ARM64: slightly modify weight for profiling counter

src/arm64/full-codegen-arm64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 373 matching lines @@
 
 void FullCodeGenerator::EmitBackEdgeBookkeeping(IterationStatement* stmt,
                                                 Label* back_edge_target) {
   ASSERT(jssp.Is(__ StackPointer()));
   Comment cmnt(masm_, "[ Back edge bookkeeping");
   // Block literal pools whilst emitting back edge code.
   Assembler::BlockPoolsScope block_const_pool(masm_);
   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);
   __ B(pl, &ok);
   __ Call(isolate()->builtins()->InterruptCheck(), RelocInfo::CODE_TARGET);
 
   // Record a mapping of this PC offset to the OSR id.  This is used to find
   // the AST id from the unoptimized code in order to use it as a key into
   // the deoptimization input data found in the optimized code.
   RecordBackEdge(stmt->OsrEntryId());
@@ skipping 22 matching lines @@
       // Runtime::TraceExit returns its parameter in x0.
       __ Push(result_register());
       __ CallRuntime(Runtime::kTraceExit, 1);
       ASSERT(x0.Is(result_register()));
     }
     // Pretend that the exit is a backwards jump to the entry.
     int weight = 1;
     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));
     }
     EmitProfilingCounterDecrement(weight);
     Label ok;
     __ B(pl, &ok);
     __ Push(x0);
     __ Call(isolate()->builtins()->InterruptCheck(),
             RelocInfo::CODE_TARGET);
     __ Pop(x0);
@@ skipping 4552 matching lines @@
   return previous_;
 }
 
 
 #undef __
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM64