Disable loop unswitching for some patterns containing equality comparison with undef.

This is a workaround for the bug described in PR31652 and
http://lists.llvm.org/pipermail/llvm-dev/2017-July/115497.html. The temporary
solution is to add a function EqualityPropUnSafe. In EqualityPropUnSafe, for
some simple patterns we can know the equality comparison may contains undef,
so we regard such comparison as unsafe and will not do loop-unswitching for
them. We also need to disable the select simplification when one of select
operand is undef and its result feeds into equality comparison.

The patch cannot clear the safety issue caused by the bug, but it can suppress
the issue from happening to some extent.

Differential Revision: https://reviews.llvm.org/D35811

llvm-svn: 309059

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 4eebe82..2f700cb 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1157,6 +1157,22 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
   Value *FalseVal = SI.getFalseValue();
   Type *SelType = SI.getType();
 
+  // FIXME: Remove this workaround when freeze related patches are done.
+  // For select with undef operand which feeds into an equality comparison,
+  // don't simplify it so loop unswitch can know the equality comparison
+  // may have an undef operand. This is a workaround for PR31652 caused by
+  // descrepancy about branch on undef between LoopUnswitch and GVN.
+  if (isa<UndefValue>(TrueVal) || isa<UndefValue>(FalseVal)) {
+    if (any_of(SI.users(), [&](User *U) {
+          ICmpInst *CI = dyn_cast<ICmpInst>(U);
+          if (CI && CI->isEquality())
+            return true;
+          return false;
+        })) {
+      return nullptr;
+    }
+  }
+
   if (Value *V = SimplifySelectInst(CondVal, TrueVal, FalseVal,
                                     SQ.getWithInstruction(&SI)))
     return replaceInstUsesWith(SI, V);