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

Authored by atmnpatel on Oct 25 2020, 3:24 PM.

Description

[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