C++: Fix range analysis rounding for negative zero

cpp/ql/src/semmle/code/cpp/rangeanalysis/SimpleRangeAnalysis.qll

@@ -293,13 +293,24 @@ predicate analyzableDef(RangeSsaDefinition def, LocalScopeVariable v) {
   assignmentDef(def, v, _) or defDependsOnDef(def, v, _, _)
 }
 
+/**
+ * Computes a normal form of `x` where -0.0 has changed to +0.0. This can be
+ * needed on the lesser side of a floating-point comparison or on both sides of
+ * a floating point equality because QL does not follow IEEE in floating-point
+ * comparisons but instead defines -0.0 to be less than and distinct from 0.0.
+ */
+bindingset[x]
+private float normalizeFloatUp(float x) {
+  result = x + 0.0
+}
+
 /**
  * Computes `x + y`, rounded towards +Inf. This is the general case where both
  * `x` and `y` may be large numbers.
  */
 bindingset[x, y]
 private float addRoundingUp(float x, float y) {
-  if (x + y) - x < y or (x + y) - y < x
+  if normalizeFloatUp((x + y) - x) < y or normalizeFloatUp((x + y) - y) < x
   then result = (x + y).nextUp()
   else result = (x + y)
 }
@@ -310,7 +321,7 @@ private float addRoundingUp(float x, float y) {
  */
 bindingset[x, y]
 private float addRoundingDown(float x, float y) {
-  if (x + y) - x > y or (x + y) - y > x
+  if (x + y) - x > normalizeFloatUp(y) or (x + y) - y > normalizeFloatUp(x)
   then result = (x + y).nextDown()
   else result = (x + y)
 }

cpp/ql/test/library-tests/rangeanalysis/rangeanalysis/lowerBound.expected

@@ -141,10 +141,10 @@
 | test.c:186:18:186:18 | a | -7.0 |
 | test.c:187:14:187:14 | a | -7.0 |
 | test.c:188:14:188:14 | a | -7.0 |
-| test.c:189:5:189:9 | total | -52.00000000000001 |
+| test.c:189:5:189:9 | total | -52.0 |
 | test.c:189:14:189:14 | b | -7.0 |
 | test.c:189:16:189:16 | c | 2.0 |
-| test.c:192:10:192:14 | total | -57.00000000000001 |
+| test.c:192:10:192:14 | total | -57.0 |
 | test.c:200:12:200:12 | a | -2.147483648E9 |
 | test.c:200:17:200:17 | a | 3.0 |
 | test.c:200:33:200:33 | b | -2.147483648E9 |

cpp/ql/test/library-tests/rangeanalysis/rangeanalysis/test.c

@@ -186,10 +186,10 @@ int test_unary(int a) {
   if (-7 <= a && a <= -2) {
     int b = +a;
     int c = -a;
-    total += b+c; // BUG: lower bound should be exact, not approximate
+    total += b+c;
   }
 
-  return total; // BUG: lower bound should be exact, not approximate
+  return total;
 }