negative indexes

[Richard Biener]

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.