tree f09b172e568941cf57a6de36058eef4fc780fc05
parent 77a641f6d5550a466c4bc161fab37bb427ebad14
author alx32 <103613512+alx32@users.noreply.github.com> 1710781698 -0700
committer Copybara-Service <copybara-worker@google.com> 1710781874 -0700

[lld-macho] Implement ObjC category merging (-objc_category_merging) (#82928)

This change adds a flag to lld to enable category merging for MachoO +
ObjC.
It adds the '-objc_category_merging' flag for enabling this option and
uses the existing '-no_objc_category_merging' flag for disabling it.
In ld64, this optimization is enabled by default, but in lld, for now,
we require explicitly passing the '-objc_category_merging' flag in order
to enable it.

Behavior: if in the same link unit, multiple categories are extending
the same class, then they get merged into a single category.
Ex: `Cat1(method1+method2,protocol1) + Cat2(method3+method4,protocol2,
property1) = Cat1_2(method1+method2+method3+method4,
protocol1+protocol2, property1)`

Notes on implementation decisions made in this diff:
1. There is a possibility to further improve the current implementation
by directly merging the category data into the base class (if the base
class is present in the link unit) - this improvement may be done as a
follow-up. This improved functionality is already present in ld64.
2. We do the merging on the raw inputSections - after dead-stripping
(categories can't be dead stripped anyway).
3. The changes are mostly self-contained to ObjC.cpp, except for adding
a new flag (linkerOptimizeReason) to ConcatInputSection and StringPiece
to mark that this data has been optimized away. Another way to do it
would have been to just mark the pieces as not 'live' but this would
cause the old symbols to show up in the linker map as being
dead-stripped - even if dead-stripping is disabled. This flag allows us
to match the ld64 behavior.

---------

Co-authored-by: Alex B <alexborcan@meta.com>
GitOrigin-RevId: ece2903ce730392e5236d27f1f387fa8067fcb1b