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

[Daniel Berlin via llvm-dev]

Ah yes, it can only return one instruction and you need two.

NewGVN could still handle it if instsimplify was changed to return the
right binary operators because it has infrastructure that can handle
insertion.
(it would need a little work).


(At this point, InstCombine seems to have become worse to me than GCC's
combining infrastructure, so i'd really like to see if we can stop putting
things there. Otherwise, we just go farther down the path of 'let's  just
make everything one big graph rewrite)


On Thu, Jul 13, 2017 at 4:37 PM, Sanjay Patel <spatel at rotateright.com>
wrote:

> 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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