linux-kernel - RE: [PATCH 06/12] openrisc: Use of_get_cpu

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Message-ID: <c81b9c01650e487785b02f6562791556@AcuMS.aculab.com>
Date:   Thu, 7 Oct 2021 07:53:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Segher Boessenkool' <segher@...nel.crashing.org>,
        Stafford Horne <shorne@...il.com>
CC:     Rich Felker <dalias@...c.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "x86@...nel.org" <x86@...nel.org>, Guo Ren <guoren@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "Will Deacon" <will@...nel.org>, Jonas Bonn <jonas@...thpole.se>,
        Rob Herring <robh@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Frank Rowand <frowand.list@...il.com>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
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        Albert Ou <aou@...s.berkeley.edu>, Ray Jui <rjui@...adcom.com>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        "openrisc@...ts.librecores.org" <openrisc@...ts.librecores.org>,
        Borislav Petkov <bp@...en8.de>,
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        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul Mackerras" <paulus@...ba.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: RE: [PATCH 06/12] openrisc: Use of_get_cpu_hwid()

From: Segher Boessenkool
> Sent: 06 October 2021 22:27
> 
> On Thu, Oct 07, 2021 at 05:44:00AM +0900, Stafford Horne wrote:
> > You have defined of_get_cpu_hwid to return u64, will this create compiler
> > warnings when since we are storing a u64 into a u32?
> >
> > It seems only if we make with W=3.
> 
> Yes.  This is done by -Wconversion, "Warn for implicit conversions that
> may alter a value."
> 
> > I thought we usually warned on this.

The microsoft compiler does - best to turn all those warnings off.

> This warning is not in -Wall or -Wextra either, it suffers too much from
> false positives.  It is very natural to just ignore the high bits of
> modulo types (which is what "unsigned" types *are*).  Or the bits that
> "fall off" on a conversion.  The C standard makes this required
> behaviour, it is useful, and it is the only convenient way of getting
> this!

I've also seen a compiler convert:
	struct->char_member = (char)(int_val & 0xff);
into:
	reg = int_val;
	reg &= 0xff; // for the & 0xff
	reg &= 0xff; // for the cast
	struct->char_member = low_8bits(reg);

You really don't want the extra noise.

I'll bet that (char)int_val is actually an arithmetic expression.
So its type will be 'int'.

	David

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