[PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

[David Laight]

From: Martin Uecker
> Sent: 07 December 2024 23:52
...
> While the compiler can not automatically prove every use
> of VLA bounded, it can reliably diagnose the cases where it
> can *not* see that it is bounded. Consider this example:
> 
> void oob(int n, char p[n]);
> void f(unsigned int n)
> {
>     char buf[MIN(n, 100)]; // bounded
>     oob(n + 10, buf); // warning
> }
...

The kernel stack has to have enough space for the [100]
so the full amount might as well always be allocated.
The chance of 'trading off' stack usage with another function
in the same call stack that is guaranteed to use less than
its maximum is about zero.

The VLA code also adds an extra stack frame, this pretty much
pessimises everything.
This happened for 'constant' sizes from min(16, sizeof (struct))
because min() needs to be a statement function to avoid re-evaluating
its arguments.
(The version of min() that managed to return constant from constant
input just exploded in cpp, partially responsible for 18MB lines
being fed into the compiler part.)

	David

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