Extract exponentiation operation from mathPow function.

Summary:
In anticipation of adding support for `**`, we move the exponentiation
logic out of `mathPow` and place it in `expOp`, where it will be used
to avoid duplicating logic in the interpreter loop.

Reviewed By: dulinriley

Differential Revision: D16628558

fbshipit-source-id: 83e3cfd3c41525498b2e13e31f2164f5d1947cfc

include/hermes/VM/Operations.h

@@ -213,6 +213,23 @@ bool isPrimitive(HermesValue val);
 CallResult<HermesValue>
 addOp_RJS(Runtime *runtime, Handle<> xHandle, Handle<> yHandle);
 
+/// ES9.0 12.6.4
+inline double expOp(double x, double y) {
+  constexpr double nan = std::numeric_limits<double>::quiet_NaN();
+
+  // Handle special cases that std::pow handles differently.
+  if (std::isnan(y)) {
+    return nan;
+  } else if (y == 0) {
+    return 1;
+  } else if (std::abs(x) == 1 && std::isinf(y)) {
+    return nan;
+  }
+
+  // std::pow handles the other edge cases as the ES9.0 spec requires.
+  return std::pow(x, y);
+}
+
 /// ES5.1 7.2
 inline bool isWhiteSpaceChar(char16_t c) {
   return c == u'\u0009' || c == u'\u000B' || c == u'\u000C' || c == u'\u0020' ||

lib/VM/JSLib/Math.cpp

@@ -179,7 +179,7 @@ CallResult<HermesValue> mathMin(void *, Runtime *runtime, NativeArgs args) {
   return HermesValue::encodeDoubleValue(result);
 }
 
-// ES5.1 15.8.2.13
+// ES9.0 20.2.2.26
 CallResult<HermesValue> mathPow(void *, Runtime *runtime, NativeArgs args) {
   auto res = toNumber_RJS(runtime, args.getArgHandle(runtime, 0));
   if (LLVM_UNLIKELY(res == ExecutionStatus::EXCEPTION)) {
@@ -193,23 +193,7 @@ CallResult<HermesValue> mathPow(void *, Runtime *runtime, NativeArgs args) {
   }
   const double y = res->getNumber();
 
-  const double nan = std::numeric_limits<double>::quiet_NaN();
-
-  double result;
-
-  // Handle special cases that std::pow handles differently.
-  if (std::isnan(y)) {
-    result = nan;
-  } else if (y == 0) {
-    result = 1;
-  } else if (std::abs(x) == 1 && std::isinf(y)) {
-    result = nan;
-  } else {
-    // std::pow handles the other edge cases as ES5.1 specifies.
-    result = std::pow(x, y);
-  }
-
-  return HermesValue::encodeNumberValue(result);
+  return HermesValue::encodeNumberValue(expOp(x, y));
 }
 
 // ES5.1 15.8.2.14