[aarch64] PR102376 - Emit better diagnostic for arch extensions in target attr

[Prathamesh Kulkarni via Gcc-patches]

Hi,
The attached patch emits a more verbose diagnostic for target attribute that
is an architecture extension needing a leading '+'.

For the following test,
void calculate(void) __attribute__ ((__target__ ("sve")));

With patch, the compiler now emits:
102376.c:1:1: error: arch extension ‘sve’ should be prepended with ‘+’
    1 | void calculate(void) __attribute__ ((__target__ ("sve")));
      | ^~~~

instead of:
102376.c:1:1: error: pragma or attribute ‘target("sve")’ is not valid
    1 | void calculate(void) __attribute__ ((__target__ ("sve")));
      | ^~~~

(This isn't specific to sve though).
OK to commit after bootstrap+test ?

Thanks,
Prathamesh

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index a9a1800af53..975f7faf968 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -17821,7 +17821,16 @@ aarch64_process_target_attr (tree args)
       num_attrs++;
       if (!aarch64_process_one_target_attr (token))
        {
-         error ("pragma or attribute %<target(\"%s\")%> is not valid", token);
+         /* Check if token is possibly an arch extension without
+            leading '+'.  */
+         char *str = (char *) xmalloc (strlen (token) + 2);
+         str[0] = '+';
+         strcpy(str + 1, token);
+         if (aarch64_handle_attr_isa_flags (str))
+           error("arch extension %<%s%> should be prepended with %<+%>", 
token);
+         else
+           error ("pragma or attribute %<target(\"%s\")%> is not valid", 
token);
+         free (str);
          return false;
        }