Re: Calculating array length

[Jan Kratochvil <jan dot kratochvil at redhat dot com>]

On Sun, 07 Jun 2009 12:22:53 +0200, Joost van der Sluis wrote:
> But I also have a lot of questions, I hope this is the right place for
> them.

Yes, VLA support is currently still not integrated with FSF GDB.


> First question is the calculation of the length of an array-type in
> type_length_get(), gdbtypes.c. (I'm using the archer-jankratochvil-sla
> branch to improve the handling of arrays)
> 
> The length is calculated as follows: (count-1)*byte_stride+element_size.
> 
> Ok, so why 'count-1'? Count is the actual item-count, why substract it
> with one? And why is the element_size added? byte_stride (when defined)
> should replace the element_size?
> 
> It doesn't make sense to me, as it should be: 'count*byte_stride'?

It depends which length do you want to calculate.  According to what you
describe you want to set the FULL_SPAN parameter of type_length_get to be
true.  Then it will really return 'count*byte_stride'.

Still the function uses FULL_SPAN as true only internally.  When GDB wants to
know the type length it uses it for transferring data from the debuggee memory
into a local GDB copy (for its printing to the user etc.).  For such case we
want a _minimal_ (but still complete) contiguous memory range.

Fortran example:

subroutine sub (p)
  integer :: p (2, 1)
  print *, p (1, 1)
  print *, p (2, 1)
end subroutine sub
program subarray
  integer :: a (2, 2)
  a (1, 1) = 1
  a (1, 2) = 2
  a (2, 1) = 3
  a (2, 2) = 4
  call sub (a (1:2, 2:2))
end

Array `a' in the main program has layout (here the first index is row, second
one is column):
  1 2
  3 4
  X Y (these are uninitialized / nonexisting / unused memory locations after
       the end of array)
Subroutine `sub' will print:
           2
           4
Subroutine `sub' know only about a table with 2 rows and 1 column.  To make it
working with the original array `a' memory layout without any copy the
pointers to the array are setup as:
  array start: row 1 column 2 (element content `2')
  rows, therefore number of elements of p: 2
  columns, therefore number of elements of p row: 1
  element size of p (one row byte length): sizeof (integer) * 1
  element size of p row (one element byte length): sizeof (integer)
  byte stride of p (offset to the next row): sizeof (integer) * 2
  byte stride of p row: sizeof (integer)

Now if you in `sub' do `print p' GDB has to transfer the `p' memory from
inferior.  Currently it will transfer contiguous block with content {2,3,4}.

If we would always use FULL_SPAN true then GDB would transfer in this case
a contiguous memory block with content {2,3,4,X}.  But X is after the end of
the array and for very large arrays (thousands of elements or elements of size
in kilobytes) memory for X may no longer be mapped and GDB would fail
retrieving the memory of variable being wished to be printed.  (+It would be
also less effective.)

GDB has to transfer only the memory it knows that belongs to a variable.


Regards,
Jan