On 1/18/25 14:11, David Laight wrote:
On Sat, 18 Jan 2025 13:21:39 -0800
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
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.
Well spotted :-)
One option would be to move the WARN_ON() below the clamp() and
add an OPTIMISER_HIDE_VAR(source_max) between them.
Or do something more sensible than the WARN().
Perhaps return target_min on any such errors?
This helps:
- WARN_ON(source_min > source_max);
- WARN_ON(target_min > target_max);
-
/* defensive */
source_val = clamp(source_val, source_min, source_max);
+ WARN_ON(source_min > source_max);
+ WARN_ON(target_min > target_max);
Guenter
