Added useful attributes supported by supported compilers

Merges compiler-polyfill into hal/api/toolchain.h following the existing
mbed conventions.

tested on:
- GCC
- Clang
- ARMCC (online compiler)
- IAR

attributes:
- PACK
- ALIGN
- UNUSED
- WEAK
- PURE
- FORCEINLINE
- NORETURN
- UNREACHABLE
- DEPRECATED

hal/api/toolchain.h

@@ -16,6 +16,214 @@
 #ifndef MBED_TOOLCHAIN_H
 #define MBED_TOOLCHAIN_H
 
+
+// Warning for unsupported compilers
+#if !defined(__GNUC__)   /* GCC        */ \
+ && !defined(__CC_ARM)   /* ARMCC      */ \
+ && !defined(__clang__)  /* LLVM/Clang */ \
+ && !defined(__ICCARM__) /* IAR        */
+#warning "This compiler is not yet supported."
+#endif
+
+
+// Attributes
+
+/** PACKED
+ *  Pack a structure, preventing any padding from being added between fields.
+ *
+ *  @code
+ *  #include "toolchain.h"
+ *
+ *  typedef struct {
+ *      char x;
+ *      int y;
+ *  } PACKED foo;
+ *  @endcode
+ */
+#ifndef PACKED
+#if defined(__ICCARM__)
+#define PACKED __packed
+#else
+#define PACKED __attribute__((packed))
+#endif
+#endif
+
+/** ALIGN(N)
+ *  Declare a variable to be aligned on an N-byte boundary.
+ *  
+ *  @code
+ *  #include "toolchain.h"
+ *
+ *  ALIGN(16) char a;
+ *  @endcode
+ */
+#ifndef ALIGN
+#if defined(__ICCARM__)
+#define _ALIGN(N) _Pragma(#N)
+#define ALIGN(N) _ALIGN(data_alignment=N)
+#else
+#define ALIGN(N) __attribute__((aligned(N)))
+#endif
+#endif
+
+/** UNUSED
+ *  Declare a function argument to be unused, suppressing compiler warnings
+ *
+ *  @code
+ *  #include "toolchain.h"
+ *
+ *  void foo(UNUSED int arg) {
+ *
+ *  }
+ *  @endcode
+ */
+#ifndef UNUSED
+#if defined(__GNUC__) || defined(__clang__) || defined(__CC_ARM)
+#define UNUSED __attribute__((__unused__))
+#else
+#define UNUSED
+#endif
+#endif
+
+/** WEAK
+ *  Mark a function as being weak.
+ *  
+ *  @note
+ *  weak functions are not friendly to making code re-usable, as they can only
+ *  be overridden once (and if they are multiply overridden the linker will emit
+ *  no warning). You should not normally use weak symbols as part of the API to
+ *  re-usable modules.
+ *  
+ *  @code
+ *  #include "toolchain.h"
+ *  
+ *  WEAK void foo() {
+ *      // a weak implementation of foo that can be overriden by a definition
+ *      // without  __weak
+ *  }
+ *  @endcode
+ */
+#ifndef WEAK
+#if defined(__ICCARM__)
+#define WEAK __weak
+#else
+#define WEAK __attribute__((weak))
+#endif
+#endif
+
+/** PURE
+ *  Hint to the compiler that a function depends only on parameters
+ *
+ *  @code
+ *  #include "toolchain.h"
+ *
+ *  PURE int foo(int arg){
+ *      // no access to global variables
+ *  }
+ *  @endcode
+ */
+#ifndef PURE
+#if defined(__GNUC__) || defined(__clang__) || defined(__CC_ARM)
+#define PURE __attribute__((const))
+#else
+#define PURE
+#endif
+#endif
+
+/** FORCEINLINE
+ *  Declare a function that must always be inlined. Failure to inline
+ *  such a function will result in an error.
+ *
+ *  @code
+ *  #include "toolchain.h"
+ *  
+ *  FORCEINLINE void foo() {
+ *  
+ *  }
+ *  @endcode
+ */
+#ifndef FORCEINLINE
+#if defined(__GNUC__) || defined(__clang__) || defined(__CC_ARM)
+#define FORCEINLINE static inline __attribute__((always_inline))
+#elif defined(__ICCARM__)
+#define FORCEINLINE _Pragma("inline=force") static
+#else
+#define FORCEINLINE static inline
+#endif
+#endif
+
+/** NORETURN
+ *  Declare a function that will never return.
+ *
+ *  @code
+ *  #include "toolchain.h"
+ *  
+ *  NORETURN void foo() {
+ *      // must never return
+ *      while (1) {}
+ *  }
+ *  @endcode
+ */
+#ifndef NORETURN
+#if defined(__GNUC__) || defined(__clang__) || defined(__CC_ARM)
+#define NORETURN __attribute__((noreturn))
+#elif defined(__ICCARM__)
+#define NORETURN __noreturn
+#else
+#define NORETURN
+#endif
+#endif
+
+/** UNREACHABLE
+ *  An unreachable statement. If the statement is reached,
+ *  behaviour is undefined. Useful in situations where the compiler
+ *  cannot deduce the unreachability of code.
+ *
+ *  @code
+ *  #include "toolchain.h"
+ *
+ *  void foo(int arg) {
+ *      switch (arg) {
+ *          case 1: return 1;
+ *          case 2: return 2;
+ *          ...
+ *      }
+ *      UNREACHABLE;
+ *  }
+ *  @endcode
+ */
+#ifndef UNREACHABLE
+#if (defined(__GNUC__) || defined(__clang__)) && !defined(__CC_ARM)
+#define UNREACHABLE __builtin_unreachable()
+#else
+#define UNREACHABLE while (1)
+#endif
+#endif
+
+/** DEPRECATED("message string")
+ *  Mark a function declaration as deprecated, if it used then a warning will be
+ *  issued by the compiler possibly including the provided message. Note that not
+ *  all compilers are able to display the message.
+ *  
+ *  @code
+ *  #include "toolchain.h"
+ *  
+ *  DEPRECATED("don't foo any more, bar instead")
+ *  void foo(int arg);
+ *  @endcode
+ */
+#ifndef DEPRECATED
+#if defined(__GNUC__) || defined(__clang__)
+#define DEPRECATED(M) __attribute__((deprecated(M)))
+#elif defined(__CC_ARM)
+#define DEPRECATED(M) __attribute__((deprecated))
+#else
+#define DEPRECATED(M)
+#endif
+#endif
+
+
+// FILEHANDLE declaration
 #if defined(TOOLCHAIN_ARM)
 #include <rt_sys.h>
 #endif
@@ -24,12 +232,9 @@
 typedef int FILEHANDLE;
 #endif
 
-#if defined (__ICCARM__)
-#   define WEAK     __weak
-#   define PACKED   __packed
-#else
-#   define WEAK     __attribute__((weak))
-#   define PACKED   __attribute__((packed))
+// Backwards compatibility
+#ifndef EXTERN
+#define EXTERN extern
 #endif
 
 // Backwards compatibility