[all-commits] [llvm/llvm-project] ba668e: Re-apply [lldb] Do not use LC_FUNCTION_STARTS data...

[Alex Langford via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ba668eb99c5dc37d3c5cf2775079562460fd7619
      https://github.com/llvm/llvm-project/commit/ba668eb99c5dc37d3c5cf2775079562460fd7619
  Author: Alex Langford <alangford at apple.com>
  Date:   2024-11-21 (Thu, 21 Nov 2024)

  Changed paths:
    M lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

  Log Message:
  -----------
  Re-apply [lldb] Do not use LC_FUNCTION_STARTS data to determine symbol size as symbols are created (#117079)

I backed this out due to a problem on one of the bots that myself and
others have problems reproducing locally. I'd like to try to land it
again, at least to gain more information.

Summary:
This improves the performance of ObjectFileMacho::ParseSymtab by
removing eager and expensive work in favor of doing it later in a
less-expensive fashion.

Experiment:
My goal was to understand LLDB's startup time.
First, I produced a Debug build of LLDB (no dSYM) and a
Release+NoAsserts build of LLDB. The Release build debugged the Debug
build as it debugged a small C++ program. I found that
ObjectFileMachO::ParseSymtab accounted for somewhere between 1.2 and 1.3
seconds consistently. After applying this change, I consistently
measured a reduction of approximately 100ms, putting the time closer to
1.1s and 1.2s on average.

Background:
ObjectFileMachO::ParseSymtab will incrementally create symbols by
parsing nlist entries from the symtab section of a MachO binary. As it
does this, it eagerly tries to determine the size of symbols (e.g. how
long a function is) using LC_FUNCTION_STARTS data (or eh_frame if
LC_FUNCTION_STARTS is unavailable). Concretely, this is done by
performing a binary search on the function starts array and calculating
the distance to the next function or the end of the section (whichever
is smaller).

However, this work is unnecessary for 2 reasons:
1. If you have debug symbol entries (i.e. STABs), the size of a function
is usually stored right after the function's entry. Performing this work
right before parsing the next entry is unnecessary work.
2. Calculating symbol sizes for symbols of size 0 is already performed
in `Symtab::InitAddressIndexes` after all the symbols are added to the
Symtab. It also does this more efficiently by walking over a list of
symbols sorted by address, so the work to calculate the size per symbol
is constant instead of O(log n).



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