[coroutines] Add support for llvm.coro.noop intrinsics

Summary:
A recent addition to Coroutines TS (https://wg21.link/p0913) adds a pre-defined coroutine noop_coroutine that does nothing.
To implement this feature, we implemented an llvm.coro.noop intrinsic that returns a coroutine handle to a coroutine that does nothing when resumed or destroyed.

Reviewers: EricWF, modocache, rnk, lewissbaker

Reviewed By: modocache

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45114

llvm-svn: 328986

1 files changed, 26 insertions, 0 deletions

diff --git a/llvm/docs/Coroutines.rst b/llvm/docs/Coroutines.rst
index 1bea04ebdd2..f3667585c6c 100644
--- a/llvm/docs/Coroutines.rst
+++ b/llvm/docs/Coroutines.rst
@@ -880,6 +880,32 @@ Example:
     %phi = phi i8* [ null, %entry ], [ %alloc, %coro.alloc ]
     %frame = call i8* @llvm.coro.begin(token %id, i8* %phi)
 
+.. _coro.noop:
+
+'llvm.coro.noop' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+::
+
+  declare i8* @llvm.coro.noop()
+
+Overview:
+"""""""""
+
+The '``llvm.coro.noop``' intrinsic returns an address of the coroutine frame of
+a coroutine that does nothing when resumed or destroyed.
+
+Arguments:
+""""""""""
+
+None
+
+Semantics:
+""""""""""
+
+This intrinsic is lowered to refer to a private constant coroutine frame. The
+resume and destroy handlers for this frame are empty functions that do nothing.
+Note that in different translation units llvm.coro.noop may return different pointers.
+
 .. _coro.frame:
 
 'llvm.coro.frame' Intrinsic