[PATCH] Initial code coverage mapping data structures, and reader and writers + C interface for ProfileData library

[Philip Reames]

On 06/26/2014 09:43 AM, Alex L wrote:
>
>
>
> 2014-06-25 17:57 GMT-07:00 Eric Christopher <echristo at gmail.com 
> <mailto:echristo at gmail.com>>:
>
>     So, any reason why we want the new coverage system? You didn't seem to
>     highlight what you saw wrong with gcov or the way it works.
>
>     I saw your original mail and it didn't have a lot of motivation here.
>
>     Thanks.
>
>     -eric
>
>
> I am not very familiar with GCOV, but the primary motivation for the 
> new coverage system is to provide very accurate execution counts for a 
> program. This would enable us to provide modern, high quality tools 
> for code coverage analysis.
>
> By very accurate I mean that instead of reasoning about code coverage 
> for basic blocks, branches and even lines, we would be able to reason 
> about code coverage for the regions of source code that resemble the 
> corresponding AST. This would enable the coverage tool to locate and 
> mark the exact regions of code that weren't executed, even if the IR 
> for those regions was transformed by the optimizer. I think that GCOV 
> fails to provide coverage for certain constructs when optimizations 
> are enabled. Also, I think that GCOV doesn't really show good coverage 
> for the lines that have multiple regions with different execution 
> counts, and the new system will enable us to create a tool which will 
> have a better way to deal with this particular situation.
>
> Also, the GCOV way produces separate mapping files and counter files 
> for each source file/object file, which can be somewhat inconvenient. 
> In the new system we pack the mapping data into the generated IR and 
> allow it to be merged by the linker, and as a result of that all our 
> mapping information is embedded inside the program's executable.
>
> The new coverage tool will be able to provide a more interactive 
> experience as well, by showing reports or code coverage only for 
> selected items like certain functions, classes, etc.
>
> Also, this new coverage system will provide a library which various 
> code coverage tools can use to make coverage reports without the need 
> to parse the output of GCOV.
I find nothing in the above explanation to justify including this code 
*in LLVM*.  Such a tool may be useful, but this really sounds like an 
independent project (i.e. "replace gcov").

Glancing through the code, I also see no real interaction with LLVM.  
This really seems like an independent profiling library which could be 
used to provide profiling data to LLVM, but is otherwise unrelated.  
Correct me if I'm wrong here - I did a *very* quick scan.

Your post about this topic on llvm-dev has not generated any consensus.  
If anything, there seems to be an active disinterest in your proposal.

Given the above, I would oppose the inclusion of this change set.


>     On Wed, Jun 25, 2014 at 4:32 PM, Alex L <arphaman at gmail.com
>     <mailto:arphaman at gmail.com>> wrote:
>     > Hi everyone,
>     > This is a first patch that implements the data structures,
>     readers and
>     > writers used by the new code coverage system. I added the new
>     code to the
>     > ProfileData library. I also added a very minimal C api for the
>     ProfileData
>     > library.
>     >
>     > http://reviews.llvm.org/D4301
>     >
>     >
>     >
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