Reverting PR 452

src/containers/mixed_union.c

@@ -206,37 +206,11 @@ bool array_array_container_inplace_union(
           return false;  // not a bitset
         } else {
           memmove(src_1->array + src_2->cardinality, src_1->array, src_1->cardinality * sizeof(uint16_t));
-          /*
-            Next line is safe:
-
-            We just need to focus on the reading and writing performed on array1. In `union_vector16`, both vectorized and scalar code still obey the basic rule: read from two inputs, do the union, and then write the output.
-
-            Let's say the length(cardinality) of input2 is L2:
-            ```
-                |<-  L2  ->|
-            array1: [output--- |input 1---|---]
-            array2: [input 2---]
-            ```
-            Let's define 3 __m128i pointers, `pos1` starts from `input1`, `pos2` starts from `input2`, these 2 point at the next byte to read, `out` starts from `output`, pointing at the next byte to overwrite.
-            ```
-            array1: [output--- |input 1---|---]
-                        ^          ^
-                    out        pos1
-            array2: [input 2---]
-                        ^
-                        pos2
-            ```
-            The union output always contains less or equal number of elements than all inputs added, so we have:
-            ```
-            out <= pos1 + pos2
-            ```
-            therefore:
-            ```
-            out <= pos1 + L2
-            ```
-            which means you will not overwrite data beyond pos1, so the data haven't read is safe, and we don't care the data already read.
-          */
-          src_1->cardinality = (int32_t)fast_union_uint16(src_1->array + src_2->cardinality, src_1->cardinality,
+          // In theory, we could use fast_union_uint16, but it is unsafe. It fails
+          // with Intel compilers in particular.
+          // https://github.com/RoaringBitmap/CRoaring/pull/452
+          // See report https://github.com/RoaringBitmap/CRoaring/issues/476
+          src_1->cardinality = (int32_t)union_uint16(src_1->array + src_2->cardinality, src_1->cardinality,
                                   src_2->array, src_2->cardinality, src_1->array);
           return false; // not a bitset
         }