GNU gfortran potential bug: polymorphic functions (thus defined operators) generates serious memory leaks

[Stefano Zaghi]

Dear all,
I faced with a possible serious bug: polymorphic functions (necessary
to define polymorphic operators in OOP programs) generate memory leaks
making OOP program not feasible.

I opened a bug report (80477) here
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80477

I report the details also below.

My best regards.

Issue summary

In the attached example "polymorphic_operators_memory_leaks.f90" I
show the possible bug in less than 100 lines. Essentially, the derived
type "a_type_t" defines the operators "=" and "+" in a polymorphic way
(in order to allow "a_type_t" to be extended while all other "agents"
using "a_type_t" do not need to be changed, OOP). The assignment
operator does not create memory leaks because it is subroutine that
does not allocate temporary. On the contrary, the operator "+" is a
function that allocates a temporary: this temporary generates the
memory leaks because it is not correctly destroyed after its usage.

I checked the above conclusion with "valgrind" and it confirms that
the "+" operator (function) generates the leaks. Note that the same
code compiled and run with Intel Fortran (17.0.2) does not generates
memory leaks and it works as expected. Moreover, is the "+"
operator/function is modified disallowing polymorphic (namely changing
"class(a_type_t), allocatable :: res" with "type(a_type_t) :: res" the
leaks disappear, thus I could conclude that the leaks are generated
only for polymorphic function results.

A word of advice: the test is designed to point out the memory leaks
occurrences, running it your RAM will be quickly consumed! Please, be
rapid to quickly kill it before your OS start swapping on HD. To test
it with valgrind I added a little "ui": it can be executed without
arguments, thus its loop will be very long (and your RAM totally
consumed) or you can pass one integer argument and the loop will do
the iterations you provided, e.g. "a.out 1" will execute 1 iteration
loop. This is written into the code comments.

Please, consider that if this is really a bug, all serious OOP
programs are indeed impossible to be done.

Thank you very much for your help.

Stefano Zaghi  Ph.D. Aerospace Engineer

Research Scientist, Dept. of Computational Hydrodynamics at CNR-INSEAN
p: +39 0650299260 | m: +39 3497730036 | e: stefano.zaghi@gmail.com

Member of Fortran-FOSS-Programmers

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