[Lldb-commits] [PATCH] D78489: [lldb/DWARF] Trust CU DW_AT_low/high_pc information when building address tables

Greg Clayton via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Wed Apr 22 16:20:56 PDT 2020

clayborg accepted this revision.
clayborg added a comment.
This revision is now accepted and ready to land.

In D78489#1996834 <https://reviews.llvm.org/D78489#1996834>, @labath wrote:

> In D78489#1995837 <https://reviews.llvm.org/D78489#1995837>, @clayborg wrote:
> > Sounds like a win then, as long as we don't get slower I am fine with this. I was guessing that line tables might be faster because it is generally very compressed and compact compared to debug info, but thanks for verifying.
> >
> > Might be worth checking the memory consumption of LLDB quickly too with DW_AT_ranges compiled out and just make sure we don't take up too much extra memory.
> I've done a similar benchmark to the last one, but measured memory usage ("RssAnon" as reported by linux). One notable difference is that I
> Currently (information retrieved from DW_AT_ranges) we use about ~330 MB of memory. If I switch to dwarf dies as the source, the memory goes all the way to 2890 MB. This number is suspiciously large -- it either means that our die freeing is not working properly, or that glibc is very bad at releasing memory back to the OS. Given the magnitude of the increase, i think it's a little bit of both. With line tables as the source the memory usage is 706 MB. It's an increase from 330, but definitely smaller than 2.8 GB. (the number 330 is kind of misleading here since we're not considering removing that option -- it will always be used if it is available).

Since we mmap in the entire DWARF, I am not surprised by taking up new memory because we touch those pages and won't get those back. If you remove the DIE freeing code, I will bet you see much more memory used. We definitely free the memory for the DIEs and give that back, so I would be willing to bet the increase you are seeing is from mmap loading pages in that we touch.

>> We aren't doing anything to unload these line tables like we do with DIEs are we? It might make sense to pares the line tables and then throw them away if they were not already parsed?  With the DIEs we were throwing them away if they weren't already parsed to keep memory consumption down, so might be worth throwing the line tables away after running this if we are now going to rely on it.
> That would be possible, but I don't think it's worth the trouble. I think the phrase "relying on it" overemphasizes the importance of that code. In practice, the only time when this path will be taken is when debugging code built with clang<=3.3, which is seven years old and did not even fully implement c++11. It also seems like the switch to line tables will save memory, at least until the die freeing bug is fixed. And lastly, the difference i reported is pretty much the worst possible case, as the only thing the debugger will do is parse the line tables and exit. Once the debugger starts doing other stuff too, the difference will start to fade (e.g. running to the breakpoint in main increases the memory usage to 600 MB even with DW_AT_ranges).

Ok, thanks for looking into this.

>> One other thing to verify we want to go this route is to re-enable the old DIE parsing for high/low PCs, but put the results in a separate address ranges class. After we parse the line tables, verify that all ranges we found in the DIEs are in the line tables? It would not be great if we were going to start missing some functions if a function doesn't have a line table? Not sure if/how this would happen, but you never know.
> I implemented a check like that. While doing it I've learned that the DIE-based parsing does not work since May 2018 (D47275 <https://reviews.llvm.org/D47275>) and nobody noticed. What that means is that in practice we were always going through line tables (if DW_AT_ranges were missing). It also means that my times reported earlier for die-based searching were incorrect as they also included the time to build the line tables. (However, that does not change the ordering, as even if we subtract the time it took to parse the line tables, the DIE method is still much slower).
> After fixing the die-based search, I was able to verify that the die-based ranges are an exact match to the line table ranges (for the particular compiler used -- top of tree clang).
> Given all of the above (die-based searching being slow, taking more memory, and not actually working), i think it's pretty clear that we should remove it.

Fine with me. Thanks for taking the time to ensure we don't regress.

  rG LLVM Github Monorepo



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