Adding noalias attribute to malloc return

ctx.cpp

@@ -3285,6 +3285,20 @@ FunctionEmitContext::CallInst(llvm::Value *func, const FunctionType *funcType,
         // pointer, so just emit the IR directly.
         llvm::Instruction *ci =
             llvm::CallInst::Create(func, argVals, name ? name : "", bblock);
+
+        // Copy noalias attribute to call instruction, to enable better
+        // alias analysis.
+        // TODO: what other attributes needs to be copied?
+        // TODO: do the same for varing path.
+#if defined (LLVM_3_3)
+        llvm::CallInst *cc = llvm::dyn_cast<llvm::CallInst>(ci);
+        if (cc &&
+            cc->getCalledFunction() &&
+            cc->getCalledFunction()->doesNotAlias(0)) {
+            cc->addAttribute(0, llvm::Attribute::NoAlias);
+        }
+#endif
+
         AddDebugPos(ci);
         return ci;
     }