[MLIR] Add llvm.noalias attribute to nGraph func args (#3232)

Aliasing prevents vectorization and other optimizations in LLVM (dot2d,
for example). I found that LLVM dialect contemplates the possibility of
previous dialects adding llvm.noalias attribute to function arguments so
that the NoAlias attribute is generated in LLVM-IR. This should be good for
now although we should discuss how to model this in Standard dialect and co.

src/contrib/mlir/lowerer.cpp

@@ -125,6 +125,7 @@ namespace
 
         void findOutputValues();
         void processFakeInstrs();
+        void insertNoAliasArgAttrs();
         Value* insertMemMgrDef(PatternRewriter* rewriter = nullptr);
 
     private:
@@ -163,6 +164,8 @@ namespace
         }
 
         processFakeInstrs();
+
+        insertNoAliasArgAttrs();
     }
 
     void DialectLoweringPass::populateNGraphToAffineConversionPatterns(
@@ -314,6 +317,23 @@ namespace
         }
     }
 
+    /// Add llvm.noalias attribute to all the memref function arguments. We know that this is safe
+    /// by nGraph op semantics.
+    void DialectLoweringPass::insertNoAliasArgAttrs()
+    {
+        auto func = getModule().getNamedFunction("main");
+        unsigned int argIdx = 0;
+        for (auto* arg : func->getArguments())
+        {
+            if (arg->getType().isa<MemRefType>())
+            {
+                func->setArgAttr(argIdx, "llvm.noalias", BoolAttr::get(true, &getContext()));
+            }
+
+            ++argIdx;
+        }
+    }
+
     mlir::Function* DialectLoweringPass::getCallDecl(StringRef name,
                                                      ArrayRef<Type> args,
                                                      ArrayRef<Type> output,