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

[Kugan <kugan.vivekanandarajah@linaro.org>]

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?

Thanks again,
Kugan