[llvm-dev] gcov vs llvm coverage mapping on line execution counts

[Vedant Kumar via llvm-dev]

The coverage mapping for a function contains a list of nested regions that bears a rough resemblance to the AST. Each region has a counter expression: the expression is either a reference to a counter from the profile, or an arithmetic expression which recursively references other expressions. During IRGen for a statement, the compiler looks up the counter for the statements AST node and emits an increment for it. In effect, after profile collection, all counter expressions which transitively reference that underlying counter are updated at once.

vedant


> On May 13, 2020, at 10:30 PM, Fangrui Song via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> (Quick example: gcc a.c --coverage; ./a.out; gcov a. => a.c.gcov)
> gcov computes line execution counts with this approach:
> 
> https://github.com/gcc-mirror/gcc/blob/master/gcc/gcov.c#L2684-L2690
>      /* The user expects the line count to be the number of times
>     a line has been executed.  Simply summing the block count
>     will give an artificially high number.  The Right Thing
>     is to sum the entry counts to the graph of blocks on this
>     line, then find the elementary cycles of the local graph
>     and add the transition counts of those cycles.  */
> 
> i.e. count from blocks outside the line + cycle count
> 
> cycle count is convoluted:
> 
> * enumerate cycles using "Finding all the elementary circuits of a directed graph." (Donald B. Johnson)
>  (gcov.c:circuit); O((V+E)*c) (unfortunately the implementation is slower: O((V+E)*V*c) where V: vertices; E: edges; c: cycles
> * for each cycle, handle_cycle() computes the minimum edge and add that to the cycle count
>  (I don't fully understand its cs_count.)
> * (For the curious, its path_contains_zero_or_negative_cycle_arc is strange: http://gcc.gnu.org/PR91601)
> 
> 
> llvm coverage mapping uses a very different approach (https://github.com/llvm/llvm-project/blob/master/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp#L769)
> How do we compute ExecutionCount?
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