c-family: Avoid unnecessary work when -Wpragmas is being ignored

This speeds up handle_pragma_diagnostic by avoiding computing a spelling
suggestion for an unrecognized option inside a #pragma directive when
-Wpragmas warnings are being suppressed.

In the range-v3 library, which contains many instances of

  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wpragmas"
  #pragma GCC diagnostic ignored "-Wfoo"
  ...
  #pragma GCC diagnostic pop

(where -Wfoo stands for a warning option we don't recognize), this
reduces compile time by 33% for some of its tests.

gcc/c-family/ChangeLog:

	* c-pragma.c (handle_pragma_diagnostic): Split the
	unknown-option -Wpragmas diagnostic into a warning and a
	subsequent note containing a spelling suggestion.  Avoid
	computing the suggestion if -Wpragmas warnings are being
	suppressed.

gcc/testsuite/ChangeLog:

	* gcc.dg/pragma-diag-6.c: Adjust expected diagnostics
	accordingly.

2 files changed, 15 insertions, 13 deletions

diff --git a/gcc/c-family/c-pragma.c b/gcc/c-family/c-pragma.c
index dc52ee8..d68985c 100644
--- a/gcc/c-family/c-pragma.c
+++ b/gcc/c-family/c-pragma.c
@@ -809,16 +809,15 @@ handle_pragma_diagnostic(cpp_reader *ARG_UNUSED(dummy))
   unsigned int option_index = find_opt (option_string + 1, lang_mask);
   if (option_index == OPT_SPECIAL_unknown)
     {
-      option_proposer op;
-      const char *hint = op.suggest_option (option_string + 1);
-      if (hint)
-	warning_at (loc, OPT_Wpragmas,
-		    "unknown option after %<#pragma GCC diagnostic%> kind;"
-		    " did you mean %<-%s%>?", hint);
-      else
-	warning_at (loc, OPT_Wpragmas,
-		    "unknown option after %<#pragma GCC diagnostic%> kind");
-
+      auto_diagnostic_group d;
+      if (warning_at (loc, OPT_Wpragmas,
+		      "unknown option after %<#pragma GCC diagnostic%> kind"))
+	{
+	  option_proposer op;
+	  const char *hint = op.suggest_option (option_string + 1);
+	  if (hint)
+	    inform (loc, "did you mean %<-%s%>?", hint);
+	}
       return;
     }
   else if (!(cl_options[option_index].flags & CL_WARNING))
diff --git a/gcc/testsuite/gcc.dg/pragma-diag-6.c b/gcc/testsuite/gcc.dg/pragma-diag-6.c
index 0dca1dc..f2df88d 100644
--- a/gcc/testsuite/gcc.dg/pragma-diag-6.c
+++ b/gcc/testsuite/gcc.dg/pragma-diag-6.c
@@ -2,7 +2,10 @@
 #pragma GCC diagnostic error "-Wnoexcept" /* { dg-warning "is valid for C../ObjC.. but not for C" } */
 #pragma GCC diagnostic error "-fstrict-aliasing" /* { dg-warning "not an option that controls warnings" } */
 #pragma GCC diagnostic error "-Werror" /* { dg-warning "not an option that controls warnings" } */
-#pragma GCC diagnostic error "-Wvla2" /* { dg-warning "unknown option after '#pragma GCC diagnostic' kind; did you mean '-Wvla'" } */
-#pragma GCC diagnostic error "-Walla" /* { dg-warning "unknown option after '#pragma GCC diagnostic' kind; did you mean '-Wall'" } */
-#pragma GCC diagnostic warning "-Walla" /* { dg-warning "unknown option after '#pragma GCC diagnostic' kind; did you mean '-Wall'" } */
+#pragma GCC diagnostic error "-Wvla2" /* { dg-warning "unknown option after '#pragma GCC diagnostic' kind" } */
+/* { dg-message "did you mean '-Wvla'" "" { target *-*-* } .-1 } */
+#pragma GCC diagnostic error "-Walla" /* { dg-warning "unknown option after '#pragma GCC diagnostic' kind" } */
+/* { dg-message "did you mean '-Wall'" "" { target *-*-* } .-1 } */
+#pragma GCC diagnostic warning "-Walla" /* { dg-warning "unknown option after '#pragma GCC diagnostic' kind" } */
+/* { dg-message "did you mean '-Wall'" "" { target *-*-* } .-1 } */
 int i;