fdlibm-for-pow.js - mozsearch

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// Test that Math.pow uses fdlibm and produces the same bit-exact result on

// every platform.

//

// Before Bug 2037094, Math.pow went through std::pow, which on Android API

// <= 28 ARM devices with unpatched OEM bionic returns 1.9275814160560204e-50

// for Math.pow(Math.PI, -100) — a 1-ULP-different result from the fdlibm

// value 1.9275814160560206e-50. Math.pow now unconditionally calls fdlibm_pow.

const FDLIBM_POW_PI_NEG_100 = 1.9275814160560206e-50;

assertEq(Math.pow(Math.PI, -100), FDLIBM_POW_PI_NEG_100);

// A few other inputs that hit the libm path (non-integer exponent, or an

// integer that overflows the integer-fast-path early). All must be bit-exact.

assertEq(Math.pow(2, 0.5), Math.SQRT2);

assertEq(Math.pow(10, 0.3010299956639812), 2);

assertEq(Math.pow(Math.E, 2), 7.3890560989306495);

assertEq(Math.pow(0.5, 100), 7.888609052210118e-31);

// Special-case paths must continue to work and return finite IEEE values.

assertEq(Math.pow(NaN, 0), 1);

assertEq(Math.pow(2, 0), 1);

assertEq(Math.pow(-0, 1), -0);

assertEq(Math.pow(4, 0.5), 2);   // sqrt fast-path

assertEq(Math.pow(4, -0.5), 0.5);

assertEq(Math.pow(2, Infinity), Infinity);

assertEq(Math.pow(1, Infinity), NaN);

assertEq(Math.pow(-1, -Infinity), NaN);

// Exercise hot paths so baseline + ion both compile pow, and accumulate the

// result over 1000 iterations so any per-call bit difference (across tiers or

// platforms) compounds into the sum and is caught. The expected value is the

// bit-exact accumulated sum; it is deliberately not 1000 * FDLIBM_POW_PI_NEG_100

// because floating-point summation rounds at each step.

function loop() {

  let acc = 0;

  for (let i = 0; i < 1000; i++) {

    acc += Math.pow(Math.PI, -100);

  return acc;

assertEq(loop(), 1.9275814160560497e-47);

// Integer-exponent fast path (powi) is bit-deterministic — the integer fast

// path itself doesn't call libm at all for small n.

assertEq(Math.pow(3, 4), 81);

assertEq(Math.pow(2, 10), 1024);

assertEq(Math.pow(-2, 3), -8);