RE: Mode change for bswap pattern expansion

[Paulo Matos <pmatos at broadcom dot com>]

> -----Original Message-----
> From: Richard Sandiford [mailto:rdsandiford@googlemail.com]
> Sent: 27 January 2014 16:06
> To: Paulo Matos
> Cc: gcc@gcc.gnu.org
> Subject: Re: Mode change for bswap pattern expansion
> 
> Paulo Matos <pmatos@broadcom.com> writes:
> > On a vector processor we can do a bswapsi with two instructions, by first
> rotating half-words (16 bits) by 8 and then rotating full words by 16.
> > However, this means expanding:
> > (set (match_operand:SI 0 "register_operand" "")
> >      (bswap:SI (match_operand:SI 1 "register_operand" "")))
> >
> > to:
> > (set (match_dup:V2HI 0)
> >      (rotate:V2HI (match_dup:V2HI 1)
> >                   (const_int 8)))
> > (set (match_dup:SI 0)
> >      (rotate:SI (match_dup:SI 0)
> >                 (const_int 16)))
> >
> > This is obviously not correct, because match_dup cannot set the mode. The point
> I am trying to make is that I can't find a good way to deal with the mode
> changes. I don't think GCC is too happy if I change the modes of the same operand
> from one instruction to the other right? The only other way is to emit
> paradoxical subregs. So something along these lines:
> > (set (subreg:V2HI (match_dup 0) 0)
> >      (rotate:V2HI (subreg:V2HI (match_dup 1) 0)
> >                   (const_int 8)))
> > (set (match_dup 0)
> >      (rotate:SI (match_dup 0)
> >                 (const_int 16)))
> 
> It's usually better not to hard-code the subregs in the pattern.
> Instead you could use C code to create the subregs, e.g.:
> 
>   [(set (match_dup 3)
>         (rotate:V2HI (match_dup 2)
>                      (const_int 8)))
>    (set (match_dup 0)
>         (rotate:SI (match_dup 4)
>                    (const_int 16)))]
>   ""
> {
>   operands[2] = gen_lowpart (V2HImode, operands[1]);
>   operands[3] = gen_reg_rtx (V2HImode);
>   operands[4] = gen_lowpart (SImode, operands[3]);
> }
> 
> so that any hard regs are correctly handled.  Or it might be easier to code
> it using emit_insn (gen_* (...))s instead.
> 
> BTW, paradoxical subregs are where the outer mode is strictly larger
> than the inner mode.
>

That's right. My mis-understanding.
 
> MIPS uses essentially the same sequence, except that it has a special
> instruction to do the first rotate (WSBH), rather than it being an instance
> of a general vector rotate.  For MIPS we just model it as an unspec SImode
> operation.  Maybe that would be easier here too.
> 

I will look at how MIPS is doing it.

However, the unspec SI has severe performance penalties on my port since it is able to issue more that one instruction per cycle, therefore having each instruction separately allows the scheduler to issue each of the bswapsi parts into different slots with other instructions.


Thanks,

Paulo Matos

> Thanks,
> Richard