[llvm-dev] [DebugInfo] The current status of debug values using multiple machine locations

Fri Jan 22 11:30:53 PST 2021

Hi Stephen,

Is it possible to quantify this coverage in absolute terms, at least the PC bytes portion? It would be helpful to understand how close this is bringing us to 100% coverage, for example.

—Owen

> On Jan 22, 2021, at 7:23 AM, via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Following the previous discussion on the mailing list[0], I have been writing a series of patches that implement the proposed instructions[1], enabling multi-location debug values (debug value lists) in LLVM. Although the patches are still in review, the basic implementation is finished (except for the removal and replacement of the old DBG_VALUE instruction, as discussed on the mailing list[2]).
> 
> Given below is the change in debug info output for the LLVM test-suite CTMark, from before and after the debug value list patch:
> Project               Available variables           PC bytes covered        
>                       Old     New  Change         Old       New  Change
> 7zip                40252   40501   0.62%     7112336   7142255   0.42%
> bullet              32655   33296   1.96%     6272034   6323049   0.81%
> ClamAV               8795    8842   0.53%     5090634   5099634   0.18%
> consumer-typeset     4354    4356   0.05%     3171498   3171605   0.00%
> kimwitu++           30006   30177   0.57%     1736826   1755152   1.06%
> lencod              14176   14319   1.01%     6123957   6177106   0.87%
> mafft                6854    6859   0.07%    12045196  12046744   0.01%
> SPASS               38477   38492   0.04%     3396246   3399668   0.10%
> sqlite3             29479   30301   2.79%     7964547   8024747   0.76%
> tramp3d-v4          91732  105588  15.10%     7925131   8106167   2.28%
> As most of the patches have been approved, I am hopeful that the full set of patches will be merged into main in the near future. Part of the purpose of this email is to give notice of the upcoming merge, as the changes are significant and may conflict with any private changes concerning debug info. In terms of output, the patch should not change any existing variable locations; it should only add new locations for some variables. This may break tests that expect certain variables to be missing or optimized out, but should not be disruptive otherwise. If you want to test this patch, either to benchmark compiler performance, gather DWARF statistics, or test its merging with private changes, there is a single patch comprising the entirety of the current work on Phabricator[3].
> 
> The other purpose of this email is to request further reviews on the patches, as all but 5 have been accepted and most of the remaining patches have been well-reviewed by now. Due to the size of the patches, there will likely be conflicts between any in-development debug-info work and these patches, creating extra work for any developers that need to update their patches to handle the new instruction. It will also allow current and future work to take advantage of the new functionality to preserve more debug information.
> 
> With the patch implementations essentially complete, we can see more precisely the effect of these patches. With respect to the results above, it is important to note that although this patch is functional it does not cover all the potential ground of this feature. The only direct improvement added by this patch is enabling the salvage of non-constant binary operator and GetElementPtr instructions within the existing salvage function during opt passes. This is a significant improvement, but more may come: follow-up patches can enable this improved salvaging during instruction selection, and enable the salvage of cmp and select instructions; there are also some gaps in the instruction selection implementation, such as resolving dangling debug value lists. Some of these are easy wins that can be implemented immediately after landing the patch, and some are longer term projects that can progress when this work has merged.
> 
> The patch currently leads to a medium-sized improvement for most cases, with some very small and one very large improvement. As mentioned previously, this set of patches primarily lays the groundwork for more complex variable locations; once it has landed there will be further work to salvage from more places, as well as improving the handling of list dbg.values to better preserve variable locations through optimizations. Compilation times do not appear to be significantly affected; I've been measuring compile times for the CTMark projects in the llvm test-suite (using the best practices from the benchmarking guide[4]), but so far any change in compile time is much smaller than the measurement noise, so I don't have an exact number to give. I estimate from the results so far that the increase will be no more than 1% in the worst case, but it could be smaller - I'm testing further to verify.
> 
> [0] https://lists.llvm.org/pipermail/llvm-dev/2020-February/139376.html <https://lists.llvm.org/pipermail/llvm-dev/2020-February/139376.html>
> [1] https://reviews.llvm.org/D82363 <https://reviews.llvm.org/D82363>
> [2] https://lists.llvm.org/pipermail/llvm-dev/2020-August/144589.html <https://lists.llvm.org/pipermail/llvm-dev/2020-August/144589.html>
> [3] https://reviews.llvm.org/D94631 <https://reviews.llvm.org/D94631>
> [4] https://llvm.org/docs/Benchmarking.html <https://llvm.org/docs/Benchmarking.html>
> 
> 
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