Default architecture according --target=...configury?

Default architecture according --target=...configury?

[Andrew Cagney]

Hello,

Is it possible to reliably determine the default architecture (struct
bfd_arch_info) as selected by the configuration option --target=....
(even when --enable-targets was specified)?

I suspect not but would like to double check.

My guess at the change required is for archures.c:

  static const bfd_arch_info_type * const bfd_archures_list[] =
  {
  #ifdef SELECT_ARCHITECTURES
    SELECT_ARCHITECTURES,
  #else
    &bfd_a29k_arch,

to be modified to be more like targets.c vs:

  const bfd_target * const bfd_target_vector[] = {

  #ifdef SELECT_VECS

	  SELECT_VECS,

  #else /* not SELECT_VECS */

  #ifdef DEFAULT_VECTOR
	  &DEFAULT_VECTOR,
  #endif

(Add DEFAULT_ARCHITECTURE_VECTOR say).

The function bfd_lookup_arch() could then be modified to detect a user
asking for a default architecture (arch == 0 (bfd_arch_unknown)?).

Comments?

	Andrew

PS: Why?  I'd like GDB to make a better guess at a default
architecture.  bfd_default_arch_struct just isn't very interesting.

Re: Default architecture according --target=...configury?

[Ian Lance Taylor]

   Date: Wed, 18 Aug 1999 19:05:43 +1000
   From: Andrew Cagney <ac131313@cygnus.com>

   Is it possible to reliably determine the default architecture (struct
   bfd_arch_info) as selected by the configuration option --target=....
   (even when --enable-targets was specified)?

Well, speaking precisely, there is no default architecture.  There is
a default BFD target, and if an object file is recognized using that
format running bfd_check_format will set the architecture
appropriately.  For example, if the default BFD target is
elf32-bigmips, BFD can recognize a file using any of the 16
architectures listed in bfd/cpu-mips.c.

So it really depends upon what you mean by the default architecture.
For example, if you configure gas for mips4111-elf, then gas will by
default generate object files which use the mips4111 architecture.

   PS: Why?  I'd like GDB to make a better guess at a default
   architecture.  bfd_default_arch_struct just isn't very interesting.

If you explain further what you mean by this, perhaps we can think of
some way to make it work for you.  When does gdb want to know the
default architecture?  What is it going to do with the information?

Ian

Re: Default architecture according --target=...configury?

[Andrew Cagney]

Ian Lance Taylor wrote:

> So it really depends upon what you mean by the default architecture.
> For example, if you configure gas for mips4111-elf, then gas will by
> default generate object files which use the mips4111 architecture.

I'll look at what that does.

>    PS: Why?  I'd like GDB to make a better guess at a default
>    architecture.  bfd_default_arch_struct just isn't very interesting.
> 
> If you explain further what you mean by this, perhaps we can think of
> some way to make it work for you.  When does gdb want to know the
> default architecture?  What is it going to do with the information?

Consider a GDB which has been built to support several different ISA's
(a mips4111, mips4300, ...).  What architecture should GDB select by
default when it goes to talk to a remote target?

	Andrew

Re: Default architecture according --target=...configury?

[Stan Shebs]

   Date: Thu, 19 Aug 1999 10:12:31 +1000
   From: Andrew Cagney <ac131313@cygnus.com>

   >    PS: Why?  I'd like GDB to make a better guess at a default
   >    architecture.  bfd_default_arch_struct just isn't very interesting.
   > 
   > If you explain further what you mean by this, perhaps we can think of
   > some way to make it work for you.  When does gdb want to know the
   > default architecture?  What is it going to do with the information?

   Consider a GDB which has been built to support several different ISA's
   (a mips4111, mips4300, ...).  What architecture should GDB select by
   default when it goes to talk to a remote target?

I don't think it should be BFD's place to choose GDB's default target
architecture.  BFD is just an object file reader.  GDB should assume
an architecture compatible with the executable it is handed, and it
should try to get it from the remote target if possible.  If neither
is helpful or available, the default should be defined in GDB's
configury somewhere.

								Stan

Re: Default architecture according --target=...configury?

[Andrew Cagney]

Stan Shebs wrote:
> 
>    Date: Thu, 19 Aug 1999 10:12:31 +1000
>    From: Andrew Cagney <ac131313@cygnus.com>
> 
>    >    PS: Why?  I'd like GDB to make a better guess at a default
>    >    architecture.  bfd_default_arch_struct just isn't very interesting.
>    >
>    > If you explain further what you mean by this, perhaps we can think of
>    > some way to make it work for you.  When does gdb want to know the
>    > default architecture?  What is it going to do with the information?
> 
>    Consider a GDB which has been built to support several different ISA's
>    (a mips4111, mips4300, ...).  What architecture should GDB select by
>    default when it goes to talk to a remote target?
> 
> I don't think it should be BFD's place to choose GDB's default target
> architecture.  BFD is just an object file reader.  GDB should assume
> an architecture compatible with the executable it is handed, and it
> should try to get it from the remote target if possible.  If neither
> is helpful or available, the default should be defined in GDB's
> configury somewhere.

When GDB is handed an executable it analyzes it (using BFD) and sets its
architecture accordingly.  For a remote target it should auto-detect the
architecture and again, for a few cases, it does that two.

The problem arises when there is no additional information available. 
As you suggest, one possibly is for GDB to configure a default arch
internally.  It just seems strange to me that GAS, BFD and GDB are all
doing it their own special way.

As an aside, GAS (well opcodes) uses BFD's config.bfd to determine
architecture information.  GDB has its own special configure.tgt.

	enjoy,
		Andrew

Re: Default architecture according --target=...configury?

[Ian Lance Taylor]

   Date: Thu, 19 Aug 1999 11:13:25 +1000
   From: Andrew Cagney <ac131313@cygnus.com>

   > I don't think it should be BFD's place to choose GDB's default target
   > architecture.  BFD is just an object file reader.  GDB should assume
   > an architecture compatible with the executable it is handed, and it
   > should try to get it from the remote target if possible.  If neither
   > is helpful or available, the default should be defined in GDB's
   > configury somewhere.

   When GDB is handed an executable it analyzes it (using BFD) and sets its
   architecture accordingly.  For a remote target it should auto-detect the
   architecture and again, for a few cases, it does that two.

   The problem arises when there is no additional information available. 
   As you suggest, one possibly is for GDB to configure a default arch
   internally.  It just seems strange to me that GAS, BFD and GDB are all
   doing it their own special way.

Well, BFD doesn't really do it at all.  BFD always either has an
object to analyze, or it requires the caller to set the architecture.

gas always needs to know which architecture to generate.  The
processor family (i386, mips, etc.) is fixed by the gas configuration.
The specific processor type is defined by the definition of
TARGET_MACH and by calls to bfd_set_arch_mach in the gas backend.  In
the MIPS case, for example, it uses a complex set of conditions at the
start of md_begin in gas/config/tc-mips.c.  The default is based on
the target configuration triplet.

If you know the processor family, maybe you can get a default
architecture by turning it into a string and calling bfd_scan_arch.

   As an aside, GAS (well opcodes) uses BFD's config.bfd to determine
   architecture information.  GDB has its own special configure.tgt.

Of course, opcodes uses config.bfd to determine which architectures
are available, but it does not use config.bfd to determine which
architecture is the default.  opcodes does not have a default.

The gdb configure.tgt file does not do the same thing as the BFD
config.bfd file.  configure.tgt selects a configuration file from
gdb/config/*.  config.bfd selects a BFD target vector and a BFD
architecture.

Ian