[Backtracing] Rename _Backtracing to Runtime.

Move the backtracing code into a new Runtime module.  This means renaming
the Swift Runtime's CMake target because otherwise there will be a name
clash.

rdar://124913332

Author: al45tair

CMakeLists.txt

@@ -727,6 +727,10 @@ option(SWIFT_ENABLE_SYNCHRONIZATION
   "Enable build of the Swift Synchronization module"
   FALSE)
 
+option(SWIFT_ENABLE_RUNTIME_MODULE
+  "Build the Swift Runtime module"
+  FALSE)
+
 option(SWIFT_ENABLE_VOLATILE
   "Enable build of the Swift Volatile module"
   FALSE)
@@ -1395,6 +1399,7 @@ if(SWIFT_BUILD_STDLIB OR SWIFT_BUILD_SDK_OVERLAY)
   message(STATUS "Synchronization Support:            ${SWIFT_ENABLE_SYNCHRONIZATION}")
   message(STATUS "Volatile Support:                   ${SWIFT_ENABLE_VOLATILE}")
   message(STATUS "Pointer Bounds Support:             ${SWIFT_ENABLE_EXPERIMENTAL_POINTER_BOUNDS}")
+  message(STATUS "Runtime Support:                    ${SWIFT_ENABLE_RUNTIME_MODULE}")
   message(STATUS "")
 else()
   message(STATUS "Not building Swift standard library, SDK overlays, and runtime")

docs/Backtracing.rst

@@ -319,3 +319,11 @@ of the backtracer using
 
 If the runtime is unable to locate the backtracer, it will allow your program to
 crash as it would have done anyway.
+
+Backtrace Storage
+-----------------
+
+Backtraces are stored internally in a format called :download:`Compact Backtrace
+Format <CompactBacktraceFormat.md>`.  This provides us with a way to store a
+large number of frames in a much smaller space than would otherwise be possible.
+

docs/CompactBacktraceFormat.md

@@ -0,0 +1,141 @@
+Compact Backtrace Format
+========================
+
+We would like to be able to efficiently store and access backtraces,
+but we also wish to minimise the memory used to store them.  Since
+backtraces typically contain a good deal of redundancy, it should be
+possible to compress the data.
+
+Compact Backtrace Format (CBF) is a binary format for holding a
+backtrace; this specification addresses only the storage of the actual
+stack backtrace, and it does not consider storage of ancillary data
+(register contents, image lists and so on).  Those will be dealt with
+separately elsewhere.
+
+## General Format
+
+Compact Backtrace Format data is byte aligned and starts with an
+information byte:
+
+~~~
+   7   6   5   4   3   2   1   0
+ ┌───────────────────────┬───────┐
+ │ version               │ size  │
+ └───────────────────────┴───────┘
+~~~
+
+The `version` field identifies the version of CBF that is in use; this
+document describes version `0`. The `size` field is encoded as
+follows:
+
+| `size` | Machine word size |
+| :----: | :---------------- |
+|   00   | 16-bit            |
+|   01   | 32-bit            |
+|   10   | 64-bit            |
+|   11   | Reserved          |
+
+This is followed by a series of instructions that tell the reader how
+to decode subsequent data.
+
+The first instruction that computes an address _must_ specify an
+absolute address (the `a` bit must be set).
+
+## Instructions
+
+The following instructions are currently defined
+
+|  `opcode`  | Mnemonic | Meaning                                   |
+| :--------: | :------- | :---------------------------------------- |
+| `00000000` | `end`    | Marks the end of the backtrace            |
+| `00000001` | `trunc`  | As above, but the backtrace was truncated |
+| `0000xxxx` | reserved | Reserved for future expansion             |
+| `0001axxx` | `pc`     | A program counter value follows           |
+| `0010axxx` | `ra`     | A return address value follows            |
+| `0011axxx` | `async`  | An async resume point follows             |
+| `01xxxxxx` | `omit`   | Indicates frames have been omitted        |
+| `1xxxxxxx` | reserved | Reserved for future expansion             |
+
+### `end`/`trunc`
+
+#### Encoding
+
+~~~
+   7   6   5   4   3   2   1   0
+ ┌───────────────────────────┬───┐
+ │ 0   0   0   0   0   0   0 │ t │  end (or trunc if t is 1)
+ └───────────────────────────┴───┘
+~~~
+
+#### Meaning
+
+Marks the end of the backtrace data.  If `t` is set, it indicates that
+the backtrace was truncated at this point (for instance because we hit
+a frame limit while capturing).
+
+It is not strictly necessary to use the `end` instruction if the
+CBF data is of a known length.
+
+### `pc`, `ra`, `async`
+
+#### Encoding
+
+~~~
+   7   6   5   4   3   2   1   0
+ ┌────────────────┬───┬──────────┐
+ │ 0   0   0   1  │ a │ count    │ pc
+ └────────────────┴───┴──────────┘
+ ┌────────────────┬───┬──────────┐
+ │ 0   0   1   0  │ a │ count    │ ra
+ └────────────────┴───┴──────────┘
+ ┌────────────────┬───┬──────────┐
+ │ 0   0   1   1  │ a │ count    │ async
+ └────────────────┴───┴──────────┘
+~~~
+
+#### Meaning
+
+Each of these instructions represents a frame on the stack.  For `pc`
+frames, the computed address is an actual program counter (aka
+instruction pointer) value.  `ra` instructions instead represent a
+_return address_, the difference being that the program has not yet
+executed that instruction.  `async` instructions point at the entry
+point of an async resume function, and are used when walking stacks on
+systems that support `async`/`await` primitives that are implemented
+by function splitting (typically an `async` instruction will point at
+the start of a function containing the code immediately following an
+`await`).
+
+The next `count + 1` bytes following the instruction are an address
+value.  If `a` is set, the computed address is equal to the address
+value.  If `a` is not set, the computed address is equal to the
+preceding computed address *plus* the address value.
+
+Address values are sign-extended to the machine word width before
+processing.  Thus a single address byte with value `0xff` on a 32-bit
+backtrace represents the address value `0xffffffff`.
+
+### `omit`
+
+#### Encoding
+
+~~~
+   7   6   5   4   3   2   1   0
+ ┌───────┬───┬───────────────────┐
+ │ 0   1 │ x │ count             │ omit
+ └───────┴───┴───────────────────┘
+~~~
+
+#### Meaning
+
+Indicates that a number of frames were skipped when capturing the
+backtrace.  This is used to allow a backtrace to include both the top
+and bottom of the stack, without carrying every intervening frame, and
+is useful to prevent the data from exploding where recursion has taken
+place.
+
+If `x` is `1`, the instruction is followed by `count + 1` bytes (up to the
+machine word length) that are zero-extended to machine word length and
+that represent a count of the number of frames that were omitted.
+
+If `x` is `0`, `count + 1` is the number of frames that were omitted.

include/swift/Runtime/CrashInfo.h

@@ -24,7 +24,6 @@
 namespace swift {
 namespace runtime {
 namespace backtrace {
-extern "C" {
 #endif
 
 // Note: The "pointers" below are pointers in a different process's address
@@ -89,7 +88,6 @@ struct thread {
 #endif
 
 #ifdef __cplusplus
-} // extern "C"
 } // namespace backtrace
 } // namespace runtime
 } // namespace swift

lib/AST/NameLookup.cpp

@@ -644,22 +644,6 @@ static void recordShadowedDeclsAfterTypeMatch(
         }
       }
 
-      // Next, prefer any other module over the _Backtracing module.
-      if (auto spModule = ctx.getLoadedModule(ctx.Id_Backtracing)) {
-        if ((firstModule == spModule) != (secondModule == spModule)) {
-          // If second module is _StringProcessing, then it is shadowed by
-          // first.
-          if (secondModule == spModule) {
-            shadowed.insert(secondDecl);
-            continue;
-          }
-
-          // Otherwise, the first declaration is shadowed by the second.
-          shadowed.insert(firstDecl);
-          break;
-        }
-      }
-
       // Next, prefer any other module over the Observation module.
       if (auto obsModule = ctx.getLoadedModule(ctx.Id_Observation)) {
         if ((firstModule == obsModule) != (secondModule == obsModule)) {

lib/DriverTool/autolink_extract_main.cpp

@@ -230,9 +230,9 @@ int autolink_extract_main(ArrayRef<const char *> Args, const char *Argv0,
       "-lswift_StringProcessing",
       "-lswiftRegexBuilder",
       "-lswift_RegexParser",
-      "-lswift_Backtracing",
       "-lswift_Builtin_float",
       "-lswift_math",
+      "-lswiftRuntime",
       "-lswiftSynchronization",
       "-lswiftGlibc",
       "-lswiftAndroid",