Re: [PATCH 4/5]: Enhancements to "flags": i386 cleanup

[Pedro Alves <palves at redhat dot com>]

[Adding Anton, this affects ARC]

On 08/11/2016 07:18 PM, Doug Evans wrote:
> On Thu, Aug 11, 2016 at 11:10 AM, Pedro Alves <palves@redhat.com> wrote:

>> Is the "type" attribute used for anything in <flags> elements?
>> Does changing the type of a flag bitfield have any user-visible
>> effect at all?  I.e., does a 1-bit uint32_t bitfield flag
>> print differently from a bool bitfield flag?
> 
> There may be some simplification possible, but note that bools do
> print differently: if false we don't print the field at all.
> 
> [digression: I'm not sure it's possible today, but while I understand
> the desire to avoid the extra verbosity if we always printed false
> fields, there are times when I *do* want the field printed even if
> false]
> 

So <https://sourceware.org/ml/gdb-patches/2016-10/msg00113.html> resulted
in this change:

diff --git a/gdb/features/arc-arcompact.c b/gdb/features/arc-arcompact.c
index d1fa4fe..a527cc2 100644
--- a/gdb/features/arc-arcompact.c
+++ b/gdb/features/arc-arcompact.c
@@ -54,19 +54,19 @@ initialize_tdesc_arc_arcompact (void)
 
   feature = tdesc_create_feature (result, "org.gnu.gdb.arc.aux-minimal");
   type = tdesc_create_flags (feature, "status32_type", 4);
-  tdesc_add_bitfield (type, "H", 0, 0);
+  tdesc_add_flag (type, 0, "H");
   tdesc_add_bitfield (type, "E", 1, 2);
   tdesc_add_bitfield (type, "A", 3, 4);
-  tdesc_add_bitfield (type, "AE", 5, 5);
-  tdesc_add_bitfield (type, "DE", 6, 6);
-  tdesc_add_bitfield (type, "U", 7, 7);
-  tdesc_add_bitfield (type, "V", 8, 8);
-  tdesc_add_bitfield (type, "C", 9, 9);
-  tdesc_add_bitfield (type, "N", 10, 10);
-  tdesc_add_bitfield (type, "Z", 11, 11);
-  tdesc_add_bitfield (type, "L", 12, 12);
-  tdesc_add_bitfield (type, "R", 13, 13);
-  tdesc_add_bitfield (type, "SE", 14, 14);
+  tdesc_add_flag (type, 5, "AE");
+  tdesc_add_flag (type, 6, "DE");
+  tdesc_add_flag (type, 7, "U");
+  tdesc_add_flag (type, 8, "V");
+  tdesc_add_flag (type, 9, "C");
+  tdesc_add_flag (type, 10, "N");
+  tdesc_add_flag (type, 11, "Z");
+  tdesc_add_flag (type, 12, "L");
+  tdesc_add_flag (type, 13, "R");
+  tdesc_add_flag (type, 14, "SE");

diff --git a/gdb/features/arc-v2.c b/gdb/features/arc-v2.c
index c963410..b2bdfb5 100644
--- a/gdb/features/arc-v2.c
+++ b/gdb/features/arc-v2.c
@@ -54,23 +54,23 @@ initialize_tdesc_arc_v2 (void)
 
   feature = tdesc_create_feature (result, "org.gnu.gdb.arc.aux-minimal");
   type = tdesc_create_flags (feature, "status32_type", 4);
-  tdesc_add_bitfield (type, "H", 0, 0);
+  tdesc_add_flag (type, 0, "H");
   tdesc_add_bitfield (type, "E", 1, 4);
-  tdesc_add_bitfield (type, "AE", 5, 5);
-  tdesc_add_bitfield (type, "DE", 6, 6);
-  tdesc_add_bitfield (type, "U", 7, 7);
-  tdesc_add_bitfield (type, "V", 8, 8);
-  tdesc_add_bitfield (type, "C", 9, 9);
-  tdesc_add_bitfield (type, "N", 10, 10);
-  tdesc_add_bitfield (type, "Z", 11, 11);
-  tdesc_add_bitfield (type, "L", 12, 12);
-  tdesc_add_bitfield (type, "DZ", 13, 13);
-  tdesc_add_bitfield (type, "SC", 14, 14);
-  tdesc_add_bitfield (type, "ES", 15, 15);
+  tdesc_add_flag (type, 5, "AE");
+  tdesc_add_flag (type, 6, "DE");
+  tdesc_add_flag (type, 7, "U");
+  tdesc_add_flag (type, 8, "V");
+  tdesc_add_flag (type, 9, "C");
+  tdesc_add_flag (type, 10, "N");
+  tdesc_add_flag (type, 11, "Z");
+  tdesc_add_flag (type, 12, "L");
+  tdesc_add_flag (type, 13, "DZ");
+  tdesc_add_flag (type, 14, "SC");
+  tdesc_add_flag (type, 15, "ES");
   tdesc_add_bitfield (type, "RB", 16, 18);
-  tdesc_add_bitfield (type, "AD", 19, 19);
-  tdesc_add_bitfield (type, "US", 20, 20);
-  tdesc_add_bitfield (type, "IE", 31, 31);
+  tdesc_add_flag (type, 19, "AD");
+  tdesc_add_flag (type, 20, "US");
+  tdesc_add_flag (type, 31, "IE");

Note how that left several flags with 2-bit and/or 4-bit
long bitfields:

   tdesc_add_bitfield (type, "E", 1, 2);
   tdesc_add_bitfield (type, "A", 3, 4);
...
   tdesc_add_bitfield (type, "E", 1, 4);

which I understand means these two fields will
be given uint32_t type instead of bool?  What does this
mean in practice?  E.g,. for "A", what do we print when both
bits 3 and 4 are clear?  What do we print if one
of the bits is set and the other is clear?

I see similar things on other archs though, it's
not just ARC.  E.g., the patch resulted in:

--- a/gdb/features/aarch64.c
+++ b/gdb/features/aarch64.c
@@ -19,10 +19,10 @@ initialize_tdesc_aarch64 (void)
   feature = tdesc_create_feature (result, "org.gnu.gdb.aarch64.core");
   type = tdesc_create_flags (feature, "cpsr_flags", 4);
   tdesc_add_flag (type, 0, "SP");
-  tdesc_add_bitfield (type, "", 1, 1);
+  tdesc_add_flag (type, 1, "");
   tdesc_add_bitfield (type, "EL", 2, 3);
   tdesc_add_flag (type, 4, "nRW");
-  tdesc_add_bitfield (type, "", 5, 5);
+  tdesc_add_flag (type, 5, "");
   tdesc_add_flag (type, 6, "F");
   tdesc_add_flag (type, 7, "I");
   tdesc_add_flag (type, 8, "A");

Which leaves "EL" as a 2-bit bitfield.

I'm still terribly confused.  :-/

Thanks,
Pedro Alves