Define a portable macro for specifying packed struct

[davidxl]

Summary

With the defined the macro, to specify a packed struct in LLVM, one should do

LLVM_PACKED (struct A {

   ...
};)

[rnk]

lgtm

[davidxl]

Updated the patch with two more macros.

The first macro can not be used when we need to include a header file in the struct body (e.g, for common definitions). The START/END pair style can be used instead.

PTAL.

[rnk]

GCC and Clang also support packing pragmas, right? I think I'd rather use those so that this code doesn't behave differently:

LLVM_PACKED_START
struct A { ... };
struct B { ... }; // packed in msvc, not gcc/clang
LLVM_PACKED_END

[davidxl]

Good point about the potential style difference in using the scope based macros.

Updated the patch according to the comment -- using _Pragma keyword for clang and gcc.

[rnk]

lgtm

llvm/trunk/include/llvm/Support/Compiler.h

 #ifdef _MSC_VER
 # define LLVM_ALIGNAS(x) __declspec(align(x))
 #elif __GNUC__ && !__has_feature(cxx_alignas) && !LLVM_GNUC_PREREQ(4, 8, 0)
 # define LLVM_ALIGNAS(x) __attribute__((aligned(x)))
 #else
 # define LLVM_ALIGNAS(x) alignas(x)
 #endif
 
+/// \macro LLVM_PACKED
+/// \brief Used to specify a packed structure.
+/// LLVM_PACKED(
+///    struct A {
+///      int i;
+///      int j;
+///      int k;
+///      long long l;
+///   });
+///
+/// LLVM_PACKED_START
+/// struct B {
+///   int i;
+///   int j;
+///   int k;
+///   long long l;
+/// };
+/// LLVM_PACKED_END 
+#ifdef _MSC_VER
+# define LLVM_PACKED(d) __pragma(pack(push, 1)) d __pragma(pack(pop))
+# define LLVM_PACKED_START __pragma(pack(push, 1))
+# define LLVM_PACKED_END   __pragma(pack(pop))
+#else
+# define LLVM_PACKED(d) d __attribute__((packed))
+# define LLVM_PACKED_START _Pragma("pack(push, 1)")
+# define LLVM_PACKED_END   _Pragma("pack(pop)")
+#endif
+
 /// \macro LLVM_PTR_SIZE
 /// \brief A constant integer equivalent to the value of sizeof(void*).
 /// Generally used in combination with LLVM_ALIGNAS or when doing computation in
 /// the preprocessor.
 #ifdef __SIZEOF_POINTER__
 # define LLVM_PTR_SIZE __SIZEOF_POINTER__
 #elif defined(_WIN64)
 # define LLVM_PTR_SIZE 8