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.

When Richard Sandiford "hookized" truly_noop_truncation in 2017
https://gcc.gnu.org/legacy-ml/gcc-patches/2017-09/msg00836.html
he mentions the inprec/outprec confusion [deciding not to add a
gcc_assert outprec < inprec here, which might be a good idea].

The next question is whether this is just
TRULY_NOOP_SIGN_EXTENSION_MODES_P
or whether there are any targets that usefully ensure some modes
are zero-extended forms of others.  TRULY_NOOP_ZERO_EXTENSION...

My vote is that a DINS instruction that updates the most significant
bit of an SImode value should then expand or define_insn_and_split
with an explicit following sign-extension operation.  To avoid this being
eliminated by the RTL optimizers/combine the DINS should return a
DImode result, with the following extension truncating it to canonical
SImode form.  This preserves the required backend invariant (and
doesn't require tweaking machine-independent code in combine).
SImode DINS instructions that don't/can't affect the MSB, can be a
single SImode instruction.

Cheers,
Roger
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