libswift: add single-threaded pass docs, fix test

Author: MaxDesiatov

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/AssumeSingleThreaded.swift

@@ -9,6 +9,21 @@
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 //===----------------------------------------------------------------------===//
+//
+// Assume that user code is single-threaded.
+//
+// Convert all reference counting operations into non-atomic ones.
+//
+// To get rid of most atomic reference counting operations, the standard
+// library should be compiled in this mode as well .
+//
+// This pass affects only reference counting operations resulting from SIL
+// instructions. It wouldn't affect places in the runtime C++ code which
+// hard-code calls to retain/release. We could take advantage of the Instruments
+// instrumentation stubs to redirect calls from the runtime if it was
+// significant, or else just build a single-threaded variant of the runtime.
+//
+//===----------------------------------------------------------------------===//
 
 import SIL
 

test/sil-passpipeline-dump/basic.test-sh

@@ -8,6 +8,5 @@
 // CHECK: passes:          [ "serialize-sil", "ownership-model-eliminator" ]
 // CHECK: ---
 // CHECK: name:            Rest of Onone
-// CHECK: passes:          [ "use-prespecialized", "sil-assume-single-threaded",
-// CHECK:                    "sil-debuginfo-gen" ]
+// CHECK: passes:          [ "use-prespecialized", "onone-prespecializer", "sil-debuginfo-gen" ]
 // CHECK: ...