[llvm-dev] failing to optimize boolean ops on cmps

[Sanjay Patel via llvm-dev]

This can't be an instsimplify though? The values we want in these cases do
not exist already:
  %res = or i8 %b, %a
  %res = or i1 %cmp, %c



On Thu, Jul 13, 2017 at 5:10 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

>
>
> On Thu, Jul 13, 2017 at 2:12 PM, Sanjay Patel <spatel at rotateright.com>
> wrote:
>
>> We have several optimizations in InstCombine for bitwise logic ops
>> (and/or/xor) that fail to handle compare patterns with the equivalent
>> bitwise logic. Example:
>>
>> define i8 @or_and_not(i8 %a, i8 %b) {
>>   %nota = xor i8 %a, -1
>>   %and = and i8 %nota, %b
>>   %res = or i8 %and, %a
>>   ret i8 %res
>> }
>>
>> define i1 @or_and_cmp_not(i32 %a, i32 %b, i1 %c) {
>>   %cmp = icmp sgt i32 %a, %b
>>   %cmp_inv = icmp sle i32 %a, %b ; this is 'not' of %cmp
>>   %and = and i1 %c, %cmp_inv
>>   %res = or i1 %cmp, %and
>>   ret i1 %res
>> }
>>
>> $ ./opt -instcombine hidden_not.ll -S
>>
>> define i8 @or_and_not(i8 %a, i8 %b) {
>>   %res = or i8 %b, %a
>>   ret i8 %res
>> }
>>
>> define i1 @or_and_cmp_not(i32 %a, i32 %b, i1 %c) {
>>   %cmp = icmp sgt i32 %a, %b
>>   %cmp_inv = icmp sle i32 %a, %b
>>   %and = and i1 %cmp_inv, %c
>>   %res = or i1 %cmp, %and
>>   ret i1 %res
>> }
>>
>> ---------------------------------------------------------------------
>>
>> Would adding to the existing InstCombine logic folds to handle this kind
>> of pattern be a welcome enhancement? I don't know if it's possible to make
>> the matchers handle this case without adding a new matcher. Eg:
>>
>>
> I would rather see this added to instsimplify than instcombine.
> If you do that, GVN/et all will get this already.
>
> This probably does require a special matcher though:
>
>
> We have:
>  if (m_c_And(m_Cmp(Pred, m_Value(), m_Value()),
>               m_Cmp(Pred, m_Value(), m_Value()))
>
> and you really want:
>  if (m_c_And(m_Cmp(Pred, m_Value(), m_Value()),
>               m_Cmp(m_Opposite(Pred), m_Value(), m_Value()))
>
>
>
>
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