Re: GCC floating point usage

[Michael Matz <matz@suse.de>]

Hi,

On Wed, 16 Oct 2002, Mark Mitchell wrote:

> > (a) Does this affect codegen when the new feature is not enabled?
> >     Zack's patch does, and it shouldn't.
>
> I certainly agree that people not using the flag should see no change.
>
> Why does Zack's patch do that, given his caveat that the sense of the
> flag would be reversed so that the flag is off by default?

Because it adds an additional insn pattern, with equal predicates as
the following one, but with different constraints.  This means, that in
the case this pattern is activated (which it is by default, given Zacks
changed default proposal) it will be preferred, which means, because it
only lists float regs in constraints, that float regs will be preferred.

Indeed I'm not even sure, that the second pattern will be used at all (if
the first is activated), because for insn recognition only predicates are
used, which make the first pattern match.  Then when reload is running it
doesn't rerecognize the insn, and instead reloads all operands of this
insn into either float regs or memory, which means, that _no_ integer regs
would be used at all for it.  That's one of the reason for the requirement
that things allowed by the predicates need to have at least one
alternative which accepts them.  And also that it's suboptimal to have
non-disjoint sets of predicates (i.e. which are active at the same time,
and match the same insns) with different constraints.


Ciao,
Michael.