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Diffstat (limited to 'clang/docs/SourceBasedCodeCoverage.rst')
| -rw-r--r-- | clang/docs/SourceBasedCodeCoverage.rst | 29 |
1 files changed, 25 insertions, 4 deletions
diff --git a/clang/docs/SourceBasedCodeCoverage.rst b/clang/docs/SourceBasedCodeCoverage.rst index cee706289284..73910e134a58 100644 --- a/clang/docs/SourceBasedCodeCoverage.rst +++ b/clang/docs/SourceBasedCodeCoverage.rst @@ -484,10 +484,31 @@ MC/DC Instrumentation --------------------- When instrumenting for Modified Condition/Decision Coverage (MC/DC) using the -clang option ``-fcoverage-mcdc``, users are limited to at most **six** leaf-level -conditions in a boolean expression. A warning will be generated for boolean -expressions that contain more than six, and they will not be instrumented for -MC/DC. +clang option ``-fcoverage-mcdc``, there are two hard limits. + +The maximum number of terms is limited to 32767, which is practical for +handwritten expressions. To be more restrictive in order to enforce coding rules, +use ``-Xclang -fmcdc-max-conditions=n``. Expressions with exceeded condition +counts ``n`` will generate warnings and will be excluded in the MC/DC coverage. + +The number of test vectors (the maximum number of possible combinations of +expressions) is limited to 2,147,483,646. In this case, approximately +256MiB (==2GiB/8) is used to record test vectors. + +To reduce memory usage, users can limit the maximum number of test vectors per +expression with ``-Xclang -fmcdc-max-test-vectors=m``. +If the number of test vectors resulting from the analysis of an expression +exceeds ``m``, a warning will be issued and the expression will be excluded +from the MC/DC coverage. + +The number of test vectors ``m``, for ``n`` terms in an expression, can be +``m <= 2^n`` in the theoretical worst case, but is usually much smaller. +In simple cases, such as expressions consisting of a sequence of single +operators, ``m == n+1``. For example, ``(a && b && c && d && e && f && g)`` +requires 8 test vectors. + +Expressions such as ``((a0 && b0) || (a1 && b1) || ...)`` can cause the +number of test vectors to increase exponentially. Also, if a boolean expression is embedded in the nest of another boolean expression but separated by a non-logical operator, this is also not supported. |