Simplify byte-extract from struct or union expressions

With rewrite_union rewriting union accesses to byte-extract operations,
simplifying such expressions must cover as many cases as possible.
Constants were mostly already being rewritten via expr2bits, but that
wouldn't be able to handle expressions involving pointers (even when
they are effectively constants when using address-of operations).  The
additional simplification rules mimic what lower_byte_extract already
did at a later stage, but such late rewrites would not benefit constant
propagation done by goto-symex.

Author: tautschnig

regression/cbmc/simplify-union/main.c

@@ -0,0 +1,49 @@
+#include <assert.h>
+#include <stdlib.h>
+
+struct O
+{
+  char *ptr;
+  unsigned len;
+};
+
+struct E
+{
+  int e;
+};
+
+union U {
+  struct O ok;
+  struct E err;
+};
+
+struct S
+{
+  union U u;
+  _Bool success;
+};
+
+void alloc(struct S *s, unsigned size)
+{
+  char *p = malloc(sizeof(char) * size);
+  if(p != 0)
+  {
+    s->u.ok = (struct O){p, size};
+    s->success = 1;
+  }
+  else
+  {
+    s->success = 0;
+  }
+}
+
+int main()
+{
+  struct S s;
+  alloc(&s, 2);
+  if(s.success)
+  {
+    assert(s.u.ok.len == 2);
+    s.u.ok.ptr = "a";
+  }
+}

regression/cbmc/simplify-union/test.desc

@@ -0,0 +1,11 @@
+CORE
+main.c
+
+^Generated 1 VCC\(s\), 0 remaining after simplification$
+^VERIFICATION SUCCESSFUL$
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--
+The len member should constant propagate with the help of simplification.

src/pointer-analysis/value_set.cpp

@@ -784,6 +784,11 @@ void value_sett::get_value_set_rec(
         get_value_set_rec(*it, dest, suffix, original_type, ns);
     }
   }
+  else if(expr.id() == ID_union)
+  {
+    get_value_set_rec(
+      to_union_expr(expr).op(), dest, suffix, original_type, ns);
+  }
   else if(expr.id()==ID_with)
   {
     const with_exprt &with_expr = to_with_expr(expr);

src/util/pointer_offset_size.cpp

@@ -16,6 +16,7 @@ Author: Daniel Kroening, [email protected]
 #include "c_types.h"
 #include "invariant.h"
 #include "namespace.h"
+#include "pointer_expr.h"
 #include "simplify_expr.h"
 #include "ssa_expr.h"
 #include "std_expr.h"
@@ -681,6 +682,27 @@ optionalt<exprt> get_subexpression_at_offset(
       }
     }
   }
+  else if(
+    object_descriptor_exprt(expr).root_object().id() == ID_union &&
+    source_type.id() == ID_union)
+  {
+    const union_typet &union_type = to_union_type(source_type);
+
+    for(const auto &component : union_type.components())
+    {
+      const auto m_size_bits = pointer_offset_bits(component.type(), ns);
+      if(!m_size_bits.has_value())
+        continue;
+
+      // if this member completely contains the target, recurse into it
+      if(offset_bytes * 8 + *target_size_bits <= *m_size_bits)
+      {
+        const member_exprt member(expr, component.get_name(), component.type());
+        return get_subexpression_at_offset(
+          member, offset_bytes, target_type_raw, ns);
+      }
+    }
+  }
 
   return make_byte_extract(
     expr, from_integer(offset_bytes, index_type()), target_type_raw);

src/util/simplify_expr.cpp

@@ -1765,6 +1765,71 @@ simplify_exprt::simplify_byte_extract(const byte_extract_exprt &expr)
       return std::move(*tmp);
   }
 
+  // push byte extracts into struct or union expressions, just like
+  // lower_byte_extract does (this is the same code, except recursive calls use
+  // simplify rather than lower_byte_extract)
+  if(expr.op().id() == ID_struct || expr.op().id() == ID_union)
+  {
+    if(expr.type().id() == ID_struct || expr.type().id() == ID_struct_tag)
+    {
+      const struct_typet &struct_type = to_struct_type(ns.follow(expr.type()));
+      const struct_typet::componentst &components = struct_type.components();
+
+      bool failed = false;
+      struct_exprt s({}, expr.type());
+
+      for(const auto &comp : components)
+      {
+        auto component_bits = pointer_offset_bits(comp.type(), ns);
+
+        // the next member would be misaligned, abort
+        if(
+          !component_bits.has_value() || *component_bits == 0 ||
+          *component_bits % 8 != 0)
+        {
+          failed = true;
+          break;
+        }
+
+        auto member_offset_opt =
+          member_offset_expr(struct_type, comp.get_name(), ns);
+
+        if(!member_offset_opt.has_value())
+        {
+          failed = true;
+          break;
+        }
+
+        exprt new_offset = simplify_rec(
+          plus_exprt{expr.offset(),
+                     typecast_exprt::conditional_cast(
+                       member_offset_opt.value(), expr.offset().type())});
+
+        byte_extract_exprt tmp = expr;
+        tmp.type() = comp.type();
+        tmp.offset() = new_offset;
+
+        s.add_to_operands(simplify_byte_extract(tmp));
+      }
+
+      if(!failed)
+        return s;
+    }
+    else if(expr.type().id() == ID_union || expr.type().id() == ID_union_tag)
+    {
+      const union_typet &union_type = to_union_type(ns.follow(expr.type()));
+      auto widest_member_opt = union_type.find_widest_union_component(ns);
+      if(widest_member_opt.has_value())
+      {
+        byte_extract_exprt be = expr;
+        be.type() = widest_member_opt->first.type();
+        return union_exprt{widest_member_opt->first.get_name(),
+                           simplify_byte_extract(be),
+                           expr.type()};
+      }
+    }
+  }
+
   // try to refine it down to extracting from a member or an index in an array
   auto subexpr =
     get_subexpression_at_offset(expr.op(), *offset, expr.type(), ns);