[Patch, fortran] Fix sizetype and size_type_node in the Fortran frontend

[Tobias Burnus]

Janne Blomqvist wrote:
> I've been looking a bit at improving our detection of overflow when
> allocating arrays (PR 28105), and I noticed that the definition of
> size_t is a bit messed up. sizetype is properly set to an unsigned
> type of the right size, but the expression is calculated in a bit
> needlessly complicated way. However, size_type_node is an alias for
> gfc_array_index_type which is a signed type.
Well spotted!

> However, this patch does introduce a small regression (which the
> testsuite doesn't test for), namely since size_type_node is now
> changed to an unsigned type, the overflow test which checks for size<
> 0 when allocating arrays is optimized away.

Frankly, while this test catches some of the overflow cases, I never 
quite liked it. The chance that this issue occurs is relatively low, it 
makes the code needlessly complicated (and slow) and the chances that it 
works are also relatively low.

For non-manual allocation, we have already removed the check, cf. PR 
42958. There, the non-negative check is only done with -fcheck=mem.

Thus, even though it is a regression, I am in favour of having no 
overflow check by default and only one - and possibly a better one - 
with -fcheck=mem. I think a better check would be "if 
(number_of_elements > (SIZE_MAX / sizeof(type)))", which assumes that 
the <number of element> does not already overflow.

Tobias

PS: If no one is faster, I will review the patch later today.