cpu/avr8_common/flash_utils: use C and linker for aliases

`flash_<funcname>()` is implemented by `<funcname>_P()` provided by
the AVR libc on AVR targets. Previously, the preprocessor was used
to do the aliasing, but this causes issues with LLVM: The signatures of
e.g. `printf_P()` expects `const char *`, whereas flash utils expects
`FLASH_ATTR const char *`. For GCC this will just implicitly drop the
`FLASH_ATTR`, while it requires an explicit cast for LLVM.

To implement the explicit cast, `static inline` function wrappers
where used instead where possible. But for the variadic functions
(e.g. `printf(fmt, ...)`) the linker is used to provide the aliases,
as there is no way to pass the variadic functions throw in C. The
alternative would be to implement `flash_printf()` by calling
`vprintf_P()`, but that increased ROM size quite a bit.

Finally, a work around for a bug in Ubuntu's toolchain has been added:
An unused function that calls to `printf_P()`, `fprintf_P()` and
`snprintf_P()`. Since this function is garbage collected anyway, it
has no impact on the generated ELF file.

cpu/avr8_common/Makefile.include

@@ -7,4 +7,8 @@ ifneq (,$(filter printf_float,$(USEMODULE)))
   LINKFLAGS += -Wl,-u,vfprintf -lprintf_flt -lm
 endif
 
+# Add aliases for flash_printf, flash_fprintf, flash_snprintf:
+LINKFLAGS += -Wl,--defsym=flash_printf=printf_P
+LINKFLAGS += -Wl,--defsym=flash_fprintf=fprintf_P
+LINKFLAGS += -Wl,--defsym=flash_snprintf=snprintf_P
 include $(RIOTMAKE)/arch/avr8.inc.mk

cpu/avr8_common/include/flash_utils_arch.h

@@ -21,7 +21,7 @@
 #define FLASH_UTILS_ARCH_H
 
 #include <stdio.h>
-#include <string.h>
+#include <stdarg.h>
 #include <avr/pgmspace.h>
 
 #ifdef __cplusplus
@@ -35,20 +35,68 @@ extern "C" {
 #define FLASH_ATTR __flash
 #define PRIsflash "S"
 #define TO_FLASH(x) __extension__({static FLASH_ATTR const char __c[] = (x); &__c[0];})
-#define flash_strcmp strcmp_P
-#define flash_strncmp strncmp_P
-#define flash_strlen strlen_P
-#define flash_strcpy strcpy_P
-#define flash_strncpy strncpy_P
-#define flash_printf printf_P
-/* avrlibc seemingly forgot to provide vprintf_P(), but vfprintf_P() is there */
-#define flash_vprintf(fmt, arglist) vfprintf_P(stdout, fmt, arglist)
-#define flash_fprintf fprintf_P
-#define flash_vfprintf vfprintf_P
-#define flash_snprintf snprintf_P
-#define flash_vsnprintf vsnprintf_P
-#define flash_puts puts_P
-#define flash_memcpy memcpy_P
+
+static inline int flash_strcmp(const char *ram, FLASH_ATTR const char *flash)
+{
+    return strcmp_P(ram, (const char *)flash);
+}
+
+static inline int flash_strncmp(const char *ram, FLASH_ATTR const char *flash, size_t n)
+{
+    return strncmp_P(ram, (const char *)flash, n);
+}
+
+static inline size_t flash_strlen(FLASH_ATTR const char *flash)
+{
+    return strlen_P((const char *)flash);
+}
+
+static inline char * flash_strcpy(char *ram, FLASH_ATTR const char *flash)
+{
+    return strcpy_P(ram, (const char *)flash);
+}
+
+static inline char * flash_strncpy(char *ram, FLASH_ATTR const char *flash, size_t n)
+{
+    return strncpy_P(ram, (const char *)flash, n);
+}
+
+
+static inline int flash_vprintf(FLASH_ATTR const char *flash, va_list args)
+{
+    /* vprintf_P() is not provided by avr-libc. But vfprintf_P() with
+     * stdout as stream can be used to implement it */
+    return vfprintf_P(stdout, (const char *)flash, args);
+}
+
+static inline int flash_vfprintf(FILE *stream, FLASH_ATTR const char *flash,
+                                 va_list args)
+{
+    return vfprintf_P(stream, (const char *)flash, args);
+}
+
+static inline int flash_vsnprintf(char *buf, size_t buf_len,
+                                  FLASH_ATTR const char *flash, va_list args)
+{
+    return vsnprintf_P(buf, buf_len, (const char *)flash, args);
+}
+
+static inline void flash_puts(FLASH_ATTR const char *flash)
+{
+    puts_P((const char *)flash);
+}
+
+static inline void * flash_memcpy(void *dest, FLASH_ATTR const void *src,
+                                  size_t n)
+{
+    return memcpy_P(dest, (const void *)src, n);
+}
+
+/* aliases need to be provided by the linker, as passing through va-args is
+ * not possible */
+int flash_printf(FLASH_ATTR const char *flash, ...);
+int flash_fprintf(FILE *stream, FLASH_ATTR const char *flash, ...);
+int flash_snprintf(char *buf, size_t buf_len, FLASH_ATTR const char *flash, ...);
 
 #endif /* Doxygen */
 

cpu/avr8_common/work_around_for_shitty_ubuntu_toolchain.c

@@ -0,0 +1,15 @@
+#include <avr/pgmspace.h>
+#include <stdio.h>
+
+/* The outdated linker from Ubuntu's toolchain contains a bug in which it will
+ * garbage collect symbols referenced only by --defsym= command line options,
+ * and subsequently complain that the symbols are not defined. Adding other
+ * references to those symbols from an unused function makes that buggy linker
+ * happy. Since this function is never used, it will be garbage collected and
+ * not impact the ROM size. */
+void work_around_for_shitty_ubuntu_toolchain_and_not_expected_to_be_called(void)
+{
+    printf_P(NULL);
+    fprintf_P(stdout, NULL);
+    snprintf_P(NULL, 0, NULL);
+}