Author: [email protected]
Date: Wed Jun 17 04:06:32 2009
New Revision: 2201
Modified:
branches/bleeding_edge/src/runtime.cc
Log:
Optimize special cases of Math.pow().
Review URL: http://codereview.chromium.org/125245
Modified: branches/bleeding_edge/src/runtime.cc
==============================================================================
--- branches/bleeding_edge/src/runtime.cc (original)
+++ branches/bleeding_edge/src/runtime.cc Wed Jun 17 04:06:32 2009
@@ -4161,16 +4161,62 @@
}
+// Helper function to compute x^y, where y is known to be an
+// integer. Uses binary decomposition to limit the number of
+// multiplications; see the discussion in "Hacker's Delight" by Henry
+// S. Warren, Jr., figure 11-6, page 213.
+static double powi(double x, int y) {
+ ASSERT(y != kMinInt);
+ unsigned n = (y < 0) ? -y : y;
+ double m = x;
+ double p = 1;
+ while (true) {
+ if ((n & 1) != 0) p *= m;
+ n >>= 1;
+ if (n == 0) {
+ if (y < 0) {
+ // Unfortunately, we have to be careful when p has reached
+ // infinity in the computation, because sometimes the higher
+ // internal precision in the pow() implementation would have
+ // given us a finite p. This happens very rarely.
+ double result = 1.0 / p;
+ return (result == 0 && isinf(p)) ? pow(x, y) : result;
+ } else {
+ return p;
+ }
+ }
+ m *= m;
+ }
+}
+
+
static Object* Runtime_Math_pow(Arguments args) {
NoHandleAllocation ha;
ASSERT(args.length() == 2);
CONVERT_DOUBLE_CHECKED(x, args[0]);
+
+ // If the second argument is a smi, it is much faster to call the
+ // custom powi() function than the generic pow().
+ if (args[1]->IsSmi()) {
+ int y = Smi::cast(args[1])->value();
+ return Heap::AllocateHeapNumber(powi(x, y));
+ }
+
CONVERT_DOUBLE_CHECKED(y, args[1]);
- if (isnan(y) || ((x == 1 || x == -1) && isinf(y))) {
- return Heap::nan_value();
+ if (y == 0.5) {
+ // It's not uncommon to use Math.pow(x, 0.5) to compute the square
+ // root of a number. To speed up such computations, we explictly
+ // check for this case and use the sqrt() function which is faster
+ // than pow().
+ return Heap::AllocateHeapNumber(sqrt(x));
+ } else if (y == -0.5) {
+ // Optimized using Math.pow(x, -0.5) == 1 / Math.pow(x, 0.5).
+ return Heap::AllocateHeapNumber(1.0 / sqrt(x));
} else if (y == 0) {
return Smi::FromInt(1);
+ } else if (isnan(y) || ((x == 1 || x == -1) && isinf(y))) {
+ return Heap::nan_value();
} else {
return Heap::AllocateHeapNumber(pow(x, y));
}
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