vm: don't allocate callstack samples on data heap

Invoking a GC during a safepoint has some complications, so stuff the callstack samples in a vector instead.

vm/sampling_profiler.cpp

@@ -3,6 +3,18 @@
 namespace factor
 {
 
+profiling_sample::profiling_sample(factor_vm *vm,
+	cell sample_count,
+	cell gc_sample_count,
+	context *ctx)
+	:
+	sample_count(sample_count),
+	gc_sample_count(gc_sample_count),
+	ctx(ctx)
+{
+	vm->record_callstack_sample(&callstack_begin, &callstack_end);
+}
+
 void factor_vm::record_sample()
 {
 	cell recorded_sample_count;
@@ -18,13 +30,26 @@ void factor_vm::record_sample()
 	FACTOR_ATOMIC_SUB(&safepoint_gc_sample_count, recorded_gc_sample_count);
 
 	samples.push_back(profiling_sample(
+		this,
 		recorded_sample_count,
 		recorded_gc_sample_count,
-		ctx,
-		capture_callstack(ctx)
+		ctx
 	));
 }
 
+void factor_vm::record_callstack_sample(cell *begin, cell *end)
+{
+	*begin = sample_callstacks.size();
+	stack_frame *frame = ctx->callstack_bottom - 1;
+
+	while (frame >= ctx->callstack_top) {
+		sample_callstacks.push_back((code_block*)frame->entry_point - 1);
+		frame = frame_successor(frame);
+	}
+
+	*end = sample_callstacks.size();
+}
+
 void factor_vm::set_sampling_profiler(bool sampling_p)
 {
 	if (sampling_p == sampling_profiler_p)
@@ -42,6 +67,7 @@ void factor_vm::start_sampling_profiler()
 	safepoint_gc_sample_count = 0;
 	samples.clear();
 	samples.reserve(10*FACTOR_PROFILE_SAMPLES_PER_SECOND);
+	sample_callstacks.reserve(100*FACTOR_PROFILE_SAMPLES_PER_SECOND);
 	sampling_profiler_p = true;
 	start_sampling_profiler_timer();
 }

vm/sampling_profiler.hpp

@@ -11,20 +11,14 @@ struct profiling_sample
 	cell gc_sample_count;
 	// Active context during sample
 	context *ctx;
-	// The callstack at safepoint time
-	cell callstack;
+	/* The callstack at safepoint time. Indexes to the beginning and ending
+	code_block entries in the vm sample_callstacks array. */
+	cell callstack_begin, callstack_end;
 
-	profiling_sample(cell sample_count,
+	profiling_sample(factor_vm *vm,
+		cell sample_count,
 		cell gc_sample_count,
-		context *ctx,
-		cell callstack)
-		:
-		sample_count(sample_count),
-		gc_sample_count(gc_sample_count),
-		ctx(ctx),
-		callstack(callstack)
-	{
-	}
+		context *ctx);
 };
 
 }

vm/vm.hpp

@@ -74,6 +74,7 @@ struct factor_vm
 
 	/* State kept by the sampling profiler */
 	std::vector<profiling_sample> samples;
+	std::vector<code_block*> sample_callstacks;
 	volatile cell safepoint_sample_count;
 	volatile cell safepoint_gc_sample_count;
 
@@ -191,6 +192,7 @@ struct factor_vm
 
 	/* Sampling profiler */
 	void record_sample();
+	void record_callstack_sample(cell *begin, cell *end);
 	void start_sampling_profiler();
 	void end_sampling_profiler();
 	void set_sampling_profiler(bool sampling);