vm: bottom_frame method for contexts

ctx->callstack_bottom - 1 -- the fuck does that mean?!

vm/callstack.cpp

@@ -28,7 +28,7 @@ called by continuation implementation, and user code shouldn't
 be calling it at all, so we leave it as it is for now. */
 stack_frame *factor_vm::second_from_top_stack_frame(context *ctx)
 {
-	stack_frame *frame = ctx->callstack_bottom - 1;
+	stack_frame *frame = ctx->bottom_frame();
 	while(frame >= ctx->callstack_top
 		&& frame_successor(frame) >= ctx->callstack_top
 		&& frame_successor(frame_successor(frame)) >= ctx->callstack_top)

vm/contexts.hpp

@@ -73,6 +73,11 @@ struct context {
 		datastack += sizeof(cell);
 		replace(tagged);
 	}
+
+	stack_frame *bottom_frame()
+	{
+		return callstack_bottom - 1;
+	}
 };
 
 VM_C_API context *new_context(factor_vm *parent);

vm/cpu-x86.cpp

@@ -13,7 +13,7 @@ void factor_vm::dispatch_signal_handler(cell *sp, cell *pc, cell handler)
 		the signal handler to do its thing, and launch the handler without going
 		through the resumable subprimitive. */
 		signal_resumable = false;
-		stack_frame *frame = ctx->callstack_bottom - 1;
+		stack_frame *frame = ctx->bottom_frame();
 
 		while((cell)frame >= *sp
 			&& frame >= ctx->callstack_top

vm/sampling_profiler.cpp

@@ -40,10 +40,10 @@ void factor_vm::record_sample()
 void factor_vm::record_callstack_sample(cell *begin, cell *end)
 {
 	*begin = sample_callstacks.size();
-	stack_frame *frame = ctx->callstack_bottom - 1;
+	stack_frame *frame = ctx->bottom_frame();
 
 	while (frame >= ctx->callstack_top) {
-		sample_callstacks.push_back((code_block*)frame->entry_point - 1);
+		sample_callstacks.push_back(frame_code(frame));
 		frame = frame_successor(frame);
 	}