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

[Vineet Gupta <vineetg@rivosinc.com>]

On 3/16/24 13:21, Jeff Law wrote:
> |   59944:	add	s0,sp,2047  <----
> |   59948:	mv	a2,a0
> |   5994c:	mv	a3,a1
> |   59950:	mv	a0,sp
> |   59954:	li	a4,1
> |   59958:	lui	a1,0x1
> |   5995c:	add	s0,s0,1     <---
> |   59960:	jal	59a3c
>
> SP here becomes unaligned, even if transitively which is undesirable as
> well as incorrect:
>   - ABI requires stack to be 8 byte aligned
>   - asm code looks weird and unexpected
>   - to the user it might falsely seem like a compiler bug even when not,
>     specially when staring at asm for debugging unrelated issue.
> It's not ideal, but I think it's still ABI compliant as-is.  If it 
> wasn't, then I suspect things like virtual origins in Ada couldn't be 
> made ABI compliant.

To be clear are u suggesting ADD sp, sp, 2047 is ABI compliant ?
I'd still like to avoid it as I'm sure someone will complain about it.

>> With the patch, we get following correct code instead:
>>
>> | ..
>> | 59944:	add	s0,sp,2032
>> | ..
>> | 5995c:	add	s0,s0,16
> Alternately you could tighten the positive side of the range of the 
> splitter from patch 1/3 so that you could always use 2032 rather than 
> 2047 on the first addi.   ie instead of allowing 2048..4094, allow 
> 2048..4064.

2033..4064 vs. 2048..4094

Yeah I was a bit split about this as well. Since you are OK with either,
I'll keep them as-is and perhaps add this observation to commitlog.

Thx,
-Vineet