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Re: Matching and testing against smulhsm3
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:").
> 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.
Segher