Owen Anderson via llvm-dev
2021-Jan-22 19:30 UTC
[llvm-dev] [DebugInfo] The current status of debug values using multiple machine locations
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> > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>-------------- next part -------------- An HTML attachment was scrubbed... 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David Blaikie via llvm-dev
2021-Jan-22 19:45 UTC
[llvm-dev] [DebugInfo] The current status of debug values using multiple machine locations
It's hard to know the upper bound on what's possible.
eg: code like this:
int x = f1();
f2(x);
f2(4);
With optimized code, there's no way to recover the value of 'x'
during the
second f2 call. We can compute an absolute upper bound that's certainly
unreachable - by looking at the scope of variables (assuming our scope
tracking is perfect - which, it's not bad, but can get weird under
optimizations) and comparing total scope bytes of variables compared to the
bytes for which a location is described. We do have those two stats in
llvm-dwarfdump --statistics. But generally we don't bother looking at that
because it's a fair way off and the limitations of any such measurement as
I've described here. (we also don't currently track where a
variable's
scope starts - so the upper bound for "x" in "{ f1(); int x =
...; f1(); }"
includes both calls to f1, even though the location shouldn't ever extend
to cover the first f1 call)
On Fri, Jan 22, 2021 at 11:31 AM Owen Anderson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> 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
> [1] https://reviews.llvm.org/D82363
> [2] https://lists.llvm.org/pipermail/llvm-dev/2020-August/144589.html
> [3] https://reviews.llvm.org/D94631
> [4] https://llvm.org/docs/Benchmarking.html
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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