Re: Largest size array in Gnat 2005 for the PC?

On Oct 30, 2:40 pm, Georg Bauhaus <rm.tsoh+bauh...@xxxxxxxxxxxxxxxxxx>
wrote:
On Tue, 2007-10-30 at 13:24 -0700, Adam Beneschan wrote:
On Oct 30, 12:26 pm, Georg Bauhaus <rm.tsoh
Is there a bug in the following program or maybe I'm just dense?

EAX is a 32-bit register, right?

Yes, i686 Linux in this case. When I ask the compiler for
64bit code, then I see occurrences of $280375465082880,
which is 2**48 - 2**40.

So to assign into the first element of Fst, the offset from the
beginning of the array is 0; to assign into the last element, the
offset will be (2**48 - 2**40) which is 255 * 2**40, which is way too
big to fit into a 32-bit register, so apparently the compiler is
truncating to 0 before setting EAX.

Ah, yes. Could exceeding size have produced the -1 in
-1(%ebp,%eax), %eax
too? It is
-10002(%ebp,%eax), %eax
for the "normal size" array of 10_000 elements?

Your array is 10_001 elements, actually. I haven't studied code
produced by GNAT very much, but -1 just looks like the starting point
for where things are allocated (downward) from the current stack frame
pointer (EBP); exceeding size would have produced -1 in that if the
compiler truncates the array size and thinks the size is 0, then the
resulting offset would have been -1. I'll bet that if you declared an
array of one byte, you would have seen -2(%ebp,%eax); if it were two
bytes, -3(%ebp,%eax), and so on. I'm just speculating, though... I
haven't tried this myself.

-- Adam


.

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