Index: lib/Transforms/Utils/MetaRenamer.cpp
===================================================================
--- lib/Transforms/Utils/MetaRenamer.cpp
+++ lib/Transforms/Utils/MetaRenamer.cpp
@@ -16,6 +16,7 @@
 #include "llvm/Transforms/IPO.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Module.h"
@@ -67,6 +68,7 @@
     }
 
     void getAnalysisUsage(AnalysisUsage &AU) const override {
+      AU.addRequired<TargetLibraryInfoWrapperPass>();
       AU.setPreservesAll();
     }
 
@@ -110,9 +112,15 @@
       }
 
       // Rename all functions
+      const TargetLibraryInfo &TLI =
+          getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
       for (auto &F : M) {
         StringRef Name = F.getName();
-        if (Name.startswith("llvm.") || (!Name.empty() && Name[0] == 1))
+        LibFunc Tmp;
+        // Leave library functions alone because their presence or absence could
+        // affect the behavior of other passes.
+        if (Name.startswith("llvm.") || (!Name.empty() && Name[0] == 1) ||
+            TLI.getLibFunc(F, Tmp))
           continue;
 
         F.setName(renamer.newName());
@@ -139,8 +147,11 @@
 }
 
 char MetaRenamer::ID = 0;
-INITIALIZE_PASS(MetaRenamer, "metarenamer", 
-                "Assign new names to everything", false, false)
+INITIALIZE_PASS_BEGIN(MetaRenamer, "metarenamer",
+                      "Assign new names to everything", false, false)
+INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
+INITIALIZE_PASS_END(MetaRenamer, "metarenamer",
+                    "Assign new names to everything", false, false)
 //===----------------------------------------------------------------------===//
 //
 // MetaRenamer - Rename everything with metasyntactic names.
Index: test/Transforms/MetaRenamer/metarenamer.ll
===================================================================
--- test/Transforms/MetaRenamer/metarenamer.ll
+++ test/Transforms/MetaRenamer/metarenamer.ll
@@ -96,3 +96,18 @@
   store i32 %arg_2_xxx, i32* %2, align 4
   ret i32 6
 }
+
+declare noalias i8* @malloc(i32)
+declare void @free(i8* nocapture)
+
+define void @dont_rename_lib_funcs() {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    [[TMP:%.*]] = call i8* @malloc(i32 23)
+; CHECK-NEXT:    call void @free(i8* [[TMP]])
+; CHECK-NEXT:    ret void
+;
+  %x = call i8* @malloc(i32 23)
+  call void @free(i8* %x)
+  ret void
+}