[PATCH] D56151: [DebugInfo] PR40010: Avoid register coalesing altering DBG_VALUE valuations

Thu Jun 27 02:35:59 PDT 2019

bjope added a comment.

LGTM as well.

I'll also share some stats after running some tests last night:

Compiled 4458 C files (using our DSP-C frontend and customized DSP backend).
Without this patch we get 1791988 "DEBUG_VALUE" comments in the resulting .s files, with the patch 1791453 (-535).
By doing a simple diff between all the .s files it seems like 753 old DEBUG_VALUE comments have been replaced by 218 new ones. And out of the new ones 183 now got "undef" as the value.

I haven't looked through all of those in detail (and I do not intend to do it either right now, not always easy to map things back to the source code to understand if it is "more correct" now or not).
What I've seen is that there are a few cases where I believe the old DEBUG_VALUE was correct and now we get "undef", and some cases where it looks like we would get faulty debug info without this patch.

FWIW, this analysis was of course not perfect when it comes to comparing the quality of debug info, but it gives some kind of measure.
My feeling is that the cases where we now lose some debug info is quite rare (at least for our DSP-C/DSP target), and the benefit of making the register coalescers handling of DBG_VALUE more sound outweigh that problem.

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https://reviews.llvm.org/D56151