[PATCH] D156881: [InstSimplify] Check the NonZero for power of two value

[Allen zhong via Phabricator via llvm-commits]

Allen marked an inline comment as done.
Allen added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:2022
+  if (Q.CxtI &&
+      (match(V, m_VScale()) || match(V, m_Shl(m_VScale(), m_Value())))) {
     const Function *F = Q.CxtI->getFunction();
----------------
goldstein.w.n wrote:
> Allen wrote:
> > goldstein.w.n wrote:
> > > What is this change for? Is this part of the bugfix?
> > Yes.  This is similar to the following logic in line 2045. 
> > ```
> >   // A shift left or a logical shift right of a power of two is a power of two
> >   // or zero.
> >   if (OrZero && (match(V, m_Shl(m_Value(X), m_Value())) ||
> >                  match(V, m_LShr(m_Value(X), m_Value()))))
> >     return isKnownToBeAPowerOfTwo(X, /*OrZero*/ true, Depth, Q);
> > ```
> > 
> > This is try to match following code, because we can assume the vscale always bigger than 1, and it is a power of 2 if it has the attribute of vscale_range
> > ```
> > %vscale = call i64 @llvm.vscale.i64()
> >   %shift = shl nuw nsw i64 %vscale, 11
> > ```
> > 
> > 
> > 
> But is it related to making the existing codes correct or just improving the coverage? I don't see how its related to the bug. If its not related to the bug, imo it should be split to a seperate patch.
Thanks very much again. I'll seperate this patch before landing.

This change is necessary to match the following code. 
otherwise the case **and_add_shl_vscale_not_power2_negative** will don't have a chance to fold the return value because we only check the **shl ** recursively when the **OrZero** is true now.
```
%vscale = call i64 @llvm.vscale.i64()
  %shift = shl nuw nsw i64 %vscale, 11
```


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D156881/new/

https://reviews.llvm.org/D156881