[IR] CallBrInst: scan+update arg list when indirect dest list changes

Authored by nickdesaulniers on Sep 6 2019, 2:50 PM.


[IR] CallBrInst: scan+update arg list when indirect dest list changes

There's an unspoken invariant of callbr that the list of BlockAddress
Constants in the "function args" list match the BasicBlocks in the
"other labels" list. (This invariant is being added to the LangRef in

When modifying the any of the indirect destinations of a callbr
instruction (possible jump targets), we need to update the function
arguments if the argument is a BlockAddress whose BasicBlock refers to
the indirect destination BasicBlock being replaced. Otherwise, many
transforms that modify successors will end up violating that invariant.
A recent change to the arm64 Linux kernel exposed this bug, which
prevents the kernel from booting.

I considered maintaining a mapping from indirect destination BasicBlock
to argument operand BlockAddress, but this ends up being a one to
potentially many (though usually one) mapping. Also, the list of
arguments to a function (or more typically inline assembly) ends up
being less than 10. The implementation is significantly simpler to just
rescan the full list of arguments. Because of the one to potentially
many relationship, the full arg list must be scanned (we can't stop at
the first instance).

Thanks to the following folks that reported the issue and helped debug

  • Nathan Chancellor
  • Will Deacon
  • Andrew Murray
  • Craig Topper

Link: https://bugs.llvm.org/show_bug.cgi?id=43222
Link: https://github.com/ClangBuiltLinux/linux/issues/649
Link: https://lists.infradead.org/pipermail/linux-arm-kernel/2019-September/678330.html

Reviewers: craig.topper, chandlerc

Reviewed By: craig.topper

Subscribers: void, javed.absar, kristof.beyls, hiraditya, llvm-commits, nathanchance, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67252

llvm-svn: 371262