Re: Option to elide single-bit bitfields when printing structures

[Eli Zaretskii <eliz at delorie dot com>]

> From: "Zack Weinberg" <zackw@stanford.edu>
> Date: Thu, 22 Feb 2001 11:56:33 -0800
> 
> (gdb) p decl->common
> $1 = {chain = 0x40253000, type = 0x40253138, code = FUNCTION_DECL, 
>   side_effects_flag = 0, constant_flag = 0, addressable_flag = 0, 
>   volatile_flag = 0, readonly_flag = 0, unsigned_flag = 0, 
>   asm_written_flag = 0, used_flag = 0, nothrow_flag = 0, static_flag = 0, 
>   public_flag = 1, private_flag = 0, protected_flag = 0, bounded_flag = 0, 
>   lang_flag_0 = 0, lang_flag_1 = 0, lang_flag_2 = 0, lang_flag_3 = 0, 
>   lang_flag_4 = 0, lang_flag_5 = 0, lang_flag_6 = 0, dummy = 0}
> 
> It's hard to see which are set and which aren't.  With this patch, you
> can get it printed like this:
> 
> (gdb) set print elide-bitflags on
> (gdb) p decl->common
> $2 = {chain = 0x40253000, type = 0x40253138, code = FUNCTION_DECL, public_flag}
> 
> which is, IMHO, much easier to read.

What if someone wants to know which flags are _reset_?  What if there
are more than one set flag in your example?  What if the bit fields
span more than one bit?  I don't see how your suggestion would deal
with these cases in a way that the resulting display is
self-explaining.  That trailing "public_flag" is already hard to
understand; I stared at it for a few moments without understanding
what's going on here, before it hit me.

Perhaps we need a special command or a special format that would put
the burden on the user.  For example, if the user specifies exactly
what flags (set or unset) does she want to see, the display doesn't
need to explain that, because the user asked for it.

>	* cp-valprint.c (elide_bitflag_print): New flag.
>	(cp_print_value_fields): If elide_bitflag_print, skip printing
>	single-bit fields whose value is zero entirely, and skip
>	printing the value for single-bit fields whose value is one.
>	(_initialize_cp_valprint): Add 'set print elide-bitflags'
>	command, default to old behavior.

If these changes are approved by the cp-valprint maintainer, please
add some minimal changes in gdb.texinfo which describe this feature.