This patch implements Chandler's idea [0] for supporting languages that
require support for infinite loops with side effects, such as Rust, providing
part of a solution to bug 965 [1].
Specifically, it adds an `llvm.sideeffect()` intrinsic, which has no actual
effect, but which appears to optimization passes to have obscure side effects,
such that they don't optimize away loops containing it. It also teaches
several optimization passes to ignore this intrinsic, so that it doesn't
significantly impact optimization in most cases.
As discussed on llvm-dev [2], this patch is the first of two major parts.
The second part, to change LLVM's semantics to have defined behavior
on infinite loops by default, with a function attribute for opting into
potential-undefined-behavior, will be implemented and posted for review in
a separate patch.
[0] http://lists.llvm.org/pipermail/llvm-dev/2015-July/088103.html
[1] https://bugs.llvm.org/show_bug.cgi?id=965
[2] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118632.html
Differential Revision: https://reviews.llvm.org/D38336
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317729 91177308-0d34-0410-b5e6-96231b3b80d8
This moves some logic added to EarlyCSE in rL268120 into
`llvm::isInstructionTriviallyDead`. Adds a test case for DCE to
demonstrate that passes other than EarlyCSE can now pick up on the new
information.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268126 91177308-0d34-0410-b5e6-96231b3b80d8
Also add a very basic test, since apparently there aren't any tests
for DCE whatsoever to add the new pass version to.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267196 91177308-0d34-0410-b5e6-96231b3b80d8
