selftests/bpf: adjust OP_EQ/OP_NE handling to use subranges for branch taken

Similar to kernel-side BPF verifier logic enhancements, use 32-bit
subrange knowledge for is_branch_taken() logic in reg_bounds selftests.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231112010609.848406-9-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/reg_bounds.c

@@ -748,16 +748,38 @@ static int reg_state_branch_taken_op(enum num_t t, struct reg_state *x, struct r
 		/* OP_EQ and OP_NE are sign-agnostic */
 		enum num_t tu = t_unsigned(t);
 		enum num_t ts = t_signed(t);
-		int br_u, br_s;
+		int br_u, br_s, br;
 
 		br_u = range_branch_taken_op(tu, x->r[tu], y->r[tu], op);
 		br_s = range_branch_taken_op(ts, x->r[ts], y->r[ts], op);
 
 		if (br_u >= 0 && br_s >= 0 && br_u != br_s)
 			ASSERT_FALSE(true, "branch taken inconsistency!\n");
-		if (br_u >= 0)
-			return br_u;
-		return br_s;
+
+		/* if 64-bit ranges are indecisive, use 32-bit subranges to
+		 * eliminate always/never taken branches, if possible
+		 */
+		if (br_u == -1 && (t == U64 || t == S64)) {
+			br = range_branch_taken_op(U32, x->r[U32], y->r[U32], op);
+			/* we can only reject for OP_EQ, never take branch
+			 * based on lower 32 bits
+			 */
+			if (op == OP_EQ && br == 0)
+				return 0;
+			/* for OP_NEQ we can be conclusive only if lower 32 bits
+			 * differ and thus inequality branch is always taken
+			 */
+			if (op == OP_NE && br == 1)
+				return 1;
+
+			br = range_branch_taken_op(S32, x->r[S32], y->r[S32], op);
+			if (op == OP_EQ && br == 0)
+				return 0;
+			if (op == OP_NE && br == 1)
+				return 1;
+		}
+
+		return br_u >= 0 ? br_u : br_s;
 	}
 	return range_branch_taken_op(t, x->r[t], y->r[t], op);
 }