linux-kernel - Re: [PATCH] docs: add extra integer types to printk-formats

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Date:   Tue, 05 Mar 2019 12:15:08 -0800
From:   Joe Perches <joe@...ches.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Louis Taylor <louis@...gniz.eu>,
        Jonathan Corbet <corbet@....net>, pmladek@...e.com,
        geert+renesas@...der.be, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux@...glegroups.com,
        Jon Flatley <jflat@...omium.org>
Subject: Re: [PATCH] docs: add extra integer types to printk-formats

On Tue, 2019-03-05 at 12:07 -0800, Matthew Wilcox wrote:
> On Tue, Mar 05, 2019 at 11:59:30AM -0800, Joe Perches wrote:
> > Many of the -Wformat warnings are bogus too.
> > 
> > There's nothing wrong with using %x for a unsigned int
> > of less than long size. (u8/u16)
> 
> I believe you to be incorrect.
> 
> 6.5.2.2 Function calls
> 
> 7 If the expression that denotes the called function has a type that does
>   include a prototype, the arguments are implicitly converted, as if by
>   assignment, to the types of the corresponding parameters, taking the
>   type of each parameter to be the unqualified version of its declared
>   type. The ellipsis notation in a function prototype declarator causes
>   argument type conversion to stop after the last declared parameter. The
>   default argument promotions are performed on trailing arguments.

Look at default argument promotions for varargs functions.
(integer promotions)

> I could define a calling convention for my CPU which says to pack u8s
> and u16s as tightly as possible in registers (or on the stack), rather
> than the prevailing calling convention of having each argument take up
> at least one register-sized slot.

Doesn't matter for varargs.  See above.

cheers, Joe