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())

[Linus Torvalds]

On Sat, 18 Jan 2025 at 09:49, Guenter Roeck <linux@xxxxxxxxxxxx> 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