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From: "Paul Edwards" <mutazilah@gmail.com>
To: "Joe Monk" <joemonk64@gmail.com>
Cc: <gcc@gcc.gnu.org>
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Subject: Re: s390 port
Date: Wed, 8 Sep 2021 13:46:31 +1000
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Hi Joe.

Thanks for your comments.

> It is unclear how this would even work.=20

> For instance, the LA instruction clears the top bit.

In AM64, LA does not clear any bits.

> Also, instructions like LPR, LNR,

These operate on data registers, not addresses,
and will continue to work unchanged.

> BXLE, BXH all treat the value in the register as signed,
> so the top bit is not available.

These are already a problem if you are putting
addresses in them and it is approaching the 2 GiB
mark. The POP has a special mention of that.

Fun fact: The z/Arch POP has the same problem with
the G version of those instructions, when it hits the
63-bit mark, but the POP incorrectly states that the
problem occurs near the 64-bit mark. I reported the
problem with the POP but nothing seems to have
been done.

The solution is to drop these instructions from the
repertoire. C-generated assembler for both i370 and
s390 targets does not use these.

BFN. Paul.