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Wed, 7 Oct 2026 17:32:43 -0400 (EDT) From: Junio C Hamano To: Johannes Schindelin Cc: Scott Chacon , git@vger.kernel.org Subject: Re: [PATCH 2/4] sha1dc-accel: vectorize the unavoidable-bitconditions check In-Reply-To: <3d640489-5db4-5527-0ec1-c2abac7a2de3@gmx.de> (Johannes Schindelin's message of "Wed, 7 Oct 2026 14:17:41 +0200 (CEST)") References: <20260929112544.86511-1-scott@gitbutler.net> <20260929112544.86511-3-scott@gitbutler.net> <3d640489-5db4-5527-0ec1-c2abac7a2de3@gmx.de> Date: Wed, 07 Oct 2026 14:32:42 -0700 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Johannes Schindelin writes: > This Perl script reproduces the tables (although with different > formatting, and without the inline comments, I verified it with > `--patience --color-words="[A-Za-z0-9_]+|."`). > ... > With all that out of the way, I would like to ask to include this script > in the patch (or in a follow-up patch) so that the lengthy `ubc_check.c` > file's tables can be validated/regenerated independently. > ... > While this code is correct, I think it is slightly misleading: depending > on `want`, it either subtracts `set` from `dvs`, or takes the minimum. But > that only happens to be what is desired because each lane of `set` is all > ones or all zero. What we actually want is to mask either those lanes or > everything but those lanes, i.e. `dvs & ~set` or `dvs & set`, > respectively. That would be: > > fail = g->want ? vbicq_u32(dvs, set) : vandq_u32(dvs, set); > > This has no speed impact nor does it produce a "more correct" result, but > it might improve readability a bit. > > I haven't looked very closely whether there are similar issues elsewhere > (it is relatively tedious for me to learn all this NEON stuff on the go, > this is all new to me). If you're familiar with NEON, it might be > worthwhile looking for similarly "correct but misleading" statements. Thanks for offering a very thoughtful help and offering to work well together.