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

[Daniel Berlin via llvm-dev]

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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