Flang runtime uses uninitialized value, causing memory leak and segfault

[difference-scheme]

The following sample code exposes the problem (this might actually have the same cause as the segfault of Issue #851. However, even if the polymorphic assignment below is replaced by sourced allocation, to eliminate the segfault, a memory leak remains present):

subroutine calc()

   implicit none
   
   type, abstract :: Base
   end type Base
   
   type, extends(Base) :: Derived
   end type Derived
   
   class(Base),    allocatable :: b
   class(Derived), allocatable :: d
   
   allocate(Derived :: d)
   
   ! polymorphic assignment
   b = d
   
end subroutine calc

program testf
   implicit none   
   call calc()
end program testf

When compiled with gfortran 9.0.1 and run using valgrind --track-origins=yes --leak-check=full ./a.out valgrind's output is:

==7303== Memcheck, a memory error detector
==7303== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==7303== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==7303== Command: ./a.out
==7303== 
==7303== 
==7303== HEAP SUMMARY:
==7303==     in use at exit: 0 bytes in 0 blocks
==7303==   total heap usage: 23 allocs, 23 frees, 13,562 bytes allocated
==7303== 
==7303== All heap blocks were freed -- no leaks are possible
==7303== 
==7303== For counts of detected and suppressed errors, rerun with: -v
==7303== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

When compiled with flang, valgrind --track-origins=yes --leak-check=full ./a.out gives:

==7309== Memcheck, a memory error detector
==7309== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==7309== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==7309== Command: ./a.out
==7309== 
==7309== Conditional jump or move depends on uninitialised value(s)
==7309==    at 0x50E73A1: get_source_and_dest_sizes (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x50E8265: f90_poly_asn_i8 (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x400CFF: calc_ (memtest.f90:17)
==7309==    by 0x400E39: MAIN_ (memtest.f90:23)
==7309==    by 0x400E75: main (in /home/Compiler_bugs/Flang_issues/Issue_8xx/a.out)
==7309==  Uninitialised value was created by a stack allocation
==7309==    at 0x4009C0: calc_ (memtest.f90:1)
==7309== 
==7309== Use of uninitialised value of size 8
==7309==    at 0x50E745E: get_source_and_dest_sizes (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x50E8265: f90_poly_asn_i8 (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x400CFF: calc_ (memtest.f90:17)
==7309==    by 0x400E39: MAIN_ (memtest.f90:23)
==7309==    by 0x400E75: main (in /home/Compiler_bugs/Flang_issues/Issue_8xx/a.out)
==7309==  Uninitialised value was created by a stack allocation
==7309==    at 0x4009C0: calc_ (memtest.f90:1)
==7309== 
==7309== Invalid read of size 1
==7309==    at 0x4C341ED: memcpy@GLIBC_2.2.5 (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7309==    by 0x50E8498: f90_poly_asn_i8 (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x400CFF: calc_ (memtest.f90:17)
==7309==    by 0x400E39: MAIN_ (memtest.f90:23)
==7309==    by 0x400E75: main (in /home/Compiler_bugs/Flang_issues/Issue_8xx/a.out)
==7309==  Address 0x47175d90 is not stack'd, malloc'd or (recently) free'd
==7309== 
==7309== 
==7309== Process terminating with default action of signal 11 (SIGSEGV)
==7309==  Access not within mapped region at address 0x47175D90
==7309==    at 0x4C341ED: memcpy@GLIBC_2.2.5 (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7309==    by 0x50E8498: f90_poly_asn_i8 (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x400CFF: calc_ (memtest.f90:17)
==7309==    by 0x400E39: MAIN_ (memtest.f90:23)
==7309==    by 0x400E75: main (in /home/Compiler_bugs/Flang_issues/Issue_8xx/a.out)
==7309==  If you believe this happened as a result of a stack
==7309==  overflow in your program's main thread (unlikely but
==7309==  possible), you can try to increase the size of the
==7309==  main thread stack using the --main-stacksize= flag.
==7309==  The main thread stack size used in this run was 16777216.
==7309== 
==7309== HEAP SUMMARY:
==7309==     in use at exit: 12,352 bytes in 3 blocks
==7309==   total heap usage: 4 allocs, 1 frees, 85,056 bytes allocated
==7309== 
==7309== 32 bytes in 1 blocks are possibly lost in loss record 1 of 3
==7309==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7309==    by 0x4F400A0: __fort_gcalloc_without_abort (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x5041ED3: __alloc04_i8 (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x5043209: f90_ptr_src_calloc04a_i8 (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x400A8F: calc_ (memtest.f90:14)
==7309==    by 0x400E39: MAIN_ (memtest.f90:23)
==7309==    by 0x400E75: main (in /home/Compiler_bugs/Flang_issues/Issue_8xx/a.out)
==7309== 
==7309== 32 bytes in 1 blocks are possibly lost in loss record 2 of 3
==7309==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7309==    by 0x4F400A0: __fort_gcalloc_without_abort (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x5041ED3: __alloc04_i8 (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x5043209: f90_ptr_src_calloc04a_i8 (in /home/Compilers/Flang/install/lib/libflang.so)
==7309==    by 0x400CC2: calc_ (memtest.f90:17)
==7309==    by 0x400E39: MAIN_ (memtest.f90:23)
==7309==    by 0x400E75: main (in /home/Compiler_bugs/Flang_issues/Issue_8xx/a.out)
==7309== 
==7309== LEAK SUMMARY:
==7309==    definitely lost: 0 bytes in 0 blocks
==7309==    indirectly lost: 0 bytes in 0 blocks
==7309==      possibly lost: 64 bytes in 2 blocks
==7309==    still reachable: 12,288 bytes in 1 blocks
==7309==         suppressed: 0 bytes in 0 blocks
==7309== Reachable blocks (those to which a pointer was found) are not shown.
==7309== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==7309== 
==7309== For counts of detected and suppressed errors, rerun with: -v
==7309== ERROR SUMMARY: 5 errors from 5 contexts (suppressed: 0 from 0)
Segmentation fault (core dumped)

Notice that the number of allocs/frees is asymmetric (4 allocs were performed, but only 1 free).