ARM64: slightly modify weight for profiling counter

src/arm64/full-codegen-arm64.cc

Index: src/arm64/full-codegen-arm64.cc
diff --git a/src/arm64/full-codegen-arm64.cc b/src/arm64/full-codegen-arm64.cc
index 97e80676f28725377f436ce1fca13a5364975773..2099fde186898ae49305d47dfaa83667b7fbbeca 100644
--- a/src/arm64/full-codegen-arm64.cc
+++ b/src/arm64/full-codegen-arm64.cc
@@ -391,7 +391,12 @@ void FullCodeGenerator::EmitBackEdgeBookkeeping(IterationStatement* stmt,
   Label ok;
 
   ASSERT(back_edge_target->is_bound());
-  int distance = masm_->SizeOfCodeGeneratedSince(back_edge_target);
+  // We want to do a round rather than a floor of distance/kCodeSizeMultiplier
+  // to reduce the absolute error due to the integer division. To do that,
+  // we add kCodeSizeMultiplier/2 to the distance (equivalent to adding 0.5 to
+  // the result).
+  int distance =
+    masm_->SizeOfCodeGeneratedSince(back_edge_target) + kCodeSizeMultiplier / 2;
   int weight = Min(kMaxBackEdgeWeight,
                    Max(1, distance / kCodeSizeMultiplier));
   EmitProfilingCounterDecrement(weight);
@@ -434,7 +439,7 @@ void FullCodeGenerator::EmitReturnSequence() {
     if (info_->ShouldSelfOptimize()) {
       weight = FLAG_interrupt_budget / FLAG_self_opt_count;
     } else {
-      int distance = masm_->pc_offset();
+      int distance = masm_->pc_offset() + kCodeSizeMultiplier / 2;
       weight = Min(kMaxBackEdgeWeight,
                    Max(1, distance / kCodeSizeMultiplier));
     }