Hi Richard. Thanks for your reply, but if I understand
you correctly, you are saying this fix is for situations
where the size of an integer is different from the size
of a pointer?
That is not my issue. The size is the same. Absolutely
everything is 32-bits in the program (long, int, char *,
void *, code addresses).
However, since I am running as AMODE 64, if someone
attempts to do an index by adding two 32-bit registers
together in a single instruction, that reference will
actually take effect, and go up into the 4 GiB to 8 GiB
region.
Is your answer still applicable (I don't really understand
your answer. :-) ).
Thanks. Paul.
-----Original Message-----
From: Richard Biener
Sent: Sunday, March 14, 2021 7:05 PM
To: Paul Edwards ; Paul Edwards via Gcc ; gcc@gcc.gnu.org
Subject: Re: negative indexes
On March 14, 2021 6:55:32 AM GMT+01:00, Paul Edwards via Gcc
<gcc@gcc.gnu.org> wrote:
If I have code like this:
char foo(char *p)
{
return (p[-1]);
}
It generates a negative index, like this:
* Function foo code
L 2,=F'-1'
L 3,0(11)
SLR 15,15
IC 15,0(2,3)
* Function foo epilogue
See that (2,3) - that is adding both R2 + R3.
R3 is a pointer to a location in 4 GiB space.
R2 is now 0xFFFFFFFF
In 64-bit mode, both of those values are added
together and there is no address wrap, so it
accesses memory above the 4 GiB boundary
(between 4 GiB and 8 GiB to be precise)
which I don't have access to.
Is there a way of constraining index registers to positive
values?
I want it to instead generate
ALR 3,2
to add these two values together using 32-bit arithmetic,
causing truncation at 32 bits, then it can do
IC 15,0(3)
(ie no index)
I'm using GCC 3.2.3 using the i370 target if it makes a difference.
You are likely missing a fix that sign extends offsets on Pmode!=ptr_mode
targets. 3.2.3 is really old now ;)
Richard.
Thanks. Paul.
