[all-commits] [llvm/llvm-project] f88a79: [LoopDeletion] Allows deletion of possibly infinit...

[Atmn Patel via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: f88a7975210fc995197af4b393e3bb5030e97a5c
      https://github.com/llvm/llvm-project/commit/f88a7975210fc995197af4b393e3bb5030e97a5c
  Author: Atmn Patel <a335pate at uwaterloo.ca>
  Date:   2021-01-05 (Tue, 05 Jan 2021)

  Changed paths:
    M clang/test/Misc/loop-opt-setup.c
    M llvm/include/llvm/Transforms/Utils/LoopUtils.h
    M llvm/lib/Transforms/Scalar/LoopDeletion.cpp
    M llvm/lib/Transforms/Utils/LoopUtils.cpp
    A llvm/test/Transforms/LoopDeletion/mustprogress.ll
    M llvm/test/Transforms/LoopDeletion/no-exit-blocks.ll

  Log Message:
  -----------
  [LoopDeletion] Allows deletion of possibly infinite side-effect free loops

>From C11 and C++11 onwards, a forward-progress requirement has been
introduced for both languages. In the case of C, loops with non-constant
conditionals that do not have any observable side-effects (as defined by
6.8.5p6) can be assumed by the implementation to terminate, and in the
case of C++, this assumption extends to all functions. The clang
frontend will emit the `mustprogress` function attribute for C++
functions (D86233, D85393, D86841) and emit the loop metadata
`llvm.loop.mustprogress` for every loop in C11 or later that has a
non-constant conditional.

This patch modifies LoopDeletion so that only loops with
the `llvm.loop.mustprogress` metadata or loops contained in functions
that are required to make progress (`mustprogress` or `willreturn`) are
checked for observable side-effects. If these loops do not have an
observable side-effect, then we delete them.

Loops without observable side-effects that do not satisfy the above
conditions will not be deleted.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D86844