gdb: fix printing of flag enums with multi-bit enumerators

GDB has this feature where if an enum looks like it is meant to
represent binary flags, it will present the values of that type as a
bitwise OR of the flags that are set in the value.

The original motivation for this patch is to fix this behavior:

  enum hello { AAA = 0x1, BBB = 0xf0 };

  (gdb) p (enum hello) 0x11
  $1 = (AAA | BBB)

This is wrong because the bits set in BBB (0xf0) are not all set in the
value 0x11, but GDB presents it as if they all were.

I think that enumerations with enumerators that have more than one bit
set should simply not qualify as "flag enum", as far as this
heuristic is concerned.  I'm not sure what it means to have flags of
more than one bit.  So this is what this patch implements.

I have added an assert in generic_val_print_enum_1 to make sure the flag
enum types respect that, in case they are used by other debug info
readers, in the future.

I've enhanced the gdb.base/printcmds.exp test to cover this case.  I've
also added tests for printing flag enums with value 0, both when the
enumeration has and doesn't have an enumerator for value 0.

gdb/ChangeLog:

	* dwarf2/read.c: Include "count-one-bits.h".
	(update_enumeration_type_from_children): If an enumerator has
	multiple bits set, don't treat the enumeration as a "flag enum".
	* valprint.c (generic_val_print_enum_1): Assert that enumerators
	of flag enums have 0 or 1 bit set.

gdb/testsuite/ChangeLog:

	* gdb.base/printcmds.c (enum flag_enum): Prefix enumerators with
	FE_, add FE_NONE.
	(three): Update.
	(enum flag_enum_without_zero): New enum.
	(flag_enum_without_zero): New variable.
	(enum not_flag_enum): New enum.
	(three_not_flag): New variable.
	* gdb.base/printcmds.exp (test_artificial_arrays): Update.
	(test_print_enums): Add more tests for printing flag enums.

1 files changed, 9 insertions, 3 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index e74383e..5a77b62 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -82,6 +82,7 @@
 #include "gdbsupport/selftest.h"
 #include "rust-lang.h"
 #include "gdbsupport/pathstuff.h"
+#include "count-one-bits.h"
 
 /* When == 1, print basic high level tracing messages.
    When > 1, be more verbose.
@@ -15526,10 +15527,15 @@ update_enumeration_type_from_children (struct die_info *die,
 	  unsigned_enum = 0;
 	  flag_enum = 0;
 	}
-      else if ((mask & value) != 0)
-	flag_enum = 0;
       else
-	mask |= value;
+	{
+	  if (count_one_bits_ll (value) >= 2)
+	    flag_enum = 0;
+	  else if ((mask & value) != 0)
+	    flag_enum = 0;
+	  else
+	    mask |= value;
+	}
 
       /* If we already know that the enum type is neither unsigned, nor
 	 a flag type, no need to look at the rest of the enumerates.  */