Merge pull request #7597 from tautschnig/bugfixes/7596-attributes

C front-end: fix the interaction between packing and alignment

Author: tautschnig

regression/ansi-c/gcc_attributes15/main.c

@@ -0,0 +1,45 @@
+#define PADDING_SIZE (64 - sizeof(char) - sizeof(int))
+
+typedef struct __attribute__((packed, aligned(64)))
+{
+  char uchar;
+  int uint;
+  char padding[PADDING_SIZE];
+} a_t;
+
+typedef struct __attribute__((packed, aligned(64)))
+{
+  int uint;
+  char uchar;
+  char padding[PADDING_SIZE];
+} b_t;
+
+typedef struct __attribute__((packed))
+{
+  int uint;
+  char uchar;
+  struct
+  {
+    char a;
+    int b;
+  } not_packed __attribute__((aligned(8)));
+} c_t;
+
+typedef struct __attribute__((packed, aligned(1)))
+{
+  int uint;
+  char uchar;
+  struct
+  {
+    char a;
+    int b;
+  } not_packed __attribute__((aligned(8)));
+} d_t;
+
+int main()
+{
+  _Static_assert(sizeof(a_t) == sizeof(b_t), "");
+  _Static_assert(sizeof(c_t) == 8 + 2 * sizeof(int), "");
+  _Static_assert(sizeof(d_t) == 8 + 2 * sizeof(int), "");
+  return 0;
+}

regression/ansi-c/gcc_attributes15/test.desc

@@ -0,0 +1,8 @@
+CORE gcc-only
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

regression/ansi-c/pragma_pack4/main.c

@@ -0,0 +1,27 @@
+struct S1
+{
+  char a;
+  struct
+  {
+    short b;
+  } c;
+};
+
+#pragma pack(4)
+// note that the above is different from __attribute__((packed, aligned(4))) in
+// that it sets the _maximum_ alignment of members - it does not affect the
+// overall alignment of the struct
+
+struct S2
+{
+  char a;
+  struct
+  {
+    short b;
+  } c;
+};
+
+int main()
+{
+  _Static_assert(sizeof(struct S1) == sizeof(struct S2), "");
+}

regression/ansi-c/pragma_pack4/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/padding.cpp

@@ -330,14 +330,10 @@ static void add_padding_gcc(struct_typet &type, const namespacet &ns)
     }
   }
 
-  // Is the struct packed, without any alignment specification?
-  if(type.get_bool(ID_C_packed) &&
-     type.find(ID_C_alignment).is_nil())
-    return; // done
-
   mp_integer offset=0;
   mp_integer max_alignment=0;
   std::size_t bit_field_bits=0;
+  const bool struct_is_packed = type.get_bool(ID_C_packed);
 
   for(struct_typet::componentst::iterator
       it=components.begin();
@@ -347,9 +343,6 @@ static void add_padding_gcc(struct_typet &type, const namespacet &ns)
     const typet it_type=it->type();
     mp_integer a=1;
 
-    const bool packed=it_type.get_bool(ID_C_packed) ||
-                      ns.follow(it_type).get_bool(ID_C_packed);
-
     if(it_type.id()==ID_c_bit_field)
     {
       a = alignment(to_c_bit_field_type(it_type).underlying_type(), ns);
@@ -392,13 +385,19 @@ static void add_padding_gcc(struct_typet &type, const namespacet &ns)
       bit_field_bits == 0, "padding ensures offset at byte boundaries");
 
     // check minimum alignment
-    if(a<config.ansi_c.alignment && !packed)
+    if(
+      a < config.ansi_c.alignment && !it_type.get_bool(ID_C_packed) &&
+      !ns.follow(it_type).get_bool(ID_C_packed))
+    {
       a=config.ansi_c.alignment;
+    }
 
     if(max_alignment<a)
       max_alignment=a;
 
-    if(a!=1)
+    if(
+      a != 1 &&
+      (!struct_is_packed || it_type.find(ID_C_alignment).is_not_nil()))
     {
       // we may need to align it
       const mp_integer displacement = offset % a;
@@ -433,7 +432,7 @@ static void add_padding_gcc(struct_typet &type, const namespacet &ns)
     }
   }
   // Is the struct packed, without any alignment specification?
-  else if(type.get_bool(ID_C_packed))
+  else if(struct_is_packed)
     return; // done
 
   // There may be a need for 'end of struct' padding.

src/ansi-c/parser.y

@@ -1741,12 +1741,14 @@ member_declaring_list:
           type_specifier
           member_declarator
         {
-          if(!PARSER.pragma_pack.empty() &&
+          if(parser_stack($2).id() != ID_struct &&
+             parser_stack($2).id() != ID_union &&
+             !PARSER.pragma_pack.empty() &&
              !PARSER.pragma_pack.back().is_zero())
           {
             // communicate #pragma pack(n) alignment constraints by
-            // by both setting packing AND alignment; see padding.cpp
-            // for more details
+            // by both setting packing AND alignment for individual struct/union
+            // members; see padding.cpp for more details
             init($$);
             set($$, ID_packed);
             $2=merge($2, $$);