[llvm-dev] Filter optimization remarks by the hotness of the code region

Mehdi Amini via llvm-dev llvm-dev at lists.llvm.org
Wed May 4 16:54:44 PDT 2016

I think it is a good idea, and it reminds me a discussion about Polly at the last llvm-dev meeting, where we considered limiting compile-time impact by running polly only the code that is deemed to be "hot".
There could be the same kind of logic for things like LoopVersioningLICMPass, or specific optimizations like maybe the vectorization:  if the remark is not relevant because the user should not care about this loop, why does the optimizer care in the first place?


> On May 4, 2016, at 11:12 AM, Adam Nemet via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> This idea came up a few times recently [1][2] so I’d like start prototyping it.  To summarize, we can emit optimization remarks using the -Rpass* options.  These are currently emitted by optimizations like vectorization[3], unrolling, inlining and since last week loop distribution.
> For large programs however this can amount to a lot of diagnostics output to sift through.  Filtering this by the hotness of the region can help to focus the user on performance opportunities that are likely to pay off.
> The approach I am thinking of taking is to install a wrapper as the diagnostics handler that will only forward to the original handler if the region of code is considered hot.  This will be installed by a new pass that will use BlockFrequencyInfo to determine the top N hot regions.
> This is at very early stage right now.  I would appreciate any feedback.
> Thanks,
> Adam
> [1] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098492.html
> [2] http://lists.llvm.org/pipermail/cfe-dev/2016-April/048526.html
> [3] http://blog.llvm.org/2014/11/loop-vectorization-diagnostics-and.html
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