[all-commits] [llvm/llvm-project] c9bea5: [dsymutil] Fix O(n^2) behavior when running on ld6...

[Nico Weber via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c9bea51760015c122b57be832013d01ef1229b03
      https://github.com/llvm/llvm-project/commit/c9bea51760015c122b57be832013d01ef1229b03
  Author: Nico Weber <thakis at chromium.org>
  Date:   2022-04-06 (Wed, 06 Apr 2022)

  Changed paths:
    M llvm/tools/dsymutil/MachODebugMapParser.cpp

  Log Message:
  -----------
  [dsymutil] Fix O(n^2) behavior when running on ld64.lld's current ICF output

STABS information consists of a list of records in the linked binary
that look like this:

  OSO: path/to/some.o
  SO: path/to/some.c
  FUN: sym1
  FUN: sym2
  ...

The linked binary has one such set of records for every .o file linked
into it.

When dsymutil processes the binary's STABS information, it:

1. Reads the .o file mentioned in the OSO line
2. For each FUN entry after it in the main executable's STABS info:
  a) it looks up that symbol in the symbol of that .o file
  b) if it doesn't find it there, it goes through all symbols in the
     main binary at the same address and sees if any of those match

With ICF, ld64.lld's STABS output claims that all identical functions
that were folded are in the .o file of the one that's deemed the
canonical one. Many small functions might be folded into a single
function, so there are .o OSO entries that end up with many FUN lines,
but almost none of them exist in the .o file's symbol table.

Previously, dsymutil would do a full scan of all symbols in the main
executable _for every of these entries_.

This patch instead scans all aliases once and remembers them per name.
This reduces the alias resolution complexity from
O(number_of_aliases_in_o_file * number_of_symbols_in_main_executable) to
O(number_of_aliases_in_o_file * log(number_of_aliases_in_o_file)).

In practice, it reduces the time spent to run dsymutil on
Chromium Framework from 26 min (after https://reviews.llvm.org/D89444)
or 12 min (before https://reviews.llvm.org/D89444) to ~8m30s.

We probably want to change how ld64.lld writes STABS entries when ICF
is enabled, but making dsymutil not have pathological performance for
this input seems like a good change as well.

No expected behavior change (other than it's faster). I verified that
for Chromium Framework, the generated .dSYM is identical with and
without this patch.

Differential Revision: https://reviews.llvm.org/D123218