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Date: Wed, 14 Aug 2019 16:11:00 -0000
From: Joseph Myers <joseph@codesourcery.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
CC: Tejas Joshi <tejasjoshi9673@gmail.com>, <gcc@gcc.gnu.org>, Martin Jambor	<mjambor@suse.cz>, <hubicka@ucw.cz>
Subject: Re: Expansion of narrowing math built-ins into power instructions
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On Wed, 14 Aug 2019, Segher Boessenkool wrote:

> I think you can do one RTL code that replaces float_truncate in
> 
> > > > > (define_insn "add_truncdfsf3"
> > > > >   [(set (match_operand:SF 0 "gpc_reg_operand" "=f,wa")
> > > > >     (float_truncate:SF
> > > > >       (plus:DF (match_operand:DF 1 "gpc_reg_operand" "%d,wa")
> > > > >                (match_operand:DF 2 "gpc_reg_operand" "d,wa"))))]
> 
> but that is only meant for such explicit contraction.  This can then
> happily be used to implement all such patterns.  Is there some issue
> with that I overlook?

Yes, I think such a separate RTL code would work (as would an 
architecture-specific UNSPEC) - it just needs to avoid the pattern 
matching RTL that can arise other than from the built-in functions.

(Everything to do with needing -fno-math-errno to expand into such 
instructions should be handled in the architecture-independent compiler.)

-- 
Joseph S. Myers
joseph@codesourcery.com