[Backtracing] Updates following review.

Added some explanatory text to the Compact Backtrace Format
documentation, and also to the `CMakeLists.txt` for the runtime.

Tweaked the conversions for `Backtrace.Address` to truncate, which
should result in reasonable behaviour for negative fixed-width
integers.

Use a constant for the ASCII values for `/` and `\` in the Compact
Image Format encoder/decoder.

Make the TSD key for `ElfImageCache` non-optional, and call
`fatalError()` if we don't get one.

rdar://124913332

docs/CompactBacktraceFormat.md

@@ -57,6 +57,10 @@ The following instructions are currently defined
 | `1000xxxx` | `rep`    | Repeat the previous frame                 |
 | `1xxxxxxx` | reserved | Reserved for future expansion             |
 
+If the bit labelled `a` is set, it means that the address computation
+is absolute rather than being relative to the previously computed
+address.
+
 ### `end`/`trunc`
 
 #### Encoding

stdlib/public/RuntimeModule/Address.swift

@@ -148,13 +148,13 @@ extension Backtrace.Address {
         return T(0)
       case let .sixteenBit(addr):
         guard T.bitWidth >= 16 else { return nil }
-        return T(addr)
+        return T(truncatingIfNeeded: addr)
       case let .thirtyTwoBit(addr):
         guard T.bitWidth >= 32 else { return nil }
-        return T(addr)
+        return T(truncatingIfNeeded: addr)
       case let .sixtyFourBit(addr):
         guard T.bitWidth >= 64 else { return nil }
-        return T(addr)
+        return T(truncatingIfNeeded: addr)
     }
   }
 }
@@ -204,11 +204,11 @@ extension Backtrace.Address {
   public init?<T: FixedWidthInteger>(_ value: T) {
     switch T.bitWidth {
       case 16:
-        self.init(UInt16(value))
+        self.init(UInt16(truncatingIfNeeded: value))
       case 32:
-        self.init(UInt32(value))
+        self.init(UInt32(truncatingIfNeeded: value))
       case 64:
-        self.init(UInt64(value))
+        self.init(UInt64(truncatingIfNeeded: value))
       default:
         return nil
     }

stdlib/public/RuntimeModule/CompactImageMap.swift

@@ -16,6 +16,9 @@
 
 import Swift
 
+private let slash = UInt8(ascii: "/")
+private let backslash = UInt8(ascii: "\\")
+
 @_spi(Internal)
 public enum CompactImageMapFormat {
 
@@ -58,7 +61,7 @@ public enum CompactImageMapFormat {
     while pos < end {
       let ch = str[pos]
 
-      if pos > base && (ch == 0x2f || ch == 0x5c) {
+      if pos > base && (ch == slash || ch == backslash) {
         let range = base..<pos
         let prefix = String(str[range])!
         body(prefix)
@@ -179,7 +182,8 @@ public enum CompactImageMapFormat {
             guard let char = iterator.next() else {
               return nil
             }
-            if base + n > stringBase! && (char == 0x2f || char == 0x5c) {
+            if base + n > stringBase! && (char == slash
+                                            || char == backslash) {
               let prefix = String(decoding: resultBytes[stringBase!..<base+n],
                                   as: UTF8.self)
               #if DEBUG_COMPACT_IMAGE_MAP
@@ -226,11 +230,11 @@ public enum CompactImageMapFormat {
           print("framewk version='\(versionChar)' name='\(name)'")
           #endif
 
-          resultBytes.append(0x2f) // '/'
+          resultBytes.append(slash)
           resultBytes.append(contentsOf: nameBytes)
           resultBytes.append(contentsOf: ".framework/Versions/".utf8)
           resultBytes.append(version)
-          resultBytes.append(0x2f)
+          resultBytes.append(slash)
           resultBytes.append(contentsOf: nameBytes)
 
           return String(decoding: resultBytes, as: UTF8.self)
@@ -383,7 +387,7 @@ public enum CompactImageMapFormat {
         // Extract the name from the path
         if let path = path {
           if let lastSlashNdx = path.utf8.lastIndex(
-               where: { $0 == 0x2f || $0 == 0x5c }
+               where: { $0 == slash || $0 == backslash }
              ) {
             let nameNdx = path.index(after: lastSlashNdx)
 
@@ -726,7 +730,7 @@ public enum CompactImageMapFormat {
                   version = remainingPath[verNdx]
 
                   let slashNdx = remainingPath.index(after: verNdx)
-                  if remainingPath[slashNdx] == 0x2f {
+                  if remainingPath[slashNdx] == slash {
                     let nameNdx = remainingPath.index(after: slashNdx)
                     if remainingPath[nameNdx...].elementsEqual(name.utf8) {
                       self.remainingPath = remainingPath[nameNdx...]

stdlib/public/RuntimeModule/ElfImageCache.swift

@@ -66,7 +66,7 @@ final class ElfImageCache {
     return nil
   }
 
-  private static var key: pthread_key_t? = {
+  private static var key: pthread_key_t = {
     var theKey = pthread_key_t()
     let err = pthread_key_create(
       &theKey,
@@ -78,15 +78,15 @@ final class ElfImageCache {
       }
     )
     if err != 0 {
-      return nil
+      fatalError("Unable to create TSD key for ElfImageCache")
     }
     return theKey
   }()
 
   static var threadLocal: ElfImageCache {
-    guard let rawPtr = pthread_getspecific(key!) else {
+    guard let rawPtr = pthread_getspecific(key) else {
       let cache = Unmanaged<ElfImageCache>.passRetained(ElfImageCache())
-      pthread_setspecific(key!, cache.toOpaque())
+      pthread_setspecific(key, cache.toOpaque())
       return cache.takeUnretainedValue()
     }
     let cache = Unmanaged<ElfImageCache>.fromOpaque(rawPtr)

stdlib/public/runtime/CMakeLists.txt

@@ -144,6 +144,10 @@ foreach(sdk ${SWIFT_SDKS})
   endif()
 endforeach()
 
+# In modern CMake, we would be able to use the previous name `swiftRuntime`
+# without clashing with the `Runtime` module, but the current build system
+# is architected in such a way that we had to rename this to `swiftRuntimeCore`
+# in order to avoid a clash with the new Swift module.
 add_swift_target_library(swiftRuntimeCore OBJECT_LIBRARY
   ${swift_runtime_sources}
   ${swift_runtime_objc_sources}
@@ -323,4 +327,3 @@ if(static_binary_lnk_file_list)
   add_dependencies(stdlib ${static_binary_lnk_file_list})
   add_custom_target(static_binary_magic ALL DEPENDS ${static_binary_lnk_file_list})
 endif()
-