Avoid use of std::string where we have const char*.

Removing the ClassHelper caused std::string creation for all calls to
Class::GetDescriptor and a significant performance regression. Make the
std::string an out argument so the caller can maintain it and its life time
while allowing GetDescriptor to return the common const char* case.

Don't generate GC maps when compilation is disabled.

Remove other uses of std::string that are occuring on critical paths.
Use the cheaper SkipClass in CompileMethod in CompilerDriver.
Specialize the utf8 as utf16 comparison code for the common shorter byte
encoding.
Force a bit of inlining, remove some UNLIKELYs (they are prone to pessimizing
code), add some LIKELYs.

x86-64 host 1-thread interpret-only of 57 apks:
Before: 29.539s
After: 23.467s

Regular compile:
Before: 1m35.347s
After: 1m20.056s

Bug: 16853450
Change-Id: Ic705ea24784bee24ab80084d06174cbf87d557ad

1 files changed, 3 insertions, 1 deletions

diff --git a/runtime/class_linker-inl.h b/runtime/class_linker-inl.h
index 9921bddad8..3af90b2b86 100644
--- a/runtime/class_linker-inl.h
+++ b/runtime/class_linker-inl.h
@@ -47,7 +47,9 @@ inline mirror::Class* ClassLinker::FindArrayClass(Thread* self, mirror::Class**
     }
   }
   DCHECK(!(*element_class)->IsPrimitiveVoid());
-  std::string descriptor = "[" + (*element_class)->GetDescriptor();
+  std::string descriptor = "[";
+  std::string temp;
+  descriptor += (*element_class)->GetDescriptor(&temp);
   StackHandleScope<2> hs(Thread::Current());
   Handle<mirror::ClassLoader> class_loader(hs.NewHandle((*element_class)->GetClassLoader()));
   HandleWrapper<mirror::Class> h_element_class(hs.NewHandleWrapper(element_class));