[Bug tree-optimization/113632] Range info for a^CSTP2-1 could be improved in some cases

["amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113632

Andrew Macleod <amacleod at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |amacleod at redhat dot com

--- Comment #1 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Andrew Pinski from comment #0)
> Take:
> ```
> void dummy();
> _Bool f(unsigned long a)
> {
>         _Bool cmp = a > 8192;
>         if (cmp) goto then; else goto e;
> then:
>         unsigned long t = __builtin_clzl(a); // [0,50] 
>         t^=63; // [13,63]
>         return t >= 13;
> e:
>   dummy();
>   return 0;
> }
> ```
> 
> Currently after the t^=63; we get:
> ```
>   # RANGE [irange] int [1, 63] MASK 0x3f VALUE 0x0
>   _7 = _1 ^ 63;
> ```
> 
> But this could/should be improved to [13,63].
> 
> If we change to using minus instead:
> ```
> t = 63 - t;
> ```
> 
> We get the better range and the comparison (t >= 13) is optimized away.
> ```
> Folding statement: t_10 = 63 - t_9;
> Global Exported: t_10 = [irange] long unsigned int [13, 63] MASK 0x3f VALUE
> 0x0
> Not folded
> ```
> 
> Yes this should up in real code, see the LLVM issue for more information on
> that.

I think the current implementation of "operator_bitwise_xor::wi_fold ()" in
range-op.cc  was simply ported from the original version we used in the old VRP
code.  so it is neither multi-range awre, nor been enhanced.

If you put a break point there, you'll see its getting:

(gdb) p lh_lb.dump()
[0], precision = 32
$1 = void
(gdb) p lh_ub.dump()
[0x32], precision = 32
$2 = void
(gdb) p rh_ub.dump()
[0x3f], precision = 32
$3 = void
(gdb) p rh_lb.dump()
[0x3f], precision = 32
$4 = void

One could conceivable do something much better than the general masking stuff
that goes on if rh_lb == rh_ub.  I suspect we could probably do a better job in
general, but have never looked at it.

It also looks like we make some minor attempts with signed values in
wi_optimize_signed_bitwise_op (),   but again, I do not think anyone has tried
to make this code do anything new yet.