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From: "law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110538] [14 Regression] Dead Code Elimination
 Regression since  r14-368-ge1366a7e4ce
Date: Sat, 09 Mar 2024 07:13:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110538

--- Comment #3 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Thread references removed edge:   Cancelling jump thread: (9, 10) incoming
edge;  (10, 8) joiner (8, 6) normal;

Thread references removed edge:   Cancelling jump thread: (2, 3) incoming e=
dge;
 (3, 9) joiner (9, 10) nocopy (10, 8) normal;

Thread references removed edge:   Cancelling jump thread: (2, 3) incoming e=
dge;
 (3, 10) joiner (10, 8) normal;

Thread references removed edge:   Cancelling jump thread: (3, 10) incoming
edge;  (10, 8) normal;

[ ... ]

Merging blocks 10 and 8
Removing basic block 11
;; basic block 11, loop depth 1
;;  pred:=20=20=20=20=20=20=20=20=20
goto <bb 3>; [100.00%]
;;  succ:       3

So one of the edges in the jump threading path gets removed.  As a result t=
he
threader throws the path away.  At least that's my best guess.

We could possibly defer block merging until after threading.  No idea how h=
ard
that might be.  The sequencing is a bit painful, but IIRC the key is that we
must remove unreachables in the CFG before threading (due to how unreachabl=
es
interact with the dominator tree updates).=