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From: Kent Overstreet To: Eric Biggers Cc: Lorenzo Stoakes , Christoph Hellwig , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-bcachefs@vger.kernel.org, Kent Overstreet , Andrew Morton , Uladzislau Rezki , linux-mm@kvack.org Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec Message-ID: References: <20230509165657.1735798-1-kent.overstreet@linux.dev> <20230509165657.1735798-8-kent.overstreet@linux.dev> <20230510064849.GC1851@quark.localdomain> <20230513015752.GC3033@quark.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230513015752.GC3033@quark.localdomain> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 6B0891C0004 X-Stat-Signature: r4zg4jutx4qqqwbhfr73dxfdgtew5ir1 X-HE-Tag: 1684006104-892858 X-HE-Meta: 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 IIH737jW S68MYg04omA0Ref4xwB+XdTkULAXHzOC1JJT7p9Xj6Nt8ISIHbGkwvqul2Gu8a+voujBwFRVDy4eG6m+GW+9MPUrwi0a2DWWEmBeHtTANRbPvh6cvJNV9yg8FfrpVbkhNeYRFKkzhSZE6UZir70P/eA3TVICUPc7mzCcU X-Bogosity: Ham, tests=bogofilter, spamicity=0.000886, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, May 12, 2023 at 06:57:52PM -0700, Eric Biggers wrote: > What I would suggest instead is preprocessing the list of 6 field lengths to > create some information that can be used to extract all 6 fields branchlessly > with no dependencies between different fields. (And you clearly *can* add a > preprocessing step, as you already have one -- the dynamic code generator.) > > So, something like the following: > > const struct field_info *info = &format->fields[0]; > > field0 = (in->u64s[info->word_idx] >> info->shift1) & info->mask; > field0 |= in->u64s[info->word_idx - 1] >> info->shift2; > > ... but with the code for all 6 fields interleaved. > > On modern CPUs, I think that would be faster than your current C code. > > You could do better by creating variants that are specialized for specific > common sets of parameters. During "preprocessing", you would select a variant > and set an enum accordingly. During decoding, you would switch on that enum and > call the appropriate variant. (This could also be done with a function pointer, > of course, but indirect calls are slow these days...) > > For example, you mentioned that 8-byte packed keys is a common case. In that > case there is only a single u64 to decode from, so you could create a function > that just handles that case: > > field0 = (word >> info->shift) & info->mask; > > You could also create other variants, e.g.: > > - 16-byte packed keys (which you mentioned are common) > - Some specific set of fields have zero width so don't need to be extracted > (which it sounds like is common, or is it different fields each time?) > - All fields having specific lengths (are there any particularly common cases?) > > Have you considered any of these ideas? I like that idea. Gonna hack some code... :)