JS: Also have modulo return integer 0 when the lhs is 0

When both operands of a modulo are integers, and the right-hand side is
bigger than zero, it is fine to have the left-hand side be 0, as the
result is (positive) zero.

This also handles the somewhat common case below, where lhs is 0,
and where returning a double would result in the slow path to do the
actual array access:
	array[lhs % array.length]

Change-Id: I4ca268a08604c3c77a23a92eee193c807696d5ba
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 7 insertions, 4 deletions

diff --git a/src/qml/jsruntime/qv4runtime.cpp b/src/qml/jsruntime/qv4runtime.cpp
index 11f06d9ed1..82e1284289 100644
--- a/src/qml/jsruntime/qv4runtime.cpp
+++ b/src/qml/jsruntime/qv4runtime.cpp
@@ -1801,10 +1801,13 @@ ReturnedValue Runtime::method_mod(const Value &left, const Value &right)
 {
     TRACE2(left, right);
 
-    if (Value::integerCompatible(left, right) && left.integerValue() > 0 && right.integerValue() > 0) {
-        int intRes = left.integerValue() % right.integerValue();
-        if (intRes != 0 || left.integerValue() >= 0)
-            return Encode(intRes);
+    if (Value::integerCompatible(left, right) && left.integerValue() >= 0 && right.integerValue() > 0) {
+        // special cases are handled by fmod, among them:
+        //  - arithmic execeptions for ints in c++, eg: INT_MIN % -1
+        //  - undefined behavior in c++, e.g.: anything % 0
+        //  - uncommon cases which would complicate the condition, e.g.: negative integers
+        //    (this makes sure that -1 % 1 == -0 by passing it to fmod)
+        return Encode(left.integerValue() % right.integerValue());
     }
 
     double lval = RuntimeHelpers::toNumber(left);