negative indexes

[Paul Edwards]

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.