Re: Segments

David Brown <david.brown@hesbynett.no> schrieb:

On 17/01/2025 17:42, Thomas Koenig wrote:

David Brown <david.brown@hesbynett.no> schrieb:

On 16/01/2025 17:46, Waldek Hebisch wrote:

David Brown <david.brown@hesbynett.no> wrote:

On 16/01/2025 13:35, Michael S wrote:

On Thu, 16 Jan 2025 12:36:45 +0100
David Brown <david.brown@hesbynett.no> wrote:
>

On 15/01/2025 21:59, Thomas Koenig wrote:

Michael S <already5chosen@yahoo.com> schrieb:

On Wed, 15 Jan 2025 18:00:34 -0000 (UTC)
Thomas Koenig <tkoenig@netcologne.de> wrote:
   

>

As you can guess, in kernel drivers VLA are unwelcome.

>
I can imagine that they are - but I really don't understand why.  I've
never understood why people think there is something "dangerous" about
VLAs, or why they think using heap allocations is somehow "safer".

>
VLA normally allocate on the stack.  Which at first glance look
great.  But once one realize how small are stacks in modern
systems (compared to whole memory), this no longer looks good.
Basically, to use VLA one needs rather small bound on maximal
size of array.

>
Sure.
>

Given such bound always allocating maximal
size is simpler.  Without _small_ bound on size heap is
safer, as it is desined to handle also big allocations.

>
You don't allocate anything in a VLA without knowing the bounds and
being sure it is appropriate to put on the stack.

 
In general, that is a hard thing to know - there is no standard
way to enquire the size of the stack, how much you have already
used, how deep you are going to recurse, or how much stack
a function will use.
 

>
That would be another way of saying you have no idea when your program
is going to blow up from lack of stack space.  You don't need VLAs to
cause such problems.

I may know a few things (or I can find out), but the general user,
especially somebody who writes scientific software in Fortran,
in general does not.

In reality, you /do/ know a fair amount.  Often your knowledge is
approximate - you know you are not going to need anything like as much
stack as the system provides, and you don't worry about it.  In other
situations (such as in small embedded systems), you think about it all
the time - again, regardless of any VLAs.
>
If you are in a position where you suspect you might be pushing close to
the limits of your stack, "standard" doesn't come into it - you are
dealing with a particular target, and you can use whatever functions or
support that target provides.

Again, try look at it from the viewpoint of somebody who writes
scientific or technical code, and for whom such code should
"just work".  Also look at it from the viewpoint of somebody who
co-maintains a compiler for such people.

gfortran has the -fstack-arrays option, which can bring very
large performance improvements - 50% in some real-world code.
Do I know what code users are writing?  Not in the least,
unless they provide bug reports.

And a stack overflow has the most unfriendly user interface of all.