Optimize the assembly code generated for Math.random()

src/v8.cc

 // Copyright 2006-2009 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 192 matching lines @@
   uint64_t uint64_t_value;
 } double_int_union;
 
 
 Object* V8::FillHeapNumberWithRandom(Object* heap_number) {
   uint64_t random_bits = Random();
   // Make a double* from address (heap_number + sizeof(double)).
   double_int_union* r = reinterpret_cast<double_int_union*>(
       reinterpret_cast<char*>(heap_number) +
       HeapNumber::kValueOffset - kHeapObjectTag);
-  // Create a random number between 0.0 and 1.0 by putting random bits into
-  // the mantissa of 1.0 and subtracting 1.0.
-  r->double_value = 1.0;
-  r->uint64_t_value |= (random_bits << 20);
-  r->double_value -= 1.0;  // Force into the range [0.0, 1.0).
+  // Convert 32 random bits to 0.(32 random bits) in a double
+  // by computing:
+  // ( 1.(20 0s)(32 random bits) x 2^20 ) - (1.0 x 2^20)).
+  const double binary_million = 1048576.0;
+  r->double_value = binary_million;
+  r->uint64_t_value |=  random_bits;
+  r->double_value -= binary_million;
 
   return heap_number;
 }
 
 } }  // namespace v8::internal