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Re: [PATCH] backends: Hook abi_cfi for arm.

> Note the DWARF abi extension for ARM says that "registers intentionally
> unused" should also be initialized as if by DW_CFA_same_value.  The
> example given is "an integer-only function might be included in one
> executable file for targets with VFP and another for targets without".
> We don't currently do this yet.

Seems worthwhile to include the URL to the ABI spec that talks about
DWARF and/or the function call ABI (we never did for the others, but
I'm not sure there are canonical URLs for most of them).

We have the DW_CFA_restore hack (which I could have sworn was used
somewhere--I thought we had abi_cfi for ppc long ago).  So perhaps the
right thing is to use that plus explicit DW_CFA_undefined for the
call-clobbered registers.  

Either way, this needs to cover the VFP/NEON registers (which are half
call-clobbered and half call-saved).

> +      /* The link register contains the return address setup by caller.  */
> +      SV (14),

Should also have:

	DW_CFA_register, ULEB128_7 (15), ULEB128_7 (14), /* pc = lr */

> +  abi_info->return_address_register = 14; /* Link Register.  */

I think that's wrong.  14 is LR, which is just a general register that
happens to be where the ABI says the return address is on entry.  The
"return_address_register" is for the "caller's PC" (a "virtual
register" on machines where the PC is not directly addressable), so it
should use 15 (PC).


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