Re: [GCC][PATCH 13/15, v5] arm: Add support for dwarf debug directives and pseudo hard-register for PAC feature.

["Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>]

On 13/01/2023 18:02, Jakub Jelinek via Gcc-patches wrote:
> On Fri, Jan 13, 2023 at 05:44:15PM +0000, Srinath Parvathaneni via Gcc-patches wrote:
>> Hello,
>>
>> This patch teaches the DWARF support in gcc about RA_AUTH_CODE pseudo hard-register and also
>> updates the ".save", ".cfi_register", ".cfi_offset", ".cfi_restore" directives accordingly.
>> This patch also adds support to emit ".pacspval" directive when "pac ip, lr, sp" instruction
>> in generated in the assembly.
>>
>> RA_AUTH_CODE register number is 107 and it's dwarf register number is 143.
> 
> I'm afraid increasing number of DWARF registers is ABI incompatible change.
> E.g. libgcc __frame_state_for function fills in:
> typedef struct frame_state
> {
>    void *cfa;
>    void *eh_ptr;
>    long cfa_offset;
>    long args_size;
>    long reg_or_offset[PRE_GCC3_DWARF_FRAME_REGISTERS+1];
>    unsigned short cfa_reg;
>    unsigned short retaddr_column;
>    char saved[PRE_GCC3_DWARF_FRAME_REGISTERS+1];
> } frame_state;
> 
> structure, where PRE_GCC3_DWARF_FRAME_REGISTERS defaults to
> __LIBGCC_DWARF_FRAME_REGISTERS__, which is defined to
> DWARF_FRAME_REGISTERS, which defaults to FIRST_PSEUDO_REGISTER.
> So, changing FIRST_PSEUDO_REGISTER is an ABI change unless you arrange for
> PRE_GCC3_DWARF_FRAME_REGISTERS to be defined to the old value.
> 
> 	Jakub
> 

So where's the red flag that warns about this?

I also note that Richard Sandiford made a similar type of change for 
AArch64 in r10-4195 (183bfdafc6f1f98711c5400498a7268cc1441096) and 
nothing was said about that at the time.

It seems incredibly fragile to me to have some ABI based off the number 
of machine registers.

R.