linux-kernel - Re: Buiild error in i915/xe (was: [PATCH next 4/7] minmax.h: Use BUILD_BUG_ON

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Message-ID: <CAHk-=whAxUvW-APU42yWfCKHF5NhPLoTida9Jo=A9ZGGgjb18Q@mail.gmail.com>
Date: Sat, 18 Jan 2025 13:21:39 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Guenter Roeck <linux@...ck-us.net>
Cc: David Laight <david.laight.linux@...il.com>, David Laight <David.Laight@...lab.com>, 
	Arnd Bergmann <arnd@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>, 
	Matthew Wilcox <willy@...radead.org>, Christoph Hellwig <hch@...radead.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Dan Carpenter <dan.carpenter@...aro.org>, "Jason A . Donenfeld" <Jason@...c4.com>, 
	"pedro.falcato@...il.com" <pedro.falcato@...il.com>, Mateusz Guzik <mjguzik@...il.com>, 
	"linux-mm@...ck.org" <linux-mm@...ck.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	intel-xe@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Jani Nikula <jani.nikula@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>
Subject: Re: Buiild error in i915/xe (was: [PATCH next 4/7] minmax.h: Use
 BUILD_BUG_ON_MSG() for the lo < hi test in clamp())

On Sat, 18 Jan 2025 at 09:49, Guenter Roeck <linux@...ck-us.net> wrote:
>
> No idea why the compiler would know that the values are invalid.

It's not that the compiler knows tat they are invalid, but I bet what
happens is in scale() (and possibly other places that do similar
checks), which does this:

        WARN_ON(source_min > source_max);
        ...
        source_val = clamp(source_val, source_min, source_max);

and the compiler notices that the ordering comparison in the first
WARN_ON() is the same as the one in clamp(), so it basically converts
the logic to

        if (source_min > source_max) {
                WARN(..);
                /* Do the clamp() knowing that source_min > source_max */
                source_val = clamp(source_val, source_min, source_max);
        } else {
                /* Do the clamp knowing that source_min <= source_max */
                source_val = clamp(source_val, source_min, source_max);
        }

(obviously I dropped the other WARN_ON in the conversion, it wasn't
relevant for this case).

And now that first clamp() case is done with source_min > source_max,
and it triggers that build error because that's invalid.

So the condition is not statically true in the *source* code, but in
the "I have moved code around to combine tests" case it now *is*
statically true as far as the compiler is concerned.

              Linus