gdb: change print format of flag enums with value 0

If a flag enum has value 0 and the enumeration type does not have an
enumerator with value 0, we currently print:

  $1 = (unknown: 0x0)

I don't like the display of "unknown" here, since for flags, 0 is a
an expected value.  It just means that no flags are set.  This patch
makes it so that we print it as a simple 0 in this situation:

  $1 = 0

If there is an enumerator with value 0, it is still printed using that
enumerator, for example (from the test):

  $1 = FE_NONE

gdb/ChangeLog:

	* valprint.c (generic_val_print_enum_1): When printing a flag
	enum with value 0 and there is no enumerator with value 0, print
	just "0" instead of "(unknown: 0x0)".

gdb/testsuite/ChangeLog:

	* gdb.base/printcmds.exp (test_print_enums): Update expected
	output.

4 files changed, 36 insertions, 8 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index cc47e2d..9d29568 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,11 @@
 2020-02-18  Simon Marchi  <simon.marchi@efficios.com>
 
+	* valprint.c (generic_val_print_enum_1): When printing a flag
+	enum with value 0 and there is no enumerator with value 0, print
+	just "0" instead of "(unknown: 0x0)".
+
+2020-02-18  Simon Marchi  <simon.marchi@efficios.com>
+
 	* valprint.c (generic_val_print_enum_1): Print unknown part of
 	flag enum in hex.
 
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 087714b..8c09052 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2020-02-18  Simon Marchi  <simon.marchi@efficios.com>
 
+	* gdb.base/printcmds.exp (test_print_enums): Update expected
+	output.
+
+2020-02-18  Simon Marchi  <simon.marchi@efficios.com>
+
 	* gdb.base/printcmds.exp (test_print_enums): Expect hex values
 	for "unknown".
 
diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp
index d6f5c75..bd2afc8 100644
--- a/gdb/testsuite/gdb.base/printcmds.exp
+++ b/gdb/testsuite/gdb.base/printcmds.exp
@@ -743,7 +743,7 @@ proc test_print_enums {} {
     gdb_test "print (enum flag_enum) 0x0" [string_to_regexp " = FE_NONE"]
 
     # Print a flag enum with value 0, where no enumerator has value 0.
-    gdb_test "print flag_enum_without_zero" [string_to_regexp " = (unknown: 0x0)"]
+    gdb_test "print flag_enum_without_zero" [string_to_regexp " = 0"]
 
     # Print a flag enum with unknown bits set.
     gdb_test "print (enum flag_enum) 0xf1" [string_to_regexp " = (FE_ONE | unknown: 0xf0)"]
diff --git a/gdb/valprint.c b/gdb/valprint.c
index 67049e7..ee37022 100644
--- a/gdb/valprint.c
+++ b/gdb/valprint.c
@@ -635,7 +635,6 @@ generic_val_print_enum_1 (struct type *type, LONGEST val,
 	 appropriate.  The enum may have multiple enumerators representing
 	 the same bit, in which case we choose to only print the first one
 	 we find.  */
-      fputs_filtered ("(", stream);
       for (i = 0; i < len; ++i)
 	{
 	  QUIT;
@@ -647,24 +646,42 @@ generic_val_print_enum_1 (struct type *type, LONGEST val,
 
 	  if ((val & enumval) != 0)
 	    {
-	      if (!first)
+	      if (first)
+		{
+		  fputs_filtered ("(", stream);
+		  first = 0;
+		}
+	      else
 		fputs_filtered (" | ", stream);
-	      first = 0;
 
 	      val &= ~TYPE_FIELD_ENUMVAL (type, i);
 	      fputs_filtered (TYPE_FIELD_NAME (type, i), stream);
 	    }
 	}
 
-      if (first || val != 0)
+      if (val != 0)
 	{
-	  if (!first)
+	  /* There are leftover bits, print them.  */
+	  if (first)
+	    fputs_filtered ("(", stream);
+	  else
 	    fputs_filtered (" | ", stream);
+
 	  fputs_filtered ("unknown: 0x", stream);
 	  print_longest (stream, 'x', 0, val);
+	  fputs_filtered (")", stream);
+	}
+      else if (first)
+	{
+	  /* Nothing has been printed and the value is 0, the enum value must
+	     have been 0.  */
+	  fputs_filtered ("0", stream);
+	}
+      else
+	{
+	  /* Something has been printed, close the parenthesis.  */
+	  fputs_filtered (")", stream);
 	}
-
-      fputs_filtered (")", stream);
     }
   else
     print_longest (stream, 'd', 0, val);