allocatable arrays and -fmax-stack-var-size
So, it seems that at some point in the past, the option
-fmax-stack-var-size was expanded to allow the placement
of an allocatable array into static memory. This has
a possibly unintended consequence in that automatic
deallocation of an allocatable array does not (or can
not) occur.
program foo
implicit none
call testAutoDealloc(20)
call testAutoDealloc(200)
contains
subroutine testAutoDealloc(n)
integer, intent(in) :: n
real, allocatable :: temp(:)
allocate(temp(n))
temp = n
if (temp(n) /= n) stop n
end
end program foo
% gfcx -o u a.f90 && ./u
% gfcx -o u -fmax-stack-var-size=10 -fdump-tree-original a.f90 && ./u
At line 9 of file a.f90
Fortran runtime error: Attempting to allocate already allocated variable 'temp'
% head -4 u-a.f90.005t.original
__attribute__((fn spec (". r ")))
void testautodealloc (integer(kind=4) & restrict n)
{
static struct array01_real(kind=4) temp = {.data=0B};
Now, it seems that an explicit deallocation of temp at the end of
the subroutine testAutoDealloc suppresses the runtime error. Looking
at a -fdump-tree-original with the modified code shows
if ((real(kind=4)[0:] * restrict) temp.data == 0B)
{
_gfortran_runtime_error_at (...Attempt to DEALLOCATE unallocated...);
}
else
{
__builtin_free ((void *) temp.data);
(real(kind=4)[0:] * restrict) temp.data = 0B;
}
Should the automatic deallocation of allocatable arrays be restore?
I'll let someone who cares enough to pursue this route. Until then,
here's a patch to the manual to caution the unwary.
diff --git a/gcc/fortran/invoke.texi b/gcc/fortran/invoke.texi
index 6435dc4d4de..b5002d2a31a 100644
--- a/gcc/fortran/invoke.texi
+++ b/gcc/fortran/invoke.texi
@@ -1786,13 +1786,19 @@ The default value for @var{n} is 65535.
@item -fmax-stack-var-size=@var{n}
@opindex @code{fmax-stack-var-size}
This option specifies the size in bytes of the largest array that will be put
-on the stack; if the size is exceeded static memory is used (except in
-procedures marked as RECURSIVE). Use the option @option{-frecursive} to
-allow for recursive procedures which do not have a RECURSIVE attribute or
-for parallel programs. Use @option{-fno-automatic} to never use the stack.
+on the stack. If the size of an array exceeds @var{n}, then the array is
+placed in static memory (except in procedures marked as RECURSIVE). Use
+the option @option{-frecursive} to allow for recursive procedures which
+do not have a RECURSIVE attribute or for parallel programs.
+Use @option{-fno-automatic} to never use the stack.
+
+The @option{-Wsurprising} option can be used to determine which arrays
+have been placed into static memory.
+
+@option{-fmax-stack-var-size} can inhibit the automatic deallocation of
+allocatable arrays. Proper memory management is required if this option
+is used (i.e., explicit deallocation is encouraged).
-This option currently only affects local arrays declared with constant
-bounds, and may not apply to all character variables.
Future versions of GNU Fortran may improve this behavior.
The default value for @var{n} is 65536.
--
Steve
Re: allocatable arrays and -fmax-stack-var-size
Hi Steve, So, it seems that at some point in the past, the option -fmax-stack-var-size was expanded to allow the placement of an allocatable array into static memory. This has a possibly unintended consequence in that automatic deallocation of an allocatable array does not (or can not) occur. Sounds like a bug to me, and if your test program worked in a previous release, it's a regression. Probably best to open a PR. Best regards Thomas
Re: allocatable arrays and -fmax-stack-var-size
On Thu, Mar 31, 2022 at 08:36:37PM +0200, Thomas Koenig wrote:
> Hi Steve,
>
> > So, it seems that at some point in the past, the option
> > -fmax-stack-var-size was expanded to allow the placement
> > of an allocatable array into static memory. This has
> > a possibly unintended consequence in that automatic
> > deallocation of an allocatable array does not (or can
> > not) occur.
>
> Sounds like a bug to me, and if your test program worked
> in a previous release, it's a regression.
>
> Probably best to open a PR.
>
Thomas
Seems someone from Fortran Discourse forum beat me to it.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105117
With all of the work on openmp, I wasn't sure if this
was intended or not. Either way it is surprising to me
that an allocatable array is placed in static memory.
I went looking and found this chunk of code in trans-decl.cc
(lines 743-774 where I removed the paragraph warning).
/* Keep variables larger than max-stack-var-size off stack. */
if (!(sym->ns->proc_name && sym->ns->proc_name->attr.recursive)
&& !sym->attr.automatic
&& sym->attr.save != SAVE_EXPLICIT
&& sym->attr.save != SAVE_IMPLICIT
&& INTEGER_CST_P (DECL_SIZE_UNIT (decl))
&& !gfc_can_put_var_on_stack (DECL_SIZE_UNIT (decl))
/* Put variable length auto array pointers always into stack. */
&& (TREE_CODE (TREE_TYPE (decl)) != POINTER_TYPE
|| sym->attr.dimension == 0
|| sym->as->type != AS_EXPLICIT
|| sym->attr.pointer
|| sym->attr.allocatable)
&& !DECL_ARTIFICIAL (decl))
{
if (flag_max_stack_var_size > 0
&& !(sym->ns->proc_name
&& sym->ns->proc_name->attr.is_main_program))
gfc_warning (OPT_Wsurprising,
...
sym->name, &sym->declared_at);
TREE_STATIC (decl) = 1;
If I set the last line to 0, I get what I expect as far as an
allocatable array. I have been unable to decipher the 12 line
conditional.
--
Steve
