Re: [PATCH] MIPS gas: Cleanup address insn conditionals

["Maciej W. Rozycki" <macro at ds2 dot pg dot gda dot pl>]

On Mon, 16 Jun 2003, Thiemo Seufer wrote:

> >  Hmm, this effectively disables addresses in the ranges of 0x7fff8000 -
> > 0x7fffffff and 0xffff8000 - 0xffffffff for certain operations for the n32
> > ABI.  The latter is probably out of the scope of the ABI, but the former
> > certainly is not.
> 
> Well, IRIX works this way, and the n32 ABI documentation mandates the
> use of add/addi.

 The only n32 reference I have is "MIPSPro N32 ABI Handbook" and it indeed
uses trapping instructions, namely "add" and "sub", but only for address
arithmetics and not loading.  Which seems to make more sense; actually
regardless of the ABI used.  But only a little, as such a trap would only
be of any use for a kernel; for userland an address error exception would
be triggered if the resulting address was ever used.  And that's the job
of a compiler, not gas, anyway. 

> > Does any of the ABI documents contain any specific
> > comment on a special treatment of the range?  Why do you think such an
> > overflow check should be done -- what advantages are there?  Certainly
> > "addu" and "addiu" cannot result in improper sign-extension of low 32-bit
> > halfs of registers. 
> 
> Right, the idea of catching improper sign-extension was only valid
> for n64 (where it isn't done). I'll fix the comment.

 Well, then the same note applies to 0x7fffffffffff8000 -
0x7fffffffffffffff (invalid for old 64-bit processors such as R4k or R10k,
but perfectly fine for the MIPS64 ISA) and 0xffffffffffff8000 -
0xffffffffffffffff.  I fail to see any advantage for trapping on address
loads for these, too.

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +