JavaScriptのMathクラスの実装をV8エンジンソースから読み解く╭( ･ㅂ･)و ̑̑ ref: http://qiita.com/timakin/items/2b0436c60e43e78894a2

file0.txt

var $floor = MathFloor;

function MathFloor(x) {
  x = TO_NUMBER_INLINE(x);
  // It's more common to call this with a positive number that's out
  // of range than negative numbers; check the upper bound first.
  if (x < 0x80000000 && x > 0) {
    // Numbers in the range [0, 2^31) can be floored by converting
    // them to an unsigned 32-bit value using the shift operator.
    // We avoid doing so for -0, because the result of Math.floor(-0)
    // has to be -0, which wouldn't be the case with the shift.
    return TO_UINT32(x);
  } else {
    return %MathFloorRT(x);
  }
}

file1.txt

macro TO_NUMBER_INLINE(arg) = (IS_NUMBER(%IS_VAR(arg)) ? arg : NonNumberToNumber(arg));

file2.txt

RUNTIME_FUNCTION(Runtime_MathFloorRT) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 1);
  isolate->counters()->math_floor()->Increment();

  CONVERT_DOUBLE_ARG_CHECKED(x, 0);
  return *isolate->factory()->NewNumber(Floor(x));
}

file3.txt

#define CONVERT_DOUBLE_ARG_CHECKED(name, index) \
  RUNTIME_ASSERT(args[index]->IsNumber());      \
  double name = args.number_at(index);

file4.c

double number_at(int index) {
    return (*this)[index]->Number();
}

file4.txt

double number_at(int index) {
    return (*this)[index]->Number();
}

macros.py

macro TO_NUMBER_INLINE(arg) = (IS_NUMBER(%IS_VAR(arg)) ? arg : NonNumberToNumber(arg));

math.js

var $floor = MathFloor;

function MathFloor(x) {
  x = TO_NUMBER_INLINE(x);
  // It's more common to call this with a positive number that's out
  // of range than negative numbers; check the upper bound first.
  if (x < 0x80000000 && x > 0) {
    // Numbers in the range [0, 2^31) can be floored by converting
    // them to an unsigned 32-bit value using the shift operator.
    // We avoid doing so for -0, because the result of Math.floor(-0)
    // has to be -0, which wouldn't be the case with the shift.
    return TO_UINT32(x);
  } else {
    return %MathFloorRT(x);
  }
}

runtime-maths.cc

RUNTIME_FUNCTION(Runtime_MathFloorRT) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 1);
  isolate->counters()->math_floor()->Increment();

  CONVERT_DOUBLE_ARG_CHECKED(x, 0);
  return *isolate->factory()->NewNumber(Floor(x));
}

runtime-utils.h

#define CONVERT_DOUBLE_ARG_CHECKED(name, index) \
  RUNTIME_ASSERT(args[index]->IsNumber());      \
  double name = args.number_at(index);