Re: define pg_structiszero(addr, s, r)

Hi,

On Fri, Nov 08, 2024 at 11:18:09PM +1300, David Rowley wrote:
> I tried with [1] and the
> latest gcc does not seem to be smart enough to figure this out. I
> tried adding some additional len checks that the compiler can use as a
> cue and won't need to emit code for the checks providing the function
> does get inlined. That was enough to get the compiler to not emit the
> loops when they'll not be used. See the -DCHECK_LEN flag I'm passing
> in the 2nd compiler window. I just don't know if putting something
> like that into the code is a good idea as if the function wasn't
> inlined for some reason, the extra len checks would have to be
> compiled into the function.
>
> David
>
> [1] https://godbolt.org/z/xa81ro8GK

Looking at it, that looks like an issue.

I mean, without the -DCHECK_LEN flag then the SIMD code will read up to 48 bytes
beyond the struct's memory (which is 16 bytes):

This is fine:
"
movdqu xmm0, XMMWORD PTR [rdi]
"

But I don't think it is:

"
movdqu xmm2, XMMWORD PTR [rdi+16]
movdqu xmm1, XMMWORD PTR [rdi+32]
movdqu xmm3, XMMWORD PTR [rdi+48]
"

given that the struct size is only 16 bytes.

Thoughts?

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com