Implement packed structures (RFC 32)

This change adds program annotations allowing programmers to declare
packed structures, i.e. structures without implementation-specific
padding between members.

Closes #1523.

src/libponyc/codegen/gentype.c

@@ -407,6 +407,7 @@ static bool make_struct(compile_t* c, reach_type_t* t)
 {
   LLVMTypeRef type;
   int extra = 0;
+  bool packed = false;
 
   switch(t->underlying)
   {
@@ -421,12 +422,18 @@ static bool make_struct(compile_t* c, reach_type_t* t)
       break;
 
     case TK_STRUCT:
+    {
       // Pointer and Maybe will have no structure.
       if(t->structure == NULL)
         return true;
 
       type = t->structure;
+      ast_t* def = (ast_t*)ast_data(t->ast);
+      if(ast_has_annotation(def, "packed"))
+        packed = true;
+
       break;
+    }
 
     case TK_PRIMITIVE:
       // Machine words will have a primitive.
@@ -481,7 +488,7 @@ static bool make_struct(compile_t* c, reach_type_t* t)
     }
   }
 
-  LLVMStructSetBody(type, elements, t->field_count + extra, false);
+  LLVMStructSetBody(type, elements, t->field_count + extra, packed);
   ponyint_pool_free_size(buf_size, elements);
   return true;
 }

test/libponyc/codegen.cc

@@ -0,0 +1,105 @@
+#include <gtest/gtest.h>
+#include <platform.h>
+
+#include <reach/reach.h>
+
+#include "util.h"
+
+#ifdef _MSC_VER
+// Stop MSVC from complaining about conversions from LLVMBool to bool.
+# pragma warning(disable:4800)
+#endif
+
+#define TEST_COMPILE(src) DO(test_compile(src, "ir"))
+
+
+class CodegenTest : public PassTest
+{};
+
+
+TEST_F(CodegenTest, PackedStructIsPacked)
+{
+  const char* src =
+    "struct \\packed\\ Foo\n"
+    "  var a: U8 = 0\n"
+    "  var b: U32 = 0\n"
+
+	"actor Main\n"
+	"  new create(env: Env) =>\n"
+	"    Foo";
+
+  TEST_COMPILE(src);
+
+  reach_t* reach = compile->reach;
+  reach_type_t* foo = reach_type_name(reach, "Foo");
+  ASSERT_TRUE(foo != NULL);
+
+  LLVMTypeRef type = foo->structure;
+  ASSERT_TRUE(LLVMIsPackedStruct(type));
+}
+
+
+TEST_F(CodegenTest, NonPackedStructIsntPacked)
+{
+  const char* src =
+    "struct Foo\n"
+    "  var a: U8 = 0\n"
+    "  var b: U32 = 0\n"
+
+	"actor Main\n"
+	"  new create(env: Env) =>\n"
+	"    Foo";
+
+  TEST_COMPILE(src);
+
+  reach_t* reach = compile->reach;
+  reach_type_t* foo = reach_type_name(reach, "Foo");
+  ASSERT_TRUE(foo != NULL);
+
+  LLVMTypeRef type = foo->structure;
+  ASSERT_TRUE(!LLVMIsPackedStruct(type));
+}
+
+
+TEST_F(CodegenTest, ClassCannotBePacked)
+{
+  const char* src =
+    "class \\packed\\ Foo\n"
+    "  var a: U8 = 0\n"
+    "  var b: U32 = 0\n"
+
+	"actor Main\n"
+	"  new create(env: Env) =>\n"
+	"    Foo";
+
+  TEST_COMPILE(src);
+
+  reach_t* reach = compile->reach;
+  reach_type_t* foo = reach_type_name(reach, "Foo");
+  ASSERT_TRUE(foo != NULL);
+
+  LLVMTypeRef type = foo->structure;
+  ASSERT_TRUE(!LLVMIsPackedStruct(type));
+}
+
+
+TEST_F(CodegenTest, ActorCannotBePacked)
+{
+  const char* src =
+    "actor \\packed\\ Foo\n"
+    "  var a: U8 = 0\n"
+    "  var b: U32 = 0\n"
+
+	"actor Main\n"
+	"  new create(env: Env) =>\n"
+	"    Foo";
+
+  TEST_COMPILE(src);
+
+  reach_t* reach = compile->reach;
+  reach_type_t* foo = reach_type_name(reach, "Foo");
+  ASSERT_TRUE(foo != NULL);
+
+  LLVMTypeRef type = foo->structure;
+  ASSERT_TRUE(!LLVMIsPackedStruct(type));
+}