test/profile/Linux/coverage_short_circuit.cpp (+36)

diff --git a/clang/lib/CodeGen/CoverageMappingGen.cpp b/clang/lib/CodeGen/CoverageMappingGen.cpp
index 4aafac349e3e9..f55290a5feee6 100644
--- a/clang/lib/CodeGen/CoverageMappingGen.cpp
+++ b/clang/lib/CodeGen/CoverageMappingGen.cpp
@@ -2269,6 +2269,11 @@ struct CounterCoverageMappingBuilder
     // Track LHS True/False Decision.
     const auto DecisionLHS = MCDCBuilder.pop();
 
+    if (auto Gap =
+            findGapAreaBetween(getEnd(E->getLHS()), getStart(E->getRHS()))) {
+      fillGapAreaWithCount(Gap->getBegin(), Gap->getEnd(), getRegionCounter(E));
+    }
+
     // Counter tracks the right hand side of a logical and operator.
     extendRegion(E->getRHS());
     propagateCounts(getRegionCounter(E), E->getRHS());
@@ -2330,6 +2335,11 @@ struct CounterCoverageMappingBuilder
     // Track LHS True/False Decision.
     const auto DecisionLHS = MCDCBuilder.pop();
 
+    if (auto Gap =
+            findGapAreaBetween(getEnd(E->getLHS()), getStart(E->getRHS()))) {
+      fillGapAreaWithCount(Gap->getBegin(), Gap->getEnd(), getRegionCounter(E));
+    }
+
     // Counter tracks the right hand side of a logical or operator.
     extendRegion(E->getRHS());
     propagateCounts(getRegionCounter(E), E->getRHS());
diff --git a/compiler-rt/test/profile/Linux/coverage_short_circuit.cpp b/compiler-rt/test/profile/Linux/coverage_short_circuit.cpp
new file mode 100644
index 0000000000000..cc4022bc3c286
--- /dev/null
+++ b/compiler-rt/test/profile/Linux/coverage_short_circuit.cpp
@@ -0,0 +1,36 @@
+// RUN: %clangxx_profgen -std=c++17 -fuse-ld=lld -fcoverage-mapping -o %t %s
+// RUN: env LLVM_PROFILE_FILE=%t.profraw %run %t
+// RUN: llvm-profdata merge -o %t.profdata %t.profraw
+// RUN: llvm-cov show %t -instr-profile=%t.profdata 2>&1 | FileCheck %s
+
+void foo() {          // CHECK:       [[@LINE]]| 1|void foo() {
+  bool cond1 = false; // CHECK-NEXT:  [[@LINE]]| 1|  bool cond1 = false;
+  bool cond2 = true;  // CHECK-NEXT:  [[@LINE]]| 1|  bool cond2 = true;
+  if (cond1 &&        // CHECK-NEXT:  [[@LINE]]| 1|  if (cond1 &&
+      cond2) {        // CHECK-NEXT:  [[@LINE]]| 0|      cond2) {
+  }                   // CHECK-NEXT:  [[@LINE]]| 0|  }
+} // CHECK-NEXT:  [[@LINE]]| 1|}
+
+void bar() {          // CHECK:       [[@LINE]]| 1|void bar() {
+  bool cond1 = true;  // CHECK-NEXT:  [[@LINE]]| 1|  bool cond1 = true;
+  bool cond2 = false; // CHECK-NEXT:  [[@LINE]]| 1|  bool cond2 = false;
+  if (cond1 &&        // CHECK-NEXT:  [[@LINE]]| 1|  if (cond1 &&
+      cond2) {        // CHECK-NEXT:  [[@LINE]]| 1|      cond2) {
+  }                   // CHECK-NEXT:  [[@LINE]]| 0|  }
+} // CHECK-NEXT:  [[@LINE]]| 1|}
+
+void baz() {          // CHECK:       [[@LINE]]| 1|void baz() {
+  bool cond1 = false; // CHECK-NEXT:  [[@LINE]]| 1|  bool cond1 = false;
+  bool cond2 = true;  // CHECK-NEXT:  [[@LINE]]| 1|  bool cond2 = true;
+  if (cond1           // CHECK-NEXT:  [[@LINE]]| 1|  if (cond1
+      &&              // CHECK-NEXT:  [[@LINE]]| 0|      &&
+      cond2) {        // CHECK-NEXT:  [[@LINE]]| 0|      cond2) {
+  }                   // CHECK-NEXT:  [[@LINE]]| 0|  }
+} // CHECK-NEXT:  [[@LINE]]| 1|}
+
+int main() {
+  foo();
+  bar();
+  baz();
+  return 0;
+}

github-actions · 2025-07-16T12:16:01Z

✅ With the latest revision this PR passed the C/C++ code formatter.

evodius96 · 2025-07-16T15:51:12Z

Thank you! I assume this is only a problem with line coverage? In other words, region coverage is still correct when visualized.

int-zjt · 2025-07-17T02:43:13Z

Thank you! I assume this is only a problem with line coverage? In other words, region coverage is still correct when visualized.

Yeah, region coverage is still correct, just line coverage is wrong.

[llvm-cov] Add gap region after binary operator && and ||

763293b

llvmbot added clang Clang issues not falling into any other category compiler-rt clang:codegen IR generation bugs: mangling, exceptions, etc. PGO Profile Guided Optimizations labels Jul 16, 2025

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[llvm-cov] Add gap region between binary operator '&& and ||' and RHS #149085

[llvm-cov] Add gap region between binary operator '&& and ||' and RHS #149085

int-zjt commented Jul 16, 2025

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llvmbot commented Jul 16, 2025 •

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int-zjt commented Jul 17, 2025

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[llvm-cov] Add gap region between binary operator '&& and ||' and RHS #149085

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[llvm-cov] Add gap region between binary operator '&& and ||' and RHS #149085

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int-zjt commented Jul 16, 2025

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