[PATCH] D143368: [InstCombine] Look through truncate to fold icmp with intrinsics

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:4918
 
+  if (auto *II = dyn_cast<IntrinsicInst>(X)) {
+    if (II->getIntrinsicID() == Intrinsic::ctpop ||
----------------
bcl5980 wrote:
> goldstein.w.n wrote:
> > Imo it makes more sense to do this generically.
> > 
> > `(icmp P (trunc(X), C))` if `KnownBits(X)[OrigWidth:TruncWidth] == 0` just drop the truncate. That will cover all these intrins + any other cases that happen to come up.
> I don't want to enable all cases for the trunc. That may cause a lot of potential regressions. By default we should shrink bits if possible not expand it. I prefer to limited the change to the case we can really get improvement.
> Maybe I can move this change to SDAG to avoid these concern.
Seems to only match `m_Oneuse(m_Trunc(...))` so seems like only case is the truncate is entirely eliminated, no? Not sure I see how that could cause regressions.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D143368/new/

https://reviews.llvm.org/D143368