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

[Richard Earnshaw <Richard.Earnshaw@foss.arm.com>]

On 13/01/2023 21:58, Richard Earnshaw (lists) via Gcc-patches wrote:
> 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.

Also, the Arm port does not use dwarf based unwinding, so is this really 
relevant?

R.