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Re: [PATCH] backends: Hook abi_cfi for arm.
- From: Mark Wielaard <mjw at redhat dot com>
- To: elfutils-devel at lists dot fedorahosted dot org
- Date: Thu, 29 Aug 2013 21:17:53 +0200
- Subject: Re: [PATCH] backends: Hook abi_cfi for arm.
On Thu, 2013-08-29 at 10:51 -0700, Roland McGrath wrote:
> > 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).
Good idea. It is always hard to find them. This one can be found here:
"DWARF for the ARM Architecture ABI r2.09"
> 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).
Somehow missed them because they are discussed under "5.1.2 Co-processor
Registers". But indeed they also have a calling convention defined.
"Procedure Call Standard for the ARM Architecture ABI r2.09"
I'll also add that link. Hopefully those URLs are stable.
> > + /* 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 */
Ah, right, of course.
> > + 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).
Which makes sense with the pc = lr definition above.
Though I am sure I saw r14 used instead. Now I cannot remember
where/why. I'll double check and update the patch.