Re: ARM and virtual/raw registers

[Andrew Cagney <ac131313@cygnus.com>]

> Ok, so it's clear that the current ARM code for handling virtual and raw 
> registers is wrong...
> 
> So the question is, what does it need to be?
> 
> The answer is that I'm not quite sure, since there are a number of factors 
> involved here.
> 
> 1) FPA registers are multi-precision -- that is, they are modal, holding 
> information about the type of result in the register at any particular 
> time.  The extra information is used to enable correct support of type 
> conversions with signalling NaNs.

[And I thought MIPS was bad :-)]

So an FP register could contain a single, double, ... and each would be 
represented differently - single, for instance, would not be stored as a 
rounded double?

I think the Alpha does this for int values - it has some really strange 
to/from register/int code.

If this is the only problem, then the new REGISTER_TO_VALUE et.al. 
methods should address the problem.

Those methods, do, however, assume that the format of a register spilt 
into memory is identical to the layout of the h/w register supplied by 
the OS / H/W / ...  Hmm, ...

> 2) The format of that information can depend on the hardware present, or, 
> if absent, on the emulator being used (different emulators handle this in 
> different ways).


> 3) Some of the emulators use three words to hold the raw information, some 
> use four.  RDP returns uses four words.
> 
> 4) When a floating point value is stored in memory the format of that 
> memory may depend on the instruction used to store it.  For example, the 
> sfmfd instruction used in a prologue sequence will store three words (as 
> would stfe), but there may be information in the unused bits that 
> indicates the type of the value in the register.  The format of this 
> memory may, or may not, be the same as the three-word register information 
> mentioned in 3) above.

So the way the ARM spills its FP registers into memory may not 
necessarily match the equivalent h/w register?

Does the ARM always spill these registers in the same way (at least for 
a given ISA/ABI combination?) - if GDB is fetching a saved register from 
the stack, it knows what the format is.

> 5) All of the above is poorly documented ;-(
> 
> 6) Selecting the correct conversion routine may involve some inspired 
> guess-work :-)

Andrew