From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: Re: [PATCH] ARM: delay: avoid clang -Wtautological-constant warning
Date: Tue, 23 Mar 2021 13:30:05 +0000 [thread overview]
Message-ID: <20210323133005.GC1463@shell.armlinux.org.uk> (raw)
In-Reply-To: <20210323132031.2858996-1-arnd@kernel.org>
On Tue, Mar 23, 2021 at 02:20:23PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
>
> Passing an 8-bit constant into delay() triggers a warning when building
> with 'make W=1' using clang:
>
> drivers/clk/actions/owl-pll.c:182:2: error: result of comparison of constant 2000 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
> udelay(pll_hw->delay);
> ^~~~~~~~~~~~~~~~~~~~~
> arch/arm/include/asm/delay.h:84:9: note: expanded from macro 'udelay'
> ((n) > (MAX_UDELAY_MS * 1000) ? __bad_udelay() : \
> ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~
> arch/arm/mach-omap2/wd_timer.c:89:3: error: result of comparison of constant 2000 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
> udelay(oh->class->sysc->srst_udelay);
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Shut up the warning by adding a cast to a 64-bit number. A cast to 'int'
> would usually be sufficient, but would fail to cause a link-time error
> for large 64-bit constants.
What effect (if any) does this have on code generation when the argument
is not constant?
--
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next prev parent reply other threads:[~2021-03-23 13:33 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-23 13:20 [PATCH] ARM: delay: avoid clang -Wtautological-constant warning Arnd Bergmann
2021-03-23 13:30 ` Russell King - ARM Linux admin [this message]
2021-03-24 23:14 ` David Laight
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