Hi, On Sat, Nov 09, 2024 at 08:00:35AM +0900, Michael Paquier wrote: > On Fri, Nov 08, 2024 at 11:18:09PM +1300, David Rowley wrote: > > I'm slightly worried due to the current rate we're receiving cleanup > > suggestions that someone might come along and think they'd be doing us > > a favour by submitting a patch to "fixup the inefficient bitwise-ORs > > and use boolean-OR". Maybe a comment like the following might prevent > > that from happening. > > Not sure, but OK by me to tweak things more. > > > Can you share your test case for this? 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. > > Feel free to use that (I hope it works), and see the difference once > the aligned structure is 121 bytes or more: > https://godbolt.org/z/94393nPGG > > At least, I can see that the SIMD loop is ignored.
What I see (with the godbolt you shared) is that with BLCKSZ of 120: gcc: then no SIMD instructions are used (I think that's because sizeof(AlignedBlock) is 120 which is not a multiple of 16 (SIMD xmm register size)). with BLCKSZ of 121: gcc: then SIMD instructions are used (I think that's because sizeof(AlignedBlock) is 128 which is a multiple of 16 (SIMD xmm register size)). While clang uses SIMD instructions in both cases (more complex code with more branches at least in the 120 case). Regards, -- Bertrand Drouvot PostgreSQL Contributors Team RDS Open Source Databases Amazon Web Services: https://aws.amazon.com
