Enable HASH_CODE by default to avoid repeated hash computation

On some SV-COMP benchmark categories, hashing accounts for >20% of CPU
time (with profiling enabled) - top five:

* ReachSafety-BitVectors: 29.29% (470.54 seconds, which reduces to 4.39
  seconds; for benchmarks not timing out we save 170 seconds (25%) in
  non-profiling mode)
* Systems_BusyBox_NoOverflows: 27.98% (284.15 seconds, which reduces to
  1.74 seconds; for the 1 benchmark not timing out we save 23 seconds
  (6%) in non-profiling mode)
* Systems_BusyBox_MemSafety: 24.24% (194.74 seconds, which reduces to
  0.93 seconds; no measurable difference on the 2 benchmarks not
  failing/timing out)
* NoOverflows-Other: 18.84% (1127.61 seconds, which reduces to
  23.57 seconds; for benchmarks not timing out we save 5 seconds (7%) in
  non-profiling mode)
* ReachSafety-ControlFlow: 17.75% (1194.04 seconds, which reduces to
  29.17 seconds; for benchmarks not timing out we save 200 seconds (25%)
  in non-profiling mode)

For ReachSafety-ECA it's only 4.7%, which still amounts to 3006.7
seconds. With this change this reduces to 323.07 seconds. On
ReachSafety-ECA, this enables 3055.35 symex_step calls per second over
2752.28 calls per second without this change.

Author: tautschnig

src/util/irep.h

@@ -17,7 +17,9 @@ Author: Daniel Kroening, [email protected]
 #include "irep_ids.h"
 
 #define SHARING
-// #define HASH_CODE
+#ifndef HASH_CODE
+#  define HASH_CODE 1
+#endif
 // #define NAMED_SUB_IS_FORWARD_LIST
 
 #ifdef NAMED_SUB_IS_FORWARD_LIST
@@ -138,7 +140,7 @@ class tree_nodet : public ref_count_ift<sharing>
   named_subt named_sub;
   subt sub;
 
-#ifdef HASH_CODE
+#if HASH_CODE
   mutable std::size_t hash_code = 0;
 #endif
 
@@ -147,7 +149,7 @@ class tree_nodet : public ref_count_ift<sharing>
     data.clear();
     sub.clear();
     named_sub.clear();
-#ifdef HASH_CODE
+#if HASH_CODE
     hash_code = 0;
 #endif
   }
@@ -157,7 +159,7 @@ class tree_nodet : public ref_count_ift<sharing>
     d.data.swap(data);
     d.sub.swap(sub);
     d.named_sub.swap(named_sub);
-#ifdef HASH_CODE
+#if HASH_CODE
     std::swap(d.hash_code, hash_code);
 #endif
   }
@@ -278,7 +280,7 @@ class sharing_treet
   dt &write()
   {
     detach();
-#ifdef HASH_CODE
+#if HASH_CODE
     data->hash_code = 0;
 #endif
     return *data;
@@ -319,7 +321,7 @@ class non_sharing_treet
 
   dt &write()
   {
-#ifdef HASH_CODE
+#if HASH_CODE
     data.hash_code = 0;
 #endif
     return data;