linux-kernel - Re: [PATCH] ARM: delay: avoid clang -Wtautological-constant warning

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Message-ID: <20210323133005.GC1463@shell.armlinux.org.uk>
Date:   Tue, 23 Mar 2021 13:30:05 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] ARM: delay: avoid clang -Wtautological-constant warning

On Tue, Mar 23, 2021 at 02:20:23PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.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?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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