Add utility function 'memxor'.

misc.h

@@ -430,4 +430,13 @@ LoadFileStatus lf_load(LoadedFile *lf, const Filename *filename);
 static inline ptrlen ptrlen_from_lf(LoadedFile *lf)
 { return make_ptrlen(lf->data, lf->len); }
 
+/* Set the memory block of 'size' bytes at 'out' to the bitwise XOR of
+ * the two blocks of the same size at 'in1' and 'in2'.
+ *
+ * 'out' may point to exactly the same address as one of the inputs,
+ * but if the input and output blocks overlap in any other way, the
+ * result of this function is not guaranteed. No memmove-style effort
+ * is made to handle difficult overlap cases. */
+void memxor(uint8_t *out, const uint8_t *in1, const uint8_t *in2, size_t size);
+
 #endif

utils.c

@@ -1073,3 +1073,29 @@ void write_c_string_literal(FILE *fp, ptrlen str)
             fprintf(fp, "\\%03o", (unsigned char)c);
     }
 }
+
+void memxor(uint8_t *out, const uint8_t *in1, const uint8_t *in2, size_t size)
+{
+    switch (size & 15) {
+      case 0:
+        while (size >= 16) {
+                   *out++ = *in1++ ^ *in2++;
+          case 15: *out++ = *in1++ ^ *in2++;
+          case 14: *out++ = *in1++ ^ *in2++;
+          case 13: *out++ = *in1++ ^ *in2++;
+          case 12: *out++ = *in1++ ^ *in2++;
+          case 11: *out++ = *in1++ ^ *in2++;
+          case 10: *out++ = *in1++ ^ *in2++;
+          case 9:  *out++ = *in1++ ^ *in2++;
+          case 8:  *out++ = *in1++ ^ *in2++;
+          case 7:  *out++ = *in1++ ^ *in2++;
+          case 6:  *out++ = *in1++ ^ *in2++;
+          case 5:  *out++ = *in1++ ^ *in2++;
+          case 4:  *out++ = *in1++ ^ *in2++;
+          case 3:  *out++ = *in1++ ^ *in2++;
+          case 2:  *out++ = *in1++ ^ *in2++;
+          case 1:  *out++ = *in1++ ^ *in2++;
+            size -= 16;
+        }
+    }
+}