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 0ccfd17aa858d38a0d2d413a743266e5eb3334f5..31cd10c7288eaca8fb4f8fd64c20b0af4b5f9967 100644
--- a/runtime/vm/stub_code_arm.cc
+++ b/runtime/vm/stub_code_arm.cc
@@ -1712,7 +1712,8 @@ 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);
@@ -1721,8 +1722,15 @@ 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.
   __ mov(FP, Operand(R2));  // Frame_pointer.
   __ mov(SP, Operand(IP));  // Stack pointer.

[siva, 2014/07/14 20:13:12]

The comment here is confusing, stack pointer is being set twice
according to the comments???

[Cutch, 2014/07/14 21:32:08]

I've cleaned up the comments to make it more clear.

+  // 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());

[siva, 2014/07/14 20:13:12]

Not sure if using CTX like this is a good idea it appears as though
setting CTX is part of the contract to jump to the exception
handler code whereas here you are using it as a temporary register
to access the isolate fields.

Why not use TMP (i.e IP)

[Cutch, 2014/07/14 21:32:08]

AFAIK Setting CTX is not part of the contract of jumping to the exception
handler. It's a free register that can be used in this stub. I agree it reads
weird and have replaced all usage of it with another free register.

   __ bx(LR);  // Jump to the exception handler code.
 }