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stream: code cleanup and micro-optimization #2479

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Aug 24, 2015
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stream: micro-optimize high water mark calculation
Don't iterate over all 32 bits, use some hacker's delight bit twiddling
to compute the next power of two.

The logic can be reduced to `n = 1 << 32 - Math.clz32(n)` but then it
can't easily be backported to v2.x; Math.clz32() was added in V8 4.3.

PR-URL: #2479
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
bnoordhuis committed Aug 24, 2015
commit 3af8e451f272f1398acf7336f9eff0c64340ffe3
6 changes: 5 additions & 1 deletion lib/_stream_readable.js
Original file line number Diff line number Diff line change
@@ -198,7 +198,11 @@ function roundUpToNextPowerOf2(n) {
} else {
// Get the next highest power of 2
n--;
for (var p = 1; p < 32; p <<= 1) n |= n >> p;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
n++;
}
return n;