Re: [PATCH 2/2] Enable elimination of zext/sext

[Richard Biener <richard.guenther@gmail.com>]

On Sun, Sep 7, 2014 at 11:50 AM, Kugan
<kugan.vivekanandarajah@linaro.org> wrote:
> On 05/09/14 19:50, Richard Biener wrote:
>
>> Well - the best way would be to expose the target specifics to GIMPLE
>> at some point in the optimization pipeline.  My guess would be that it's
>> appropriate after loop optimizations (but maybe before induction variable
>> optimization).
>>
>> That is, have a pass that applies register promotion to all SSA names
>> in the function, inserting appropriate truncations and extensions.  That
>> way you'd never see (set (subreg...) on RTL.  The VRP and DOM
>> passes running after that pass would then be able to aggressively
>> optimize redundant truncations and extensions.
>>
>> Effects on debug information are to be considered.  You can change
>> the type of SSA names in-place but you don't want to do that for
>> user DECLs (and we can't have the SSA name type and its DECL
>> type differ - and not sure if we might want to lift that restriction).
>
> Thanks. I will try to implement this.
>
> I still would like to keep the VRP based approach as there are some
> cases that I think can only be done with range info. For example:
>
> short foo(unsigned char c)
> {
>   c = c & (unsigned char)0x0F;
>   if( c > 7 )
>     return((short)(c - 5));
>   else
>     return(( short )c);
> }
>
>
> So, how about adding and setting the overflow/wrap around flag to
> range_info. We now set static_flag for VR_RANG/VR_ANTI_RANGE. If we go
> back to the max + 1, min - 1 for VR_ANTI_RANGE, we can use this
> static_flag to encode overflow/wrap around. Will that be something
> acceptable?

You mean tracking in the VRP lattice whether a value wrapped around
(or was assumed not to due to undefined behavior)?  I'm not sure this
is easy to do correctly (VRP is large).

Note that I don't think we'll lose the testcase you quoted if the promotion
pass runs before VRP2.   We'd have as input to VRP2 sth like (assuming
promote mode would promote to SImode)

  SImode tem_2 = (unsigned int)c_1(D);
  tem_3 = tem_3 & 0xF;
  if (tem_3 > 7)
    {
      tem_4 = tem_3 - 5;
      short _5 = (short)_4;
      tem_5 = (unsigned int)_5;
     return tem_5;
   }
else
   {
     short _6 = (short)_3;
     return _6;
   }

VRP should be able to remove the (unsigned int)(short) sign-extension
of tem_4.

note that both incoming registers and return registers are "interesting".
For simplicity I suggest to not promote them on GIMPLE.

What you'd lose in VRP2 is the smaller value-ranges you'd get from
(undefined) wrapping.  You could recover the undefinedness by
looking at SSA names recorded value-range and transfering that
in the promotion pass (but I'm not sure if you want to open the
can of latent signed overflow bugs in programs even more for
PROMOTE_MODE targets...)

Richard.

>
> Thanks again,
> Kugan