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

[David Laight]

From: Martin Uecker
> Sent: 08 December 2024 12:38
> 
> Am Sonntag, dem 08.12.2024 um 11:26 +0000 schrieb 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.
> 
> In numerical computing this is a big motivation because
> you can reduce stack usage in recursive divide-and-conquer
> algorithms.  For the kernel, I agree this is not a
> compelling use case, and the better motivation would be
> precise bounds checking and clearer semantics for buffer
> management.

Except that changing the size of the on-stack array makes
absolutely no difference.
Ideally the kernel stack would be a single 4k page, but too
much code uses on-stack buffers so it has been increased and
might be 16k (or more!).
Remember this is physical memory allocated to every user thread.
On Linux it is not swappable.

...
> > This happened for 'constant' sizes from min(16, sizeof (struct))
> > because min() needs to be a statement function to avoid re-evaluating
> > its arguments.
> 
> Can you clarify this?  If the VLA size is constant, even when
> it is not an integer constant expression according to ISO C,
> the compiler should not produce worse code.  For example,

I just tried to reproduce the failing case - and failed.
It was similar to __builtin_constant_p() initially returning 'don't know'
so the 'variable sized' array code got added, then much later
after further optimisation passes the expression became constant.
So you ended up with a 'fixed size' VLA.

Compile with -Wno-vla (and -Werror) and the compile failed.

...
> So a lot of this macro business seems to be necessary
> to avoid creating warnings for ISO VLAs when instead you really
> care about the created code not having a dynamic allocation on
> the stack.

A lot of the 'macro business' for min/max is avoiding unexpected
conversion of negative values to very large unsigned ones.
And no, -Wsign-compare is spectacularly useless.

..
> The issue here is that we miss a language feature in C to
> introduce local variables that help avoid multiple expansion
> of macro arguments.  GCC's statement expressions and __auto_type
> are a solution

or historically 'typeof(x) _x = x'

> #define foo(x) ({ __auto_type __x = (x); ... })
> 
> but this runs into the current limitations that ({ }) can not be used
> at file-scope and can not return constant expressions.
> 
> 
> For other reasons I was thinking about adding names to _Generic,
> as in
> 
> _Generic(x, int i: (i + 1));
> 
> because one design issues with _Generic is that it typechecks
> also the untaken associations and there the 'x' then has the wrong
> type.  Having an 'i' with the right type which is set to the value
> of 'x' when the branch is taken would fix this issue.

That looks even more syntactically obscure than _Generic itself.
Why does it need to do more than very simple syntax analysis of
the unwanted branches - or they could automatically be analysed
with the named variable have the specified type?

> But this feature might also allow writing macros that avoid
> double expansion without requiring statement expressions (which
> are more difficult to fix):
> 
> #define foo(x) _Generic(x, int i: (i + i));

How can that work for things like min() that have multiple arguments?
Not going to work if you need __auto_type either.

	David

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