[llvm] 9b507b2 - [LAA] We only need pointer checks if there are non-zero checks (NFC).

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2020-05-27T12:47:36+01:00
New Revision: 9b507b2127f116f29437e04a187cdca70ae9aa33

URL: https://github.com/llvm/llvm-project/commit/9b507b2127f116f29437e04a187cdca70ae9aa33
DIFF: https://github.com/llvm/llvm-project/commit/9b507b2127f116f29437e04a187cdca70ae9aa33.diff

LOG: [LAA] We only need pointer checks if there are non-zero checks (NFC).

If it turns out that we can do runtime checks, but there are no
runtime-checks to generate, set RtCheck.Need to false.

This can happen if we can prove statically that the pointers passed in
to canCheckPtrAtRT do not alias. This should not change any results, but
allows us to skip some work and assert that runtime checks are
generated, if LAA indicates that runtime checks are required.

Reviewers: anemet, Ayal

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D79969

Note: This is a recommit of 259abfc7cbc11cd98c05b1eb8e4b3fb6a9664bc0,
with some suggested renaming.

Added: 
    

Modified: 
    llvm/lib/Analysis/LoopAccessAnalysis.cpp
    llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 014cabd88fed..4cb2db58100b 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -705,7 +705,7 @@ bool AccessAnalysis::canCheckPtrAtRT(RuntimePointerChecking &RtCheck,
   // to place a runtime bound check.
   bool CanDoRT = true;
 
-  bool NeedRTCheck = false;
+  bool MayNeedRTCheck = false;
   if (!IsRTCheckAnalysisNeeded) return true;
 
   bool IsDepCheckNeeded = isDependencyCheckNeeded();
@@ -747,10 +747,10 @@ bool AccessAnalysis::canCheckPtrAtRT(RuntimePointerChecking &RtCheck,
     // 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.
+    // Note that this function computes CanDoRT and MayNeedRTCheck
+    // independently. For example CanDoRT=false, MayNeedRTCheck=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.
     bool NeedsAliasSetRTCheck = false;
     if (!(IsDepCheckNeeded && CanDoAliasSetRT && RunningDepId == 2))
       NeedsAliasSetRTCheck = (NumWritePtrChecks >= 2 ||
@@ -773,7 +773,7 @@ bool AccessAnalysis::canCheckPtrAtRT(RuntimePointerChecking &RtCheck,
     }
 
     CanDoRT &= CanDoAliasSetRT;
-    NeedRTCheck |= NeedsAliasSetRTCheck;
+    MayNeedRTCheck |= NeedsAliasSetRTCheck;
     ++ASId;
   }
 
@@ -807,15 +807,18 @@ bool AccessAnalysis::canCheckPtrAtRT(RuntimePointerChecking &RtCheck,
     }
   }
 
-  if (NeedRTCheck && CanDoRT)
+  if (MayNeedRTCheck && CanDoRT)
     RtCheck.generateChecks(DepCands, IsDepCheckNeeded);
 
   LLVM_DEBUG(dbgs() << "LAA: We need to do " << RtCheck.getNumberOfChecks()
                     << " pointer comparisons.\n");
 
-  RtCheck.Need = NeedRTCheck;
+  // If we can do run-time checks, but there are no checks, no runtime checks
+  // are needed. This can happen when all pointers point to the same underlying
+  // object for example.
+  RtCheck.Need = CanDoRT ? RtCheck.getNumberOfChecks() != 0 : MayNeedRTCheck;
 
-  bool CanDoRTIfNeeded = !NeedRTCheck || CanDoRT;
+  bool CanDoRTIfNeeded = !RtCheck.Need || CanDoRT;
   if (!CanDoRTIfNeeded)
     RtCheck.reset();
   return CanDoRTIfNeeded;

diff  --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index df1529a2f7b9..8d52ddc5b3b5 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -2795,8 +2795,8 @@ void InnerLoopVectorizer::emitMemRuntimeChecks(Loop *L, BasicBlock *Bypass) {
   std::tie(FirstCheckInst, MemRuntimeCheck) =
       addRuntimeChecks(MemCheckBlock->getTerminator(), OrigLoop,
                        RtPtrChecking.getChecks(), RtPtrChecking.getSE());
-  if (!MemRuntimeCheck)
-    return;
+  assert(MemRuntimeCheck && "no RT checks generated although RtPtrChecking "
+                            "claimed checks are required");
 
   if (MemCheckBlock->getParent()->hasOptSize()) {
     assert(Cost->Hints->getForce() == LoopVectorizeHints::FK_Enabled &&