clang: work around asm output constraint problems

Work around clang problems with "=rm" asm constraint.

clang seems to always chose the memory output, while it is almost
always the worst choice.

Add ASM_OUTPUT_RM so that we can replace "=rm" constraint
where it matters for clang, while not penalizing gcc.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: edumazet

include/linux/compiler-clang.h

@@ -145,6 +145,7 @@
  */
 #define ASM_INPUT_G "ir"
 #define ASM_INPUT_RM "r"
+#define ASM_OUTPUT_RM "=r"
 
 /*
  * Declare compiler support for __typeof_unqual__() operator.

include/linux/compiler_types.h

@@ -548,11 +548,12 @@ struct ftrace_likely_data {
 
 /*
  * Clang has trouble with constraints with multiple
- * alternative behaviors (mainly "g" and "rm").
+ * alternative behaviors ("g" , "rm" and "=rm").
  */
 #ifndef ASM_INPUT_G
   #define ASM_INPUT_G "g"
   #define ASM_INPUT_RM "rm"
+  #define ASM_OUTPUT_RM "=rm"
 #endif
 
 #ifdef CONFIG_CC_HAS_ASM_INLINE