bpf: Minor improvements for bpf_cmp.

Few minor improvements for bpf_cmp() macro:
. reduce number of args in __bpf_cmp()
. rename NOFLIP to UNLIKELY
. add a comment about 64-bit truncation in "i" constraint
. use "ri" constraint for sizeof(rhs) <= 4
. improve error message for bpf_cmp_likely()

Before:
progs/iters_task_vma.c:31:7: error: variable 'ret' is uninitialized when used here [-Werror,-Wuninitialized]
   31 |                 if (bpf_cmp_likely(seen, <==, 1000))
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../bpf/bpf_experimental.h:325:3: note: expanded from macro 'bpf_cmp_likely'
  325 |                 ret;
      |                 ^~~
progs/iters_task_vma.c:31:7: note: variable 'ret' is declared here
../bpf/bpf_experimental.h:310:3: note: expanded from macro 'bpf_cmp_likely'
  310 |                 bool ret;
      |                 ^

After:
progs/iters_task_vma.c:31:7: error: invalid operand for instruction
   31 |                 if (bpf_cmp_likely(seen, <==, 1000))
      |                     ^
../bpf/bpf_experimental.h:324:17: note: expanded from macro 'bpf_cmp_likely'
  324 |                         asm volatile("r0 " #OP " invalid compare");
      |                                      ^
<inline asm>:1:5: note: instantiated into assembly here
    1 |         r0 <== invalid compare
      |            ^
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240112220134.71209-1-alexei.starovoitov@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/bpf_experimental.h

@@ -260,11 +260,11 @@ extern void bpf_throw(u64 cookie) __ksym;
 
 #define __is_signed_type(type) (((type)(-1)) < (type)1)
 
-#define __bpf_cmp(LHS, OP, SIGN, PRED, RHS, DEFAULT)						\
+#define __bpf_cmp(LHS, OP, PRED, RHS, DEFAULT)						\
 	({											\
 		__label__ l_true;								\
 		bool ret = DEFAULT;								\
-		asm volatile goto("if %[lhs] " SIGN #OP " %[rhs] goto %l[l_true]"		\
+		asm volatile goto("if %[lhs] " OP " %[rhs] goto %l[l_true]"		\
 				  :: [lhs] "r"((short)LHS), [rhs] PRED (RHS) :: l_true);	\
 		ret = !DEFAULT;									\
 l_true:												\
@@ -276,7 +276,7 @@ l_true:												\
  * __lhs OP __rhs below will catch the mistake.
  * Be aware that we check only __lhs to figure out the sign of compare.
  */
-#define _bpf_cmp(LHS, OP, RHS, NOFLIP)								\
+#define _bpf_cmp(LHS, OP, RHS, UNLIKELY)								\
 	({											\
 		typeof(LHS) __lhs = (LHS);							\
 		typeof(RHS) __rhs = (RHS);							\
@@ -285,14 +285,17 @@ l_true:												\
 		(void)(__lhs OP __rhs);								\
 		if (__cmp_cannot_be_signed(OP) || !__is_signed_type(typeof(__lhs))) {		\
 			if (sizeof(__rhs) == 8)							\
-				ret = __bpf_cmp(__lhs, OP, "", "r", __rhs, NOFLIP);		\
+				/* "i" will truncate 64-bit constant into s32,			\
+				 * so we have to use extra register via "r".			\
+				 */								\
+				ret = __bpf_cmp(__lhs, #OP, "r", __rhs, UNLIKELY);		\
 			else									\
-				ret = __bpf_cmp(__lhs, OP, "", "i", __rhs, NOFLIP);		\
+				ret = __bpf_cmp(__lhs, #OP, "ri", __rhs, UNLIKELY);		\
 		} else {									\
 			if (sizeof(__rhs) == 8)							\
-				ret = __bpf_cmp(__lhs, OP, "s", "r", __rhs, NOFLIP);		\
+				ret = __bpf_cmp(__lhs, "s"#OP, "r", __rhs, UNLIKELY);		\
 			else									\
-				ret = __bpf_cmp(__lhs, OP, "s", "i", __rhs, NOFLIP);		\
+				ret = __bpf_cmp(__lhs, "s"#OP, "ri", __rhs, UNLIKELY);		\
 		}										\
 		ret;										\
        })
@@ -304,7 +307,7 @@ l_true:												\
 #ifndef bpf_cmp_likely
 #define bpf_cmp_likely(LHS, OP, RHS)								\
 	({											\
-		bool ret;									\
+		bool ret = 0;									\
 		if (__builtin_strcmp(#OP, "==") == 0)						\
 			ret = _bpf_cmp(LHS, !=, RHS, false);					\
 		else if (__builtin_strcmp(#OP, "!=") == 0)					\
@@ -318,7 +321,7 @@ l_true:												\
 		else if (__builtin_strcmp(#OP, ">=") == 0)					\
 			ret = _bpf_cmp(LHS, <, RHS, false);					\
 		else										\
-			(void) "bug";								\
+			asm volatile("r0 " #OP " invalid compare");				\
 		ret;										\
        })
 #endif