[InstCombine] Fix infinite loop in min/max load/store bitcast combine (PR44835)

Fixes https://bugs.llvm.org/show_bug.cgi?id=44835. Skip the transform
if it wouldn't actually do anything (apart from removing and reinserting
the same instructions).

Note that the test case doesn't loop on current master anymore, only
on the LLVM 10 release branch. The issue is already mitigated on master
due to worklist order fixes, but we should fix the root cause there as well.

As a side note, we should probably assert in combineLoadToNewType()
that it does not combine to the same type. Not doing this here, because
this assertion would also be triggered in another place right now.

Differential Revision: https://reviews.llvm.org/D74278

(cherry picked from commit 23db9724d0e5490fa5a2a726acf015f84e2c87cf)

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index c288a7d8d40..74654f7ef51 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -1336,6 +1336,11 @@ static bool removeBitcastsFromLoadStoreOnMinMax(InstCombiner &IC,
   if (!isMinMaxWithLoads(LoadAddr, CmpLoadTy))
     return false;
 
+  // Make sure the type would actually change.
+  // This condition can be hit with chains of bitcasts.
+  if (LI->getType() == CmpLoadTy)
+    return false;
+
   // Make sure we're not changing the size of the load/store.
   const auto &DL = IC.getDataLayout();
   if (DL.getTypeStoreSizeInBits(LI->getType()) !=