Re: [PATCH v1] [RFC] Improve folding for comparisons with zero in tree-ssa-forwprop.

[Andrew MacLeod <amacleod@redhat.com>]

On 3/17/23 04:31, Richard Biener wrote:
> On Thu, Mar 16, 2023 at 4:27 PM Manolis Tsamis <manolis.tsamis@vrull.eu> wrote:
>> For this C testcase:
>>
>> void g();
>> void f(unsigned int *a)
>> {
>>    if (++*a == 1)
>>      g();
>> }
>>
>> GCC will currently emit a comparison with 1 by using the value
>> of *a after the increment. This can be improved by comparing
>> against 0 and using the value before the increment. As a result
>> there is a potentially shorter dependancy chain (no need to wait
>> for the result of +1) and on targets with compare zero instructions
>> the generated code is one instruction shorter.
> The downside is we now need two registers and their lifetime overlaps.
>
> Your patch mixes changing / inverting a parameter (which seems unneeded
> for the actual change) with preferring compares against zero.
>
> What's the reason to specifically prefer compares against zero?  On x86
> we have add that sets flags, so ++*a == 0 would be preferred, but
> for your sequence we'd need a test reg, reg; branch on zero, so we do
> not save any instruction.
>
> We do have quite some number of bugreports with regards to making VRPs
> life harder when splitting things this way.  It's easier for VRP to handle
>
>    _1 = _2 + 1;
>    if (_1 == 1)
>
> than it is
>
>    _1 = _2 + 1;
>    if (_2 == 0)
>
> where VRP fails to derive a range for _1 on the _2 == 0 branch.  So besides


Heh?

     _1 = *a_5(D);
     b_6 = _1 + 1;
     if (_1 == 0)
       goto <bb 3>; [INV]
     else
       goto <bb 4>; [INV]

2->3  (T) _1 :  [irange] unsigned int [0, 0] NONZERO 0x0
2->3  (T) b_6 :         [irange] unsigned int [1, 1] NONZERO 0x1
2->4  (F) _1 :  [irange] unsigned int [1, +INF]
2->4  (F) b_6 :         [irange] unsigned int [0, 0][2, +INF]

I will grant you that if the definition of b_6 is in a different basic 
block that if (_1 == 0) we may not always get a range for it,  but 
generally this should be OK?  especialyl within a basic block.

I do have a few re-computation cases to improve upon of course :-P

Andrew