RE: [PATCH v3] EXPR: Emit an truncate if 31+ bits polluted for SImode

["Roger Sayle" <roger@nextmovesoftware.com>]

> > > What's exceedingly weird is T_N_T_M_P (DImode, SImode) isn't
> > > actually a truncation!  The output precision is first, the input
> > > precision is second.  The docs explicitly state the output precision
> > > should be smaller than the input precision (which makes sense for truncation).
> > >
> > > That's where I'd start with trying to untangle this mess.
> >
> > Thanks (both) for correcting my misunderstanding.
> > At the very least might I suggest that we introduce a new
> > TRULY_NOOP_EXTENSION_MODES_P target hook that MIPS can use for this
> > purpose?  It'd help reduce confusion, and keep the
> > documentation/function naming correct.
> >
> 
> Yes. It is good for me.
> T_N_T_M_P is a really confusion naming.

Ignore my suggestion for a new target hook.  GCC already has one.
You shouldn't be using TRULY_NOOP_TRUNCATION_MODES_P
with incorrectly ordered arguments. The correct target hook is 
TARGET_MODE_REP_EXTENDED, which the MIPS backend correctly
defines via mips_mode_rep_extended.

It's MIPS definition of (and interpretation of) mips_truly_noop_truncation
that's suspect.

My latest theory is that these sign extensions should be:
(set (reg:DI) (sign_extend:DI (truncate:SI (reg:DI))))
and not
(set (reg:DI) (sign_extend:DI (subreg:SI (reg:DI))))
If the RTL optimizer's ever split this instruction the semantics of
the SUBREG intermediate are incorrect.  Another (less desirable)
approach might be to use an UNSPEC.