[LAA] Revert a small part of r239295

This commit ([LAA] Fix estimation of number of memchecks) regressed the
logic a bit.  We shouldn't quit the analysis if we encounter a pointer
without known bounds *unless* we actually need to emit a memcheck for
it.

The original code was using NumComparisons which is now computed
differently.  Instead I compute NeedRTCheck from NumReadPtrChecks and
NumWritePtrChecks.

As side note, I find the separation of NeedRTCheck and CanDoRT
confusing, so I will try to merge them in a follow-up patch.

llvm-svn: 241756

1 files changed, 20 insertions, 6 deletions

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 3c5e4d96602..093becf1829 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -479,6 +479,9 @@ bool AccessAnalysis::canCheckPtrAtRT(
   // Accesses between different groups doesn't need to be checked.
   unsigned ASId = 1;
   for (auto &AS : AST) {
+    int NumReadPtrChecks = 0;
+    int NumWritePtrChecks = 0;
+
     // We assign consecutive id to access from different dependence sets.
     // Accesses within the same set don't need a runtime check.
     unsigned RunningDepId = 1;
@@ -489,6 +492,11 @@ bool AccessAnalysis::canCheckPtrAtRT(
       bool IsWrite = Accesses.count(MemAccessInfo(Ptr, true));
       MemAccessInfo Access(Ptr, IsWrite);
 
+      if (IsWrite)
+        ++NumWritePtrChecks;
+      else
+        ++NumReadPtrChecks;
+
       if (hasComputableBounds(SE, StridesMap, Ptr) &&
           // When we run after a failing dependency check we have to make sure
           // we don't have wrapping pointers.
@@ -516,15 +524,21 @@ bool AccessAnalysis::canCheckPtrAtRT(
       }
     }
 
+    // If we have at least two writes or one write and a read then we need to
+    // check them.  But there is no need to checks if there is only one
+    // dependence set for this alias set.
+    //
+    // Note that this function computes CanDoRT and NeedRTCheck independently.
+    // For example CanDoRT=false, NeedRTCheck=false means that we have a pointer
+    // for which we couldn't find the bounds but we don't actually need to emit
+    // any checks so it does not matter.
+    if (!(IsDepCheckNeeded && CanDoRT && RunningDepId == 2))
+      NeedRTCheck |= (NumWritePtrChecks >= 2 || (NumReadPtrChecks >= 1 &&
+                                                 NumWritePtrChecks >= 1));
+
     ++ASId;
   }
 
-  // We need a runtime check if there are any accesses that need checking.
-  // However, some accesses cannot be checked (for example because we
-  // can't determine their bounds). In these cases we would need a check
-  // but wouldn't be able to add it.
-  NeedRTCheck = !CanDoRT || RtCheck.needsAnyChecking(nullptr);
-
   // If the pointers that we would use for the bounds comparison have different
   // address spaces, assume the values aren't directly comparable, so we can't
   // use them for the runtime check. We also have to assume they could