Helper for pinning down where time in a stattimer happens

If you define INVESTIGATE_STAT_TIMER to the name of the timer you want
to investigate, we will set an itimer that raises a SIGTRAP if you are
in that particular timer, but ignores the signal otherwise.  There's no
tooling on top of it, but just running that inside gdb is already helpful.

src/codegen/entry.cpp

@@ -362,6 +362,15 @@ static void handle_sigprof(int signum) {
     sigprof_pending++;
 }
 
+//#define INVESTIGATE_STAT_TIMER "us_timer_chosen_cf_body_jitted"
+#ifdef INVESTIGATE_STAT_TIMER
+static uint64_t* stat_counter = Stats::getStatCounter(INVESTIGATE_STAT_TIMER);
+static void handle_sigprof_investigate_stattimer(int signum) {
+    if (StatTimer::getCurrentCounter() == stat_counter)
+        raise(SIGTRAP);
+}
+#endif
+
 static void handle_sigint(int signum) {
     assert(signum == SIGINT);
     // TODO: this should set a flag saying a KeyboardInterrupt is pending.
@@ -475,6 +484,14 @@ void initCodegen() {
     setitimer(ITIMER_PROF, &prof_timer, NULL);
 #endif
 
+#ifdef INVESTIGATE_STAT_TIMER
+    struct itimerval prof_timer;
+    prof_timer.it_value.tv_sec = prof_timer.it_interval.tv_sec = 0;
+    prof_timer.it_value.tv_usec = prof_timer.it_interval.tv_usec = 1000;
+    signal(SIGPROF, handle_sigprof_investigate_stattimer);
+    setitimer(ITIMER_PROF, &prof_timer, NULL);
+#endif
+
     // There are some parts of llvm that are only configurable through command line args,
     // so construct a fake argc/argv pair and pass it to the llvm command line machinery:
     std::vector<const char*> llvm_args = { "fake_name" };

src/core/stats.cpp

@@ -50,6 +50,12 @@ StatTimer* StatTimer::createStack(StatTimer& timer) {
     timer.pushTopLevel(at_time);
     return prev_stack;
 }
+
+uint64_t* StatTimer::getCurrentCounter() {
+    if (stack)
+        return stack->_statcounter;
+    return NULL;
+}
 #endif
 
 std::unordered_map<uint64_t*, std::string>* Stats::names;

src/core/stats.h

@@ -206,6 +206,8 @@ public:
     static StatTimer* createStack(StatTimer& timer);
     static StatTimer* swapStack(StatTimer* s);
 
+    static uint64_t* getCurrentCounter();
+
     static void assertActive() { ASSERT(stack && !stack->isPaused(), ""); }
 };
 class ScopedStatTimer {

src/runtime/objmodel.cpp

@@ -2021,7 +2021,7 @@ void setattrGeneric(Box* obj, BoxedString* attr, Box* val, SetattrRewriteArgs* r
 }
 
 extern "C" void setattr(Box* obj, BoxedString* attr, Box* attr_val) {
-    STAT_TIMER(t0, "us_timer_slowpath_setsattr", 10);
+    STAT_TIMER(t0, "us_timer_slowpath_setattr", 10);
 
     static StatCounter slowpath_setattr("slowpath_setattr");
     slowpath_setattr.log();