Answer to Philip Reames comments

- add check for volatile (probably unneeded, but I agree that we should be conservative about it).
- strengthen condition from isUnordered() to isSimple(), as I don't understand well enough Unordered semantics (and it also matches the comment better this way) to be confident in the previous behaviour (thanks for catching that one, I had missed the case Monotonic/Unordered).
- separate a condition in two.
- lengthen comment about aliasing and loads
- add tests in GVN/atomic.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215943 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/MemoryDependenceAnalysis.cpp

@@ -407,21 +407,33 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
 
     // Values depend on loads if the pointers are must aliased.  This means that
     // a load depends on another must aliased load from the same value.
+    // One exception is atomic loads: a value can depend on an atomic load that it
+    // does not alias with when this atomic load indicates that another thread may
+    // be accessing the location.
     if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
       // Atomic loads have complications involved.
       // A monotonic load is OK if the query inst is itself not atomic.
       // FIXME: This is overly conservative.
       if (!LI->isUnordered()) {
-        if (!QueryInst || LI->getOrdering() != Monotonic)
+        if (!QueryInst)
+          return MemDepResult::getClobber(LI);
+        if (LI->getOrdering() != Monotonic)
           return MemDepResult::getClobber(LI);
         if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
-          if (!QueryLI->isUnordered())
+          if (!QueryLI->isSimple())
             return MemDepResult::getClobber(LI);
         if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
-          if (!QuerySI->isUnordered())
+          if (!QuerySI->isSimple())
             return MemDepResult::getClobber(LI);
       }
 
+      // FIXME: this is overly conservative.
+      // While volatile access cannot be eliminated, they do not have to clobber
+      // non-aliasing locations, as normal accesses can for example be reordered
+      // with volatile accesses.
+      if (LI->isVolatile())
+        return MemDepResult::getClobber(LI);
+
       AliasAnalysis::Location LoadLoc = AA->getLocation(LI);
 
       // If we found a pointer, check if it could be the same as our pointer.
@@ -481,16 +493,25 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
       // A monotonic store is OK if the query inst is itself not atomic.
       // FIXME: This is overly conservative.
       if (!SI->isUnordered()) {
-        if (!QueryInst || SI->getOrdering() != Monotonic)
+        if (!QueryInst)
+          return MemDepResult::getClobber(SI);
+        if (SI->getOrdering() != Monotonic)
           return MemDepResult::getClobber(SI);
         if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
-          if (!QueryLI->isUnordered())
+          if (!QueryLI->isSimple())
             return MemDepResult::getClobber(SI);
         if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
-          if (!QuerySI->isUnordered())
+          if (!QuerySI->isSimple())
             return MemDepResult::getClobber(SI);
       }
 
+      // FIXME: this is overly conservative.
+      // While volatile access cannot be eliminated, they do not have to clobber
+      // non-aliasing locations, as normal accesses can for example be reordered
+      // with volatile accesses.
+      if (SI->isVolatile())
+        return MemDepResult::getClobber(SI);
+
       // If alias analysis can tell that this store is guaranteed to not modify
       // the query pointer, ignore it.  Use getModRefInfo to handle cases where
       // the query pointer points to constant memory etc.

test/Transforms/GVN/atomic.ll

@@ -78,3 +78,29 @@ entry:
   %x3 = add i32 %x, %x2
   ret i32 %x3
 }
+
+; GVN across monotonic store (allowed)
+define i32 @test7() nounwind uwtable ssp {
+; CHECK: test7
+; CHECK: add i32 %x, %x
+entry:
+  %x = load i32* @y
+  store atomic i32 %x, i32* @x monotonic, align 4
+  %y = load i32* @y
+  %z = add i32 %x, %y
+  ret i32 %z
+}
+
+; GVN of an unordered across monotonic load (not allowed)
+define i32 @test8() nounwind uwtable ssp {
+; CHECK: test8
+; CHECK: add i32 %x, %y
+entry:
+  %x = load atomic i32* @y unordered, align 4
+  %clobber = load atomic i32* @x monotonic, align 4
+  %y = load atomic i32* @y monotonic, align 4
+  %z = add i32 %x, %y
+  ret i32 %z
+}
+
+