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

Tue Apr 21 02:39:58 PDT 2020

labath added a comment.

Thanks for the tip about commenting out DW_AT_ranges code. I wanted to do some measurements, but I couldn't find a non-contrived non-trivial code base to try this on (with c++ most compile units always get DW_AT_ranges due to all of the ODR linkage brought in by templates). The results are very interesting. The test setup is: release (no assert) lldb debugging a debug clang with accelerator tables (to avoid debug info indexing overshadowing everything). I'm running `rel/lldb dbg/clang -o "image lookup -a 0x7234567" -b` as a benchmark, running it 10 times and averaging. The address is a random address inside clang's .text section.

With the current code, where we get this information via DW_AT_ranges from most compile units (only 30 CUs don't have it), the command takes 5.15 (+/- .04) seconds. With the DW_AT_ranges code commented out, where we get information by traversing the DIE tree, it takes 10.38 (+/- 0.05) seconds. With both DW_AT_ranges *and* DIE code removed (information retrieved via line tables), the time is 6.85 (+/- 0.03) seconds. These results make sense to me, as line tables are much simpler and easier to parse than DIE trees, and this makes it possible to avoid parsing a lot of compile units

So, not only does this remove code, and improve llvm consistency, it also makes things faster, if only for a limited amount of users (those building with clang<=3.3, or those building huge amounts of C code  -- the patch is nfc for the rest). Sounds like a win-win-win to me (?)

Repository:
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https://reviews.llvm.org/D78489