Move some isolate state setup from C++ code to JumpToExceptionHandler stub

runtime/vm/stub_code_arm.cc

Index: runtime/vm/stub_code_arm.cc
diff --git a/runtime/vm/stub_code_arm.cc b/runtime/vm/stub_code_arm.cc
index 3dbd41b90bc391519dc79c163357dd5a2455f796..e29b0d5cc9104f8665760d6569709062cc104aea 100644
--- a/runtime/vm/stub_code_arm.cc
+++ b/runtime/vm/stub_code_arm.cc
@@ -1685,7 +1685,8 @@ void StubCode::GenerateGetStackPointerStub(Assembler* assembler) {
 // R1: stack_pointer.
 // R2: frame_pointer.
 // R3: error object.
-// SP: address of stacktrace object.
+// SP + 0: address of stacktrace object.
+// SP + 4: isolate
 // Does not return.
 void StubCode::GenerateJumpToExceptionHandlerStub(Assembler* assembler) {
   ASSERT(kExceptionObjectReg == R0);
@@ -1694,8 +1695,15 @@ void StubCode::GenerateJumpToExceptionHandlerStub(Assembler* assembler) {
   __ mov(LR, Operand(R0));  // Program counter.
   __ mov(R0, Operand(R3));  // Exception object.
   __ ldr(R1, Address(SP, 0));  // StackTrace object.
+  __ ldr(CTX, Address(SP, 4));  // Isolate.

[regis, 2014/07/14 20:00:34]

Are you sure it is OK to trash the CTX register? I guess it is, but to be safe,
you could use R3.

[Cutch, 2014/07/14 21:32:07]

Done.

   __ mov(FP, Operand(R2));  // Frame_pointer.
   __ mov(SP, Operand(IP));  // Stack pointer.
+  // Set the tag.
+  __ LoadImmediate(R2, VMTag::kScriptTagId);
+  __ StoreToOffset(kWord, R2, CTX, Isolate::vm_tag_offset());
+  // Clear top exit frame.
+  __ LoadImmediate(R2, 0);
+  __ StoreToOffset(kWord, R2, CTX, Isolate::top_exit_frame_info_offset());
   __ bx(LR);  // Jump to the exception handler code.
 }