Re: [gcc-15 2/3] RISC-V: avoid LUI based const mat: keep stack offsets aligned

[Jeff Law <jeffreyalaw@gmail.com>]

On 3/19/24 2:05 PM, Vineet Gupta wrote:

>> Just to be clear, the changes don't misalign the stack pointer at all.
>> They merely have the potential to create *another* pointer into the
>> stack which may or may not be aligned.  Which is totally normal, it's no
>> different than taking the address of a char on the stack.
> 
> Right I never saw any sp,sp,2047 getting generated - not even in the
> first version of patch which lacked any filtering of stack regs via
> riscv_reg_frame_related () and obviously didn't have the stack variant
> of splitter. I don't know if that is just being lucky and not enough
> testing exposure (I only spot checked buildroot libc, vmlinux) or
> something somewhere enforces that.
> 
> However given that misaligned pointer off of stack is a non-issue, I
> think we can do the following:
> 
> 1. keep just one splitter with 2047 based predicates and constraint (and
> not 2032) for both stack-related and general regs.
> 2. gate the splitter on only operands[0] being not stack related
> (currently it checks for either [0] or [1]) - this allows the prominent
> case where SP is simply a src, and avoids when any potential shenanigans
> to SP itself.
Works for me.

Jeff