[MemorySSA] Allow reordering of loads that alias in the presence of volatile loads.

Summary:
Make MemorySSA allow reordering of two loads that may alias, when one is volatile.
This makes MemorySSA less conservative and behaving the same as the AliasSetTracker.
For more context, see D16875.

LLVM language reference: "The optimizers must not change the number of volatile operations or change their order of execution relative to other volatile operations. The optimizers may change the order of volatile operations relative to non-volatile operations. This is not Java’s “volatile” and has no cross-thread synchronization behavior."

Reviewers: george.burgess.iv, dberlin

Subscribers: sanjoy, reames, hfinkel, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D41525

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321382 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 29 deletions

diff --git a/lib/Analysis/MemorySSA.cpp b/lib/Analysis/MemorySSA.cpp
index 8fe190e8bcf..6e9368c49d6 100644
--- a/lib/Analysis/MemorySSA.cpp
+++ b/lib/Analysis/MemorySSA.cpp
@@ -192,8 +192,6 @@ template <> struct DenseMapInfo<MemoryLocOrCall> {
   }
 };
 
-enum class Reorderability { Always, IfNoAlias, Never };
-
 } // end namespace llvm
 
 /// This does one-way checks to see if Use could theoretically be hoisted above
@@ -202,22 +200,16 @@ enum class Reorderability { Always, IfNoAlias, Never };
 /// This assumes that, for the purposes of MemorySSA, Use comes directly after
 /// MayClobber, with no potentially clobbering operations in between them.
 /// (Where potentially clobbering ops are memory barriers, aliased stores, etc.)
-static Reorderability getLoadReorderability(const LoadInst *Use,
-                                            const LoadInst *MayClobber) {
+static bool areLoadsReorderable(const LoadInst *Use,
+                                const LoadInst *MayClobber) {
   bool VolatileUse = Use->isVolatile();
   bool VolatileClobber = MayClobber->isVolatile();
   // Volatile operations may never be reordered with other volatile operations.
   if (VolatileUse && VolatileClobber)
-    return Reorderability::Never;
-
-  // The lang ref allows reordering of volatile and non-volatile operations.
-  // Whether an aliasing nonvolatile load and volatile load can be reordered,
-  // though, is ambiguous. Because it may not be best to exploit this ambiguity,
-  // we only allow volatile/non-volatile reordering if the volatile and
-  // non-volatile operations don't alias.
-  Reorderability Result = VolatileUse || VolatileClobber
-                              ? Reorderability::IfNoAlias
-                              : Reorderability::Always;
+    return false;
+  // Otherwise, volatile doesn't matter here. From the language reference:
+  // 'optimizers may change the order of volatile operations relative to
+  // non-volatile operations.'"
 
   // If a load is seq_cst, it cannot be moved above other loads. If its ordering
   // is weaker, it can be moved above other loads. We just need to be sure that
@@ -229,9 +221,7 @@ static Reorderability getLoadReorderability(const LoadInst *Use,
   bool SeqCstUse = Use->getOrdering() == AtomicOrdering::SequentiallyConsistent;
   bool MayClobberIsAcquire = isAtLeastOrStrongerThan(MayClobber->getOrdering(),
                                                      AtomicOrdering::Acquire);
-  if (SeqCstUse || MayClobberIsAcquire)
-    return Reorderability::Never;
-  return Result;
+  return !(SeqCstUse || MayClobberIsAcquire);
 }
 
 static bool instructionClobbersQuery(MemoryDef *MD,
@@ -265,18 +255,9 @@ static bool instructionClobbersQuery(MemoryDef *MD,
     return isModOrRefSet(I);
   }
 
-  if (auto *DefLoad = dyn_cast<LoadInst>(DefInst)) {
-    if (auto *UseLoad = dyn_cast<LoadInst>(UseInst)) {
-      switch (getLoadReorderability(UseLoad, DefLoad)) {
-      case Reorderability::Always:
-        return false;
-      case Reorderability::Never:
-        return true;
-      case Reorderability::IfNoAlias:
-        return !AA.isNoAlias(UseLoc, MemoryLocation::get(DefLoad));
-      }
-    }
-  }
+  if (auto *DefLoad = dyn_cast<LoadInst>(DefInst))
+    if (auto *UseLoad = dyn_cast<LoadInst>(UseInst))
+      return !areLoadsReorderable(UseLoad, DefLoad);
 
   return isModSet(AA.getModRefInfo(DefInst, UseLoc));
 }