Change from "never retry ICs" to exponential backoff

src/asm_writing/icinfo.cpp

@@ -30,6 +30,9 @@ namespace pyston {
 
 using namespace pyston::assembler;
 
+#define MEGAMORPHIC_THRESHOLD 100
+#define MAX_RETRY_BACKOFF 1024
+
 // TODO not right place for this...
 int64_t ICInvalidator::version() {
     return cur_version;
@@ -68,7 +71,8 @@ ICSlotRewrite::~ICSlotRewrite() {
 }
 
 void ICSlotRewrite::abort() {
-    ic->failed = true;
+    ic->retry_backoff = std::max(MAX_RETRY_BACKOFF, 2 * ic->retry_backoff);
+    ic->retry_in = ic->retry_backoff;
 }
 
 void ICSlotRewrite::commit(CommitHook* hook) {
@@ -109,6 +113,11 @@ void ICSlotRewrite::commit(CommitHook* hook) {
 
     ic->times_rewritten++;
 
+    if (ic->times_rewritten == MEGAMORPHIC_THRESHOLD) {
+        static StatCounter megamorphic_ics("megamorphic_ics");
+        megamorphic_ics.log();
+    }
+
     llvm::sys::Memory::InvalidateInstructionCache(slot_start, ic->getSlotSize());
 }
 
@@ -177,7 +186,7 @@ ICInfo::ICInfo(void* start_addr, void* slowpath_rtn_addr, void* continue_addr, S
                assembler::GenericRegister return_register, TypeRecorder* type_recorder)
     : next_slot_to_try(0), stack_info(stack_info), num_slots(num_slots), slot_size(slot_size),
       calling_conv(calling_conv), live_outs(live_outs.begin(), live_outs.end()), return_register(return_register),
-      type_recorder(type_recorder), failed(false), times_rewritten(0), start_addr(start_addr),
+      type_recorder(type_recorder), retry_in(0), retry_backoff(1), times_rewritten(0), start_addr(start_addr),
       slowpath_rtn_addr(slowpath_rtn_addr), continue_addr(continue_addr) {
     for (int i = 0; i < num_slots; i++) {
         slots.push_back(ICSlotInfo(this, i));
@@ -269,6 +278,10 @@ void ICInfo::clear(ICSlotInfo* icentry) {
 }
 
 bool ICInfo::shouldAttempt() {
-    return !failed && times_rewritten < 100;
+    if (retry_in) {
+        retry_in--;
+        return false;
+    }
+    return times_rewritten < MEGAMORPHIC_THRESHOLD;
 }
 }

src/asm_writing/icinfo.h

@@ -100,7 +100,7 @@ private:
     const std::vector<int> live_outs;
     const assembler::GenericRegister return_register;
     TypeRecorder* const type_recorder;
-    bool failed;
+    int retry_in, retry_backoff;
     int times_rewritten;
 
     // for ICSlotRewrite: