Re: s390 port

["Ulrich Weigand" <Ulrich.Weigand@de.ibm.com>]

Hi Paul,

> I just checked my copy of s390.md and I don’t see
> LA being used for arithmetic.

This would be the "*la_31" and "*la_31_and" patterns.
(Note that the addition is implicit in the use of
the "address_operand" constraint.)

> If your copy of s390.md is using LA for arithmetic
> then would it be possible to have an option to
> use a normal mathematics instruction instead of
> LA?

LA was just an example.  It doesn't usually make sense
to reason on an "use instruction X" basis, that's not
how compiler optimizations work.  You rather start with
a set of semantic invariants and then make sure those
are preserved through all transformations.

Therefore again my question, what is the actual goal
you want to achieve?   I'm still not sure I understand
that ...

> Also, I just realized – if GCC is using LA for maths
> for 32-bit registers, then values will be limited to
> 2 GiB instead of 4 GiB for unsigned, but that is not
> the case.

That's why GCC makes sure to only use the instruction
when a 31-bit addition is wanted.  This can be the
case either when GCC can prove that the involved
operands are pointer values (which are by definition
restricted to 31-bit values in -m31 mode), or when
there is an explict 31-bit addition (using e.g. an
& 0x7fffffff) in the source code.

Bye,
Ulrich