Re: Need a hint or more likely help

[Mikael Morin]

Hello,

Le 11/02/2024 à 03:00, Steve Kargl a écrit :
All, consider this simple code:
   module foo
     contains
       subroutine bar
         character(len=:), allocatable :: s(:)
         call bah(s)
      end subroutine bar
   end module foo

If one compiles with -fdump-tree-original, one see (with some pruning)

   void bar ()
   {
     integer(kind=8) .s;
     struct array01_character(kind=1) s;

The above two lines seem to be ok.

     bitsizetype D.4319;
     sizetype D.4320;

     try
       {
         D.4319 = (bitsizetype) (sizetype) NON_LVALUE_EXPR <.s> * 8;
         D.4320 = (sizetype) NON_LVALUE_EXPR <.s>;
         s.data = 0B;
         s.dtype = {.elem_len=(unsigned long) .s, .version=0, .rank=1, .type=6};
         bah ((character(kind=1)[0:][1:.s] * restrict) s.data, .s);
     }

This is bad. .s is undefined.  I've trace this to trans-array.cc:11531

   if (sym->ts.type == BT_CHARACTER
       && !INTEGER_CST_P (sym->ts.u.cl->backend_decl))
     {
       gfc_conv_string_length (sym->ts.u.cl, NULL, &init);
       gfc_trans_vla_type_sizes (sym, &init);

The problem here is that sym->ts.u.cl->length == NULL.  If I change
the conditional to

   if (sym->ts.type == BT_CHARACTER
       && sym->ts.u.cl->length
       && !INTEGER_CST_P (sym->ts.u.cl->backend_decl))

No, I think sym->ts.u.cl->length == NULL is correct for deferred length.
It is set only when there is an expression defining the length:
    character(len=size(somevar)) :: s(:)

then the option -fdump-tree-original produces

   void bar ()
   {
     integer(kind=8) .s;
     struct array01_character(kind=1) s;

     try
       {
         s.data = 0B;
         s.dtype = {.elem_len=(unsigned long) .s, .version=0, .rank=1, .type=6};
         bah ((character(kind=1)[0:][1:.s] * restrict) s.data, .s);
       }

which looks good except I don't know what the reference to .s here
means.  Is this correct or should we set .s to zero by artificially
setting sym->ts.u.cl->length to, say, zero length?

Setting the variable .s to zero should be correct in any case, but it's 
not the same as setting the expression.
I think you can use
    if (sym->ts.deferred)
      gfc_add_modify (
        &init,
        sym->ts.u.cl->backend_decl,
        build_zero_cst (TREE_TYPE (sym->ts.u.cl->backen_decl))
      );
to set .s to zero.

Actually, setting to any value should be correct (including -1), as the 
value should not be used as long as the data pointer is NULL.

What is not clear is whether the values of D.4319 and D.4320  need 
update when memory is allocated for s.  I think those variables are 
referenced in the type of s (but it's not visible in the dump), and may 
need update when the value of .s changes.

I hope it helps.
Mikael