Re: internal error reading f0-f7 registers in arm-elf targets.

[Andrew Cagney <ac131313@redhat.com>]

> That's because 12 bytes are stored on the stack for each FP register.
> 
> This is all somewhat complicated.  The FPA has support for IEEE 
> extended-precision arithmetic, but it's never used by the compiler/ABI.  
> However, to ensure that register values are correctly preserved across 
> function calls and don't generate conversion traps in the prologue, they 
> have to be saved in either in extended precision or in internal format by 
> the prologue.  Old implementations of the FPE (the Emulator for the FPA) 
> did not have support for SFM instructions, so values were stored out in 
> extended precision (STFE), and that mode was configured not to trap on 
> conversion.  The FPA (and newer versions of the FPE) have the 
> store-float-multiple (SFM) which stores out the registers in their 
> internal form to guarantee that things won't trap.


> This all means that to correctly unwind floating point values we either 
> need to execute the LFM instructions (not generally possible in 
> remote/cross debugging) or understand the internal representation in use 
> (which isn't documented!).

The latter.

GDB already thinks it knows the answer: builtin_type_arm_ext_big or 
builtin_type_arm_ext_littlebyte_bigword.  If the on-stack format is 
going to be different to the target format then (in addition to from 
inflicting physical pain to the h/w designer) some playing with pseudo 
registers will be needed.

> To make things worse, some FPEs (eg the ARM-Linux one) use yet another 
> internal format... That is at least public, but it's just "different". :-(
> 
> And I haven't even mentioned the VFP yet!

I noticed that when looking for doco.  Finding anything on FPA10 / FPA11 
is hard - I eventually found something in the ARM7500FE manual.

Andrew