[Bug ada/77535] New: GNAT.Perfect_Hash_Generators access invalid memory with non-1-based strings

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77535

            Bug ID: 77535
           Summary: GNAT.Perfect_Hash_Generators access invalid memory
                    with non-1-based strings
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ada
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---

Created attachment 39585
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39585&action=edit
phg2.adb

Natasha Kerensikova reported this to comp.lang.ada.  The attached file was
downloaded from:

  http://users.instinctive.eu/nat/phg/

When running it under valgrind, I get:

==4196== Invalid read of size 1
==4196==    at 0x5032D23: ??? (in /usr/lib/x86_64-linux-gnu/libgnat-4.9.so.1)
==4196==    by 0x5035254: gnat__perfect_hash_generators__compute (in
/usr/lib/x86_64-linux-gnu/libgnat-4.9.so.1)
==4196==    by 0x402F3B: _ada_phg2 (in /tmp/g/phg2)
==4196==    by 0x402DA5: main (in /tmp/g/phg2)
==4196==  Address 0x5e61837 is 23 bytes after a block of size 16 alloc'd
==4196==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==4196==    by 0x507F403: __gnat_malloc (in
/usr/lib/x86_64-linux-gnu/libgnat-4.9.so.1)
==4196==    by 0x5033413: ??? (in /usr/lib/x86_64-linux-gnu/libgnat-4.9.so.1)
==4196==    by 0x5035098: gnat__perfect_hash_generators__compute (in
/usr/lib/x86_64-linux-gnu/libgnat-4.9.so.1)
==4196==    by 0x402F3B: _ada_phg2 (in /tmp/g/phg2)
==4196==    by 0x402DA5: main (in /tmp/g/phg2)

Others have reported a segfault.

This is apparently due to GNAT.Perfect_Hash_Generators not dealing properly
with non-1-based strings.