[LLVMdev] RFC - Adding an optimization report facility?

[Tobias Grosser]

On 03/06/2014 09:22 PM, Diego Novillo wrote:
> On Thu, Mar 6, 2014 at 2:44 PM, Tobias Grosser <tobias at grosser.es> wrote:
>
> as others already pointed out, I committed the first piece of such an
>> infrastructure in LLVM commit 202474 and clang commit 202475. This is
>> mostly the backend and printing infrastructure.
>>
>
>
> Thanks, Tobias.  I've browsed the two patches and I think they're going to
> be exactly what I need. IIUC, the patches add two new LLVM instructions
> remark and note. These are inserted in the IL by the passes and the
> compiler emits them as diagnostic if the right -W flag is enabled?

Close.

The patch adds one new LLVM diagnostic called 'remark'. 'note' has 
already been available to add extended information to warnings, but it 
could not appear independently. This still holds, but 'notes' can now be 
used to add additional information to remarks, too.

At the moment, remarks can similar to warnings be in a diagnostic group 
which is enabled with a '-Wgroupname' flag.

> One question I have from the Clang patch. If I compile with -Weverything,
> will this enable all warnings *and* remarks?

Yes, currently this enables all diagnostics, which means all warnings 
and remarks.

> In this context, I only want
> to enable all remarks.

I can see that. How to do this is really open for discussion. I could 
see us establishing two new flags '-Wall-remarks' for remarks and 
'-Wall-warnings' for warnings. Or, what I prefer, we could establish a 
new flag hierarchy e.g. with '-Reverything'.

> In fact, I want to enable a family of remarks: the
> optimization remarks.
>
> There will be other modifiers to these remarks as well:
>
>     1. Report *missed* optimizations, instead of the successful ones.
>     2. Increase verbosity of the report. This would be done using note
>     nodes, I expect. But we may want varying degrees of verbosity.
>     3. Group families of optimizations. For example, i want to report on all
>     loop-related optimizations.
>     4. IIRC, GCC's -fopt-info will also allow you to collect the reports
>     into a separate text file. Not sure how useful I find this feature myself.

5. Enable/Disable remarks with pragmas / per function

Yes, there are a lot of things to do here. The diagnostic infrastructure
already provides great features, e.g.:

   - Enabling/Disabling diagnostics with pragmas and per function
   - Nested diagnostic groups
   - Enabling a diagnostic group, but disabling individual diagnostics
     or this group.
   - Good integration in tools through libclang
   - ...

However, the command line interface might need to be improved before we 
start adding a large number of remarks. Some goals I see:

1) We want to reuse as much of the diagnostic infrastructure as possible
2) We do not want to increase the complexity of the warning flags too much
3) We still want to have a similar user interface as the warning flags 
provide
4) People raised interest in upgrading remarks to warnings or errors
    (I am personally not too sure about this one, but it already works
     with -Werror=groupname)

Several people (including Chris) warned to build up an additional 
complex diagnostic system on the side, but as Chandler pointed out, we 
can most likely reuse most of the existing infrastructure but could 
still adjust the user interface if needed.

I personally believe the best approach is to start small and let the 
design be driven by the intended use cases. If you are interested, I 
would propose to add a single diagnostic for the loop inliner using just 
the existing '-W' infrastructure e.g. with '-Winline-remarkers'.
This both increases test coverage of the exiting features and also
will help to get an idea of the needed features. At the same time, you 
could help reviewing my first patch on modularizing the clang diagnostic 
infrastructure. Again, something that may help to get an idea of where 
to head at this side.

Cheers,
Tobias