Comment describing isPresplitCorouSuspendExitEdge

Author: mtrofin

llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h

@@ -706,6 +706,22 @@ void InvertBranch(BranchInst *PBI, IRBuilderBase &Builder);
 // br/brcond/unreachable/ret
 bool hasOnlySimpleTerminator(const Function &F);
 
+// Returns true if these basic blocks belong to a presplit coroutine and the
+// edge corresponds to the 'default' case in the switch statement in the
+// pattern:
+//
+// %0 = call i8 @llvm.coro.suspend(token none, i1 false)
+// switch i8 %0, label %suspend [i8 0, label %resume
+//                               i8 1, label %cleanup]
+//
+// i.e. the edge to the `%suspend` BB. This edge is special in that it will
+// be elided by coroutine lowering (coro-split), and the `%suspend` BB needs
+// to be kept as-is. It's not a real CFG edge - post-lowering, it will end
+// up being a `ret`, and it must be thus lowerable to support symmetric
+// transfer. For example:
+//  - this edge is not a loop exit edge if encountered in a loop (and should
+//    be ignored)
+//  - must not be split for PGO instrumentation, for example.
 bool isPresplitCoroSuspendExitEdge(const BasicBlock &Src,
                                    const BasicBlock &Dest);
 } // end namespace llvm