https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101616
Bug ID: 101616
Summary: Objective-C frontend should not emit vtable/fixup
messages (at least, not by default)
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: objc
Assignee: unassigned at gcc dot gnu.org
Reporter: mhjacobson at me dot com
Target Milestone: ---
In 10.2.0, the Objective-C frontend (in NeXT v2 ABI mode) emits "fixup"
messages for all message sends. Fixup messages are an abandoned optimization
in which the runtime specializes the message send routine to a fast vtable
lookup routine for a small set of common selectors. (The set of common
selectors is hard-coded in the runtime.) For other selectors, the fixup
message simply devolves into a normal message send, albeit with extra pointer
chasing.
The Apple runtime has not actually supported this optimization since OS X
Mountain Lion, as an inspection of the Mountain Lion [0] and Mavericks [1]
sources will confirm. (Search for "fixupMessageRef".) The Mavericks-and-later
runtimes still *deal* with fixup messages, but all callsites are treated as
non-optimized selectors. Clang appears to have stopped emitting fixup messages
around 2010.
Furthermore, even when Clang *did* use fixup messages, it did so only for a
small set of selectors (that presumably tended to match the ones optimized by
the runtime). By contrast, GCC simply uses them for all messages.
GCC should probably just stop emitting fixup messages calls entirely, but at
least it should only do so behind some driver flag.
[0]
https://opensource.apple.com/source/objc4/objc4-532/runtime/objc-runtime-new.mm.auto.html
[1]
https://opensource.apple.com/source/objc4/objc4-551.1/runtime/objc-runtime-new.mm.auto.html
