Re: define pg_structiszero(addr, s, r)
От | Bertrand Drouvot |
---|---|
Тема | Re: define pg_structiszero(addr, s, r) |
Дата | |
Msg-id | ZzGiBtUWWfgga9TH@ip-10-97-1-34.eu-west-3.compute.internal обсуждение исходный текст |
Ответ на | Re: define pg_structiszero(addr, s, r) (Bertrand Drouvot <bertranddrouvot.pg@gmail.com>) |
Список | pgsql-hackers |
Hi, On Sat, Nov 09, 2024 at 04:15:04AM +0000, Bertrand Drouvot wrote: > 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? What about "simply" starting pg_memory_is_all_zeros() with? " if (len < sizeof(size_t)*8) { while (p < end) { if (*p++ != 0) return false; } return true; } " That way: - we prevents reading beyond the memory area in the SIMD section (if < 64 bytes) - we make sure that aligned_end can not be after the real end (could be if the len is < 8 bytes) - there is no need for additional size checks later in the code - len < 64 bytes will be read byte per byte but that's likely "enough" (if not faster) for those "small" sizes Thoughts? Regards, -- Bertrand Drouvot PostgreSQL Contributors Team RDS Open Source Databases Amazon Web Services: https://aws.amazon.com
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