[vm] Add (S)LEB128 encoding/decoding to BaseWriteStream.

Unlike cfc8e6d, this does _not_ replace the default variable length
encoding for {Read,Write}Streams, but insteads adds separate
{Read,Write}{S,}LEB128 methods to the appropriate classes. If we later
find the cause of the issues that led to the revert of cfc8e6d, it'll
be easy to switch over then.

Note that WriteLEB128 asserts that the value is non-negative if used
with a signed type (since negative values suggests that SLEB128 should
be used instead for minimal encoding).

Also removes the various other encoding and decoding methods for
(S)LEB128 across the codebase and changes those clients to use
{Read,Write}Streams instead.

Other cleanups:

* Various constant-related cleanups in datastream.h.

* Adds DART_FORCE_INLINE to ReadStream::ReadByte and uses it in the
  default variable length decoding methods for retrieving bytes
  from the stream instead of managing current_ by hand.

* Creates a canonical empty CompressedStackMaps instance and uses
  that instead of the null CompressedStackMaps instance in most cases.
  The only remaining (expected) use of the null CompressedStackMaps
  instance is for the global table in the object store when no global
  table exists (e.g., in JIT mode before any snapshotting).

* Moves CompressedStackMapsIterator from code_descriptors.h to an
  Iterator class within CompressedStackMaps in object.h (similar to
  PcDescriptors::Iterator), to limit friend declarations and because it
  conceptually makes more sense as part of CompressedStackMaps.

* Removed CompressedStackMaps::PayloadByte, since existing clients
  (CompressedStackMaps::Iterator, StackMapEntry in program_visitor.cc)
  are better served by just operating on the payload buffer directly
  (with appropriate NoSafepointScopes).

* WriteStreams no longer allocate their initial space on construction,
  but rather on the first write, so no allocation is performed by
  constructing a never-used WriteStream.

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Change-Id: Ice63321abaa79157fbe9f230a864c8bba0e6dea9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/166421
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Tess Strickland <[email protected]>

Author: sstrickl

disassembler.cc

@@ -220,7 +220,8 @@ void Disassembler::DisassembleCodeHelper(const char* function_fullname,
                                          const char* function_info,
                                          const Code& code,
                                          bool optimized) {
-  Zone* zone = Thread::Current()->zone();
+  Thread* thread = Thread::Current();
+  Zone* zone = thread->zone();
   LocalVarDescriptors& var_descriptors = LocalVarDescriptors::Handle(zone);
   if (FLAG_print_variable_descriptors) {
     var_descriptors = code.GetLocalVarDescriptors();
@@ -290,13 +291,16 @@ void Disassembler::DisassembleCodeHelper(const char* function_fullname,
   }
 #endif  // !defined(DART_PRECOMPILED_RUNTIME)
 
-  THR_Print("StackMaps for function '%s' {\n", function_fullname);
-  if (code.compressed_stackmaps() != CompressedStackMaps::null()) {
+  {
     const auto& stackmaps =
         CompressedStackMaps::Handle(zone, code.compressed_stackmaps());
-    THR_Print("%s\n", stackmaps.ToCString());
+    CompressedStackMaps::Iterator it(thread, stackmaps);
+    TextBuffer buffer(100);
+    buffer.Printf("StackMaps for function '%s' {\n", function_fullname);
+    it.WriteToBuffer(&buffer, "\n");
+    buffer.AddString("}\n");
+    THR_Print("%s", buffer.buffer());
   }
-  THR_Print("}\n");
 
   if (FLAG_print_variable_descriptors) {
     THR_Print("Variable Descriptors for function '%s' {\n", function_fullname);