[all-commits] [llvm/llvm-project] e23d78: [JumpThreading] Fix infinite loop (PR44611)

[kazutakahirata via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: e23d78652691f360700a08a245ee28db151844f4
      https://github.com/llvm/llvm-project/commit/e23d78652691f360700a08a245ee28db151844f4
  Author: Kazu Hirata <kazu at google.com>
  Date:   2020-03-19 (Thu, 19 Mar 2020)

  Changed paths:
    M llvm/lib/Transforms/Scalar/JumpThreading.cpp
    A llvm/test/Transforms/JumpThreading/PR44611-across-header-hang.ll

  Log Message:
  -----------
  [JumpThreading] Fix infinite loop (PR44611)

Summary:
This patch fixes https://bugs.llvm.org/show_bug.cgi?id=44611 by
preventing an infinite loop in the jump threading pass when
-jump-threading-across-loop-headers is on.  Specifically, without this
patch, jump threading through two basic blocks would trigger on the
same area of the CFG over and over, resulting in an infinite loop.

Consider testcase PR44611-across-header-hang.ll in this patch.  The
first opportunity to thread through two basic blocks is:

  from bb_body2 through bb_header and bb_body1 to bb_body2.

The pass duplicates bb_header and bb_body1 as, say, bb_header.thread1
and bb_body1.thread1.  Since bb_header contains a successor edge back
to itself, bb_header.thread1 also contains a successor edge to
bb_header, immediately giving rise to the next jump threading
opportunity:

  from bb_header.thread1 through bb_header and bb_body1 to bb_body2.

After that, we repeatedly thread an incoming edge into bb_header
through bb_header and bb_body1 to bb_body2.  In other words, we keep
peeling one iteration from bb_header's self loop.

The patch fixes the problem by preventing the pass from duplicating a
basic block containing a self loop.

Reviewers: wmi, junparser, efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76390