Re: negative indexes

[Richard Biener <richard.guenther@gmail.com>]

On March 14, 2021 9:12:01 AM GMT+01:00, Paul Edwards <mutazilah@gmail.com> wrote:
>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.

And that's exactly what I said. Pmode is DImode but ptr_mode is SImode. 

>
>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.