Fix bugzilla 24594 - ImportC: Packed struct has wrong layout (#16611)

Packed structs were already implemented with the Microsoft syntax
(e.g. `#pragma pack(push, 1)`), but the GCC syntax was ignored.

compiler/src/dmd/cparse.d

@@ -3634,6 +3634,12 @@ final class CParser(AST) : Parser!AST
                  * type on the target machine. It's the opposite of __attribute__((packed))
                  */
             }
+            else if (token.ident == Id.packed)
+            {
+                specifier.packalign.set(1);
+                specifier.packalign.setPack(true);
+                nextToken();
+            }
             else if (token.ident == Id.always_inline) // https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
             {
                 specifier.scw |= SCW.xinline;
@@ -3982,7 +3988,7 @@ final class CParser(AST) : Parser!AST
             members = new AST.Dsymbols();          // so `members` will be non-null even with 0 members
             while (token.value != TOK.rightCurly)
             {
-                cparseStructDeclaration(members);
+                cparseStructDeclaration(members, packalign);
 
                 if (token.value == TOK.endOfFile)
                     break;
@@ -3996,6 +4002,24 @@ final class CParser(AST) : Parser!AST
                  *    struct-declarator (opt)
                  */
             }
+
+            /* GNU Extensions
+             * Parse the postfix gnu-attributes (opt)
+             */
+            Specifier specifier;
+            if (token.value == TOK.__attribute__)
+                cparseGnuAttributes(specifier);
+            if (!specifier.packalign.isUnknown)
+            {
+                packalign.set(specifier.packalign.get());
+                packalign.setPack(specifier.packalign.isPack());
+                foreach (ref d; (*members)[])
+                {
+                    auto decls = new AST.Dsymbols(1);
+                    (*decls)[0] = d;
+                    d = new AST.AlignDeclaration(d.loc, specifier.packalign, decls);
+                }
+            }
         }
         else if (!tag)
             error("missing tag `identifier` after `%s`", Token.toChars(structOrUnion));
@@ -4027,8 +4051,9 @@ final class CParser(AST) : Parser!AST
      *    declarator (opt) : constant-expression
      * Params:
      *    members = where to put the fields (members)
+     *    packalign = alignment to use for struct members
      */
-    void cparseStructDeclaration(AST.Dsymbols* members)
+    void cparseStructDeclaration(AST.Dsymbols* members, structalign_t packalign)
     {
         //printf("cparseStructDeclaration()\n");
         if (token.value == TOK._Static_assert)
@@ -4039,7 +4064,7 @@ final class CParser(AST) : Parser!AST
         }
 
         Specifier specifier;
-        specifier.packalign = this.packalign;
+        specifier.packalign = packalign.isUnknown ? this.packalign : packalign;
         auto tspec = cparseSpecifierQualifierList(LVL.member, specifier);
         if (!tspec)
         {

compiler/src/dmd/frontend.h

@@ -8799,6 +8799,7 @@ struct Id final
     static Identifier* define;
     static Identifier* undef;
     static Identifier* ident;
+    static Identifier* packed;
     static void initialize();
     Id()
     {

compiler/src/dmd/id.d

@@ -574,6 +574,7 @@ immutable Msgtable[] msgtable =
     { "define" },
     { "undef" },
     { "ident" },
+    { "packed" },
 ];
 
 

compiler/test/compilable/test24594.c

@@ -0,0 +1,28 @@
+#include <stdint.h>
+
+struct S1
+{
+  uint32_t a;
+  uint64_t b;
+} __attribute__((packed));
+
+_Static_assert(sizeof(S1) == 12, "S1 size");
+_Static_assert(&((struct S1*)0)->b == 4, "S1::b offset");
+
+struct __attribute__((packed)) S2
+{
+  uint8_t a;
+  uint16_t b;
+};
+
+_Static_assert(sizeof(S2) == 3, "S2 size");
+_Static_assert(&((struct S2*)0)->b == 1, "S2::b offset");
+
+struct __attribute__((packed)) S3
+{
+  uint32_t a;
+  uint8_t b;
+};
+
+_Static_assert(sizeof(S3) == 5, "S3 size");
+_Static_assert(&((struct S3*)0)->b == 4, "S3::b offset");