test/mjsunit/math-exp-precision.js

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// Tests that the --fast-math implementation of Math.exp() has
// reasonable precision.

function exp(x) {
  return Math.exp(x);
}

var first_call_result = exp(Math.PI);
var second_call_result = exp(Math.PI);

function assertAlmostEquals(expected, actual, x) {
  if (expected == 0 && actual == 0) return;  // OK
  if (expected == Number.POSITIVE_INFINITY &&
      actual == Number.POSITIVE_INFINITY) {
    return;  // OK
  }
  relative_diff = Math.abs(expected/actual - 1);
  assertTrue(relative_diff < 1e-12, "relative difference of " + relative_diff +
                                    " for input " + x);
}

var increment = Math.PI / 35;  // Roughly 0.1, but we want to try many
                               // different mantissae.
for (var x = -708; x < 710; x += increment) {
  var ex = exp(x);
  var reference = Math.pow(Math.E, x);
  assertAlmostEquals(reference, ex, x);
  if (ex > 0 && isFinite(ex)) {
    var back = Math.log(ex);
    assertAlmostEquals(x, back, x + " (backwards)");
  }
}

// Make sure optimizing the function does not alter the result.
var last_call_result = exp(Math.PI);
assertEquals(first_call_result, second_call_result);
assertEquals(first_call_result, last_call_result);