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
Last we looked at this and couldn't come up with a reason to change
it, but with a pragma for full loop unrolling we bypass every other
loop unroll and then fail to fully unroll a loop when the pragma is set.
Move the OnlyWhenForced out of the check and into the initialization
of the full unroll pass in the new pass manager. This doesn't show up
with the old pass manager.
Add a new option to opt so that we can turn off loop unrolling
manually since this is a difference between clang and opt.
Tested with check-clang and check-llvm.
we will unroll loops. Also comment a few occasions where we need to
know whether or not we're forcing the unwinder or not.
The default before and after this patch is for LoopUnroll to be enabled,
and for it to use a cost model to determine whether to unroll the loop
(`OnlyWhenForced = false`). Before this patch, disabling loop unroll
would not run the LoopUnroll pass. After this patch, the LoopUnroll pass
is being run, but it restricts unrolling to only the loops marked by a
pragma (`OnlyWhenForced = true`).
In addition, this patch disables the UnrollAndJam pass when disabling unrolling.
Testcase is in clang because it's controlling how the loop optimizer
is being set up and there's no other way to trigger the behavior.
llvm-svn: 374838
Fangrui Song