Re: Matching and testing against smulhsm3

[Marcus Geelnard <m at bitsnbites dot eu>]

On 2020-02-09 19:02, Segher Boessenkool wrote:
> On Sun, Feb 09, 2020 at 12:15:03PM +0100, m wrote:
> > On 2020-02-07 16:44, Segher Boessenkool wrote:
> > > >    (define_insn "smulhshi3"
> > > >      [(set (match_operand:HI 0 "register_operand" "=r")
> > > >        (truncate:HI
> > > >          (ashiftrt:SI
> > > >            (mult:SI
> > > >              (sign_extend:SI (match_operand:HI 1
> > > > "register_operand" "r"))
> > > >              (sign_extend:SI (match_operand:HI 2
> > > > "register_operand" "r")))
> > > >            (const_int 15))))]
> > > >    "TARGET_PACKED_OPS"
> > > >    "mulq.h\\t%0, %1, %2")
> > > >
> > > > However, I am unable to trigger this code path. I have tried with the
> > > > following C code:
> > > >
> > > > short mulq(short op1, short op2) {
> > > >    return (short) (((int) op1 * (int) op2) >> (32 / 2 - 1));
> > > > }
> > > >
> > > > But I just get the regular 4-instruction result (2x sign extend, 1x mul,
> > > > 1x shift).
> > > What does -fdump-rtl-combine-all show it tried?  *Did* it try anything?
> > Cool option. I'm not really sure how to read the output tough. The
> > closest it seems to try to match is this:
> For every combination tried, it shows "Trying 2 -> 6:" etc., followed by
> the instructions it started with (which is very important), and then
> what worked and what didn't, and more debug information.
>
> I usually need to see that whole block (everything until the next
> "Trying:").


I've attached the full output of -fdump-rtl-combine-all.


> > Failed to match this instruction:
> > (set (reg:SI 85)
> >      (ashiftrt:SI (mult:SI (sign_extend:SI (subreg:HI (reg:SI 86) 0))
> >              (reg:SI 83 [ op2D.1381 ]))
> >          (const_int 15 [0xf])))
> >
> >
> > It seems that it has already decided to split the instruction into
> > several operations (the truncate operation is not there, and the second
> > sign_extend:SI (subreg:HI ...) is also missing).
> The code probably sign-extends the result; I need to see the full thing
> to really know.  Similarly, the sign_extend of a const_int is not
> canonical rtl, it always is written as just a const_int.
>
> You'll probably need to write a few extra patterns to recognise all the
> options here.


I haven't had much time to dig more into this, but I hope to do so soon.

Regards,

  Marcus

Attachment: mulq.c.267r.combine
Description: Text document