Re: How to express this complicated constraint in md file

[Jim Wilson <jim dot wilson at linaro dot org>]

On 07/16/2015 01:32 PM, Dmitry Grinberg wrote:
> WUMUL x, y     which will multiply 32-bit register x by 32-bit
> register y, and produce a 64-bit result, storing the high bits into
> register x and low bits into register y

You can rewrite the RTL to make this easier.  You can use a parallel to
do for instance

[(set (reg:SI x) (truncate:SI (lshiftrt:DI (mult:DI (sign_extend:DI
(reg:SI x)) (sign_extend:DI (reg:SI y))) (const_int 32)))
 (set (reg:SI y) (truncate:SI (mult:DI (sign_extend:DI (reg:SI x))
(sign_extend:DI (reg:SI y)))))]

Now you have only 32-bit regs, and you can use matching constraints to
make it work.  The truncate lshiftrt is the traditional way to write a
mulX_highpart pattern.  Some parts of the optimizer may recognize this
construct and know how to handle it.  For the second set, you might
consider just using (mult:SI ...) if that gives the correct result, or
you can use a subreg or whatever.  The optimizer is unlikely to generate
this pattern on its own, but you can have an expander and/or splitter
that generates it.  Use zero_extend instead of sign_extend if this is an
unsigned widening multiply.  You probably want to generate two SImode
temporaries in the expander, and copy the input regs into the
temporaries, as expanders aren't supposed to clobber input regs.  If you
want a 64-bit result out of this, then you would need extra instructions
to combine x and y into a 64-bit output.

Another way to do this is to arbtrarily force the result into a register
pair, then you can use a subreg to match the high part or the low part
of that register pair for the inputs.
[(set (reg:DI x) (mult:DI (sign_extend:DI (subreg:SI (reg:DI x) 0))
(sign_extend:DI (subreg:SI (reg:DI x) 1))))]
The subreg numbers may be reversed if this is little word endian instead
of big word endian.  You might need extra setup instructions to create
the register pair first.  Create a DI temp for the output, move the
inputs into the high/low word of the DI temp, and then you can do the
multiply on the DI tmep.

Jim