Faster implementation of Math.exp()

test/mjsunit/math-exp-precision.js

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-// Flags: --expose-gc
+// Tests that the --fast-math implementation of Math.exp() has
+// reasonable precision.
 
-// Test whether the runtime implementation and generated code of
-// sine and tangens return the same results.
-
-function test(f, x, name) {
-  // Reset transcendental cache.
-  gc();
-  // Initializing cache leads to a runtime call.
-  var runtime_result = f(x);
-  // Flush transcendental cache entries and optimize f.
-  for (var i = 0; i < 100000; i++) f(i);
-  // Calculate using generated code.
-  var gencode_result = f(x);
-  print(name + " runtime function: " + runtime_result);
-  print(name + " generated code  : " + gencode_result);
-  assertEquals(gencode_result, runtime_result);
+function exp(x) {
+  return Math.exp(x);
 }
 
-test(Math.tan, -1.57079632679489660000, "Math.tan");
-test(Math.sin, 6.283185307179586, "Math.sin");
+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 mantissas.
+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);