Modify FastD2I to use static_cast instead of lrint(). Benchmarks show that...

Modify FastD2I to use static_cast instead of lrint(). Benchmarks show that it's much faster this way.

Review URL: http://codereview.chromium.org/1128010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4223 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/conversions-inl.h

@@ -41,24 +41,6 @@
 namespace v8 {
 namespace internal {
 
-// The fast double-to-int conversion routine does not guarantee
-// rounding towards zero.
-static inline int FastD2I(double x) {
-#ifdef __USE_ISOC99
-  // The ISO C99 standard defines the lrint() function which rounds a
-  // double to an integer according to the current rounding direction.
-  return lrint(x);
-#else
-  // This is incredibly slow on Intel x86. The reason is that rounding
-  // towards zero is implied by the C standard. This means that the
-  // status register of the FPU has to be changed with the 'fldcw'
-  // instruction. This completely stalls the pipeline and takes many
-  // hundreds of clock cycles.
-  return static_cast<int>(x);
-#endif
-}
-
-
 // The fast double-to-unsigned-int conversion routine does not guarantee
 // rounding towards zero, or any reasonable value if the argument is larger
 // than what fits in an unsigned 32-bit integer.

src/conversions.h

@@ -36,7 +36,12 @@ namespace internal {
 // rounding towards zero.
 // The result is unspecified if x is infinite or NaN, or if the rounded
 // integer value is outside the range of type int.
-static inline int FastD2I(double x);
+static inline int FastD2I(double x) {
+  // The static_cast convertion from double to int used to be slow, but
+  // as new benchmarks show, now it is much faster than lrint().
+  return static_cast<int>(x);
+}
+
 static inline unsigned int FastD2UI(double x);