netdev - Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX

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Message-ID: <20240223125343.GD13330@nvidia.com>
Date: Fri, 23 Feb 2024 08:53:43 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: David Laight <David.Laight@...lab.com>
Cc: 'Niklas Schnelle' <schnelle@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Leon Romanovsky <leon@...nel.org>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
	Ingo Molnar <mingo@...hat.com>, Bill Wendling <morbo@...gle.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Salil Mehta <salil.mehta@...wei.com>,
	Jijie Shao <shaojijie@...wei.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
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	"x86@...nel.org" <x86@...nel.org>,
	Yisen Zhuang <yisen.zhuang@...wei.com>,
	Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <catalin.marinas@....com>,
	Leon Romanovsky <leonro@...lanox.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <mark.rutland@....com>,
	Michael Guralnik <michaelgur@...lanox.com>,
	"patches@...ts.linux.dev" <patches@...ts.linux.dev>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX_copy()

On Fri, Feb 23, 2024 at 11:05:29AM +0000, David Laight wrote:
> ...
> > > > > > +		if ((_count % 8) >= 4) {
> > > > >
> > > > > If (_count & 4) {
> > > >
> > > > That would be obfuscating, IMHO. The compiler doesn't need such things
> > > > to generate optimal code.
> > >
> > > Try it: https://godbolt.org/z/EvvGrTxv3
> > > And it isn't that obfuscated - no more so than your version.
> > 
> > The godbolt link does "n % 8 > 4" instead of "... >= 4" as in Jason's
> > original code. With ">=" the compiled code matches that for "n & 4".
> 
> Bugger :-)

Yes, I already fine tuned things to get good codegen.

Jason