From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Florian Fainelli <f.fainelli@gmail.com>
Cc: bcm-kernel-feedback-list@broadcom.com,
linux-rpi-kernel@lists.infradead.org,
linux-arm-kernel@lists.infradead.org, linux-pci@vger.kernel.org,
linux-kernel@vger.kernel.org,
"Jim Quinlan" <jim2101024@gmail.com>,
"Nicolas Saenz Julienne" <nsaenz@kernel.org>,
"Lorenzo Pieralisi" <lorenzo.pieralisi@arm.com>,
"Rob Herring" <robh@kernel.org>,
"Krzysztof Wilczyński" <kw@linux.com>,
"Bjorn Helgaas" <bhelgaas@google.com>
Subject: Re: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as __GENMASK() is for internal use only
Date: Wed, 17 Nov 2021 14:42:25 +0200 [thread overview]
Message-ID: <YZT4sdwf25XV6pKu@smile.fi.intel.com> (raw)
In-Reply-To: <41b85802-6118-33a6-692a-043d74b82d8e@gmail.com>
On Tue, Nov 16, 2021 at 12:38:39PM -0800, Florian Fainelli wrote:
> On 11/15/21 3:20 AM, Andy Shevchenko wrote:
> > Use BIT() as __GENMASK() is for internal use only. The rationale
> > of switching to BIT() is to provide better generated code. The
> > GENMASK() against non-constant numbers may produce an ugly assembler
> > code. On contrary the BIT() is simply converted to corresponding shift
> > operation.
>
> The code is not necessarily any different on ARMv8 as far as I can tell,
> before:
>
> static void brcm_msi_set_regs(struct brcm_msi *msi)
> {
> u32 val = __GENMASK(31, msi->legacy_shift);
> 84: b9406402 ldr w2, [x0,#100]
> 88: d2800021 mov x1, #0x1
> // #1
> 8c: 9ac22021 lsl x1, x1, x2
> 90: 4b0103e1 neg w1, w1
>
>
> after:
>
> static void brcm_msi_set_regs(struct brcm_msi *msi)
> {
> u32 val = ~(BIT(msi->legacy_shift) - 1);
> 84: b9406402 ldr w2, [x0,#100]
> 88: d2800021 mov x1, #0x1
> // #1
> 8c: 9ac22021 lsl x1, x1, x2
> 90: 4b0103e1 neg w1, w1
>
> and the usage of BIT() does not make this any clearer.
While I disagree on the conclusion it's good that assembly isn't bad.
Last time I have tried to compile just GENMASK() excerpts for arm32
the non-constant variants were quite bad. And it was obvious win for
BIT() over GENMASK().
Actually it maybe that I have tested something like
`GENMASK(C1 + var, C2 + var)` vs. `GENMASK(C1, C2) << var`
that time.
--
With Best Regards,
Andy Shevchenko
next prev parent reply other threads:[~2021-11-17 12:42 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-15 11:20 [PATCH v2 1/1] PCI: brcmstb: Use BIT() as __GENMASK() is for internal use only Andy Shevchenko
2021-11-15 11:57 ` Krzysztof Wilczyński
2021-11-15 13:59 ` Robin Murphy
2021-11-15 14:14 ` Andy Shevchenko
2021-11-15 14:39 ` Andy Shevchenko
2021-11-16 18:20 ` Rob Herring
2021-11-16 20:41 ` Florian Fainelli
2021-11-16 20:56 ` Florian Fainelli
2021-11-17 10:37 ` Andy Shevchenko
2021-11-17 22:46 ` Rob Herring
2021-11-16 20:38 ` Florian Fainelli
2021-11-17 12:42 ` Andy Shevchenko [this message]
2021-12-01 15:53 ` Lorenzo Pieralisi
2021-12-01 16:01 ` Andy Shevchenko
2021-12-01 17:13 ` Lorenzo Pieralisi
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