Re: On-Demand range technology [1/5] - Executive Summary

[Andrew MacLeod <amacleod@redhat.com>]

On 6/19/19 11:04 PM, Kugan Vivekanandarajah wrote:
> Hi Andrew,
>
> Thanks for working on this.
>
> Enable elimination of zext/sext with VRP patch had to be reverted in
> (https://gcc.gnu.org/ml/gcc-patches/2014-09/msg00672.html) due to the
> need for value ranges in PROMOTED_MODE precision for at least 1 test
> case for alpha.
>
> Playing with ranger suggest that it is not possible to get value
> ranges in PROMOTED_MODE precision on demand. Or is there any way we
> can use on-demand ranger here?
>
> Thanks,
> Kugan
>
>

I took a look at the thread, but I think I'm still missing some context.

Lets go back to the beginning.  What is an example of the case we arent 
getting that you want to get?

I'm going to guess to start :-)

short foo(unsigned char c)
  {
    c = c & (unsigned char)0x0F;
    if( c > 7 )
      return((short)(c - 5));
    else
      return(( short )c);
  }



A run of this thru the ranger shows me code that looks like (on x86 anyway):

  =========== BB 2 ============
c_4(D)  [0, 255] unsigned char
     <bb 2> :
     c_5 = c_4(D) & 15;
     _9 = c_4(D) & 8;
     if (_9 != 0)
       goto <bb 3>; [INV]
     else
       goto <bb 4>; [INV]

c_5 : [0, 15] unsigned char
_9 : [0, 0][8, 8] unsigned char
2->3  (T)*c_5 :         [0, 15] unsigned char
2->3  (T) _9 :  [8, 8] unsigned char
2->4  (F)*c_5 :         [0, 15] unsigned char
2->4  (F) _9 :  [0, 0] unsigned char

=========== BB 3 ============
c_5     [0, 15] unsigned char
     <bb 3> :
     _1 = (unsigned short) c_5;
     _2 = _1 + 65531;
     _7 = (short int) _2;
     // predicted unlikely by early return (on trees) predictor.
     goto <bb 5>; [INV]

_1 : [0, 15] unsigned short
_2 : [0, 10][65531, 65535] unsigned short
_7 : [-5, 10] short int

=========== BB 4 ============
c_5     [0, 15] unsigned char
     <bb 4> :
     _6 = (short int) c_5;
     // predicted unlikely by early return (on trees) predictor.


I think I see.  we aren't adjusting the range of c_5 going into blocks 3 
and 4.  its obvious from the original source where the code says < 7, 
but once its been "bitmasked" that info becomes obfuscated.
.
If you were to see a range in bb3 of c_5 = [8,15], and a range in bb4 of 
c_4 = [0,7],  would that solve your problem?

so in bb3 , we'd then see ranges that look like:

_1 : [8, 15] unsigned short
_2 : [3, 10] unsigned short
_7 : [3, 10] short int

and then later on we'd see there is no negative/wrap value, and you 
could could just drop the extension then?

SO.

yes. this is fixable,  but is it easy? :-)

We're in the process of replacing the range extraction code with the 
range-ops/gori-computes  components from the ranger.  This is the part 
which figures ranges out from individual statements on exit to a block..

We have implemented mostly the same functionality of VRP to start, and 
one of the things we have not done is enhanced the bitmask tracking side 
of it.
The gori-computes component which performs backwards calculations does 
virtually nothing with masks right now.

lets look at this hunk

     c_5 = c_4(D) & 15;
     _9 = c_4(D) & 8;
     if (_9 != 0)

on the true side of this branch, we start with _9 having a range of 
[0,0][8,8]

[0,0][8,8] = c_4(D) & 8

unfortunately we dont know much about c_4, its effectively VARYING at 
this point.   We do know c_5 has a range of [0,15].  if that statement were
     _9 = c_5 & 8
  this problem would be quite trivial  A quick tweak to the windback 
code for bitwise_and would generate the correct results  because we';d see
[1,1] = [0,15] & 8   and we'd come up with [8, 15]  for c_5 no problem.

but its not.

I do have some  "re-evalaution" code in the ranger which is only in the 
proof of concept stages which might solve this problem down the road.

what the re-evlaution code does is look at the any changes to ranges 
which are in the definition chain of the ssa-name you are looking at.
going back to the example again:
     c_5 = c_4(D) & 15;
     _9 = c_4(D) & 8;
     if (_9 != 0)
         _1 = (unsigned short) c_5;      <<--- here.

When we ask for the range of c_5 here, we calculate the range normally 
and come up with [0,15].
The re-evalution code then takes a look and notices that c_5 is also 
dependent on the value of c_4 , and queries if the range of c_4 changed 
at this point.
Well, we can know the value of c_4 must have the 0x8 bit set in this 
block.  This can be fed into a  "re-evaluation" of c_5, and also then 
know that c_5 must also have the 0x8 bit set.
THis would then reduce the calculated range from the original [0,15] to 
a range of [8, 15]

So the mechanisms within the new code are sort of there, but they have 
not been flushed out.  THis will require
  a)  the re-evaluation code to be completed.  Im not sure if it can be 
applied within EVRP (It seems it should be possible)  or whether it 
requires the ranger.
  b) some level of integration of bitfields with the range info so we 
can also track bits set and cleared along with the range, and integrate 
them with the calculations.

SO assuming I have been working on the correct problem here, No. we dont 
get this now.

Once we get the range-ops and gori-computes code integrated and working 
with EVRP and the bitfield stuff, we can again take a look and see where 
we stand.  I will keep it in mind as a practical example of how we might 
want to combine the information.

So revisit this in September maybe?

Or have i completely missed the mark of the problem you are trying to 
solve? :-)

Andrew

PS

Your original question is


    Playing with ranger suggest that it is not possible to get value
    ranges in PROMOTED_MODE precision on demand. Or is there any way we
    can use on-demand ranger here?


I'm not sure i follow completely why, but in theory it would be possible 
to add code to the ranger to walk a definition list calculating ranges 
in whatever precision you want

=========== BB 2 ============
c_4(D)  [0, 255] unsigned char
     <bb 2> :
     c_5 = c_4(D) & 15;
     _9 = c_4(D) & 8;
     if (_9 != 0)
       goto <bb 3>; [INV]
     else
       goto <bb 4>; [INV]


=========== BB 3 ============
c_5     [0, 15] unsigned char
     <bb 3> :
     _1 = (unsigned short) c_5;
     _2 = _1 + 65531;
     _7 = (short int) _2;

Im not sure how that would help?   the root of the problem would appear 
to be the range of c_5 is not properly calculated to be [8,15].  no 
matter what precision you use, the [0,4] + 65531 part of the range is 
still going to produce an undesireable set upper bit, and still require 
the zext/sext?