diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -5115,6 +5115,15 @@
 
   return PPCISD::CALL;
 }
+
+static bool isValidAIXExternalSymSDNode(StringRef SymName) {
+  return StringSwitch<bool>(SymName)
+      .Cases("__divdi3", "__fixunsdfdi", "__floatdidf", "__floatundidf",
+             "__floatundisf", "__moddi3", "__udivdi3", "__umoddi3", true)
+      .Cases("ceil", "floor", "memcpy", "memmove", "memset", "round", true)
+      .Default(false);
+}
+
 static SDValue transformCallee(const SDValue &Callee, SelectionDAG &DAG,
                                const SDLoc &dl, const PPCSubtarget &Subtarget) {
   if (!Subtarget.usesFunctionDescriptors() && !Subtarget.isELFv2ABI())
@@ -5139,42 +5148,70 @@
       Subtarget.is32BitELFABI() && !isLocalCallee() &&
       Subtarget.getTargetMachine().getRelocationModel() == Reloc::PIC_;
 
+  // On AIX, direct function calls reference the symbol for the function's
+  // entry point, which is named by prepending a "." before the function's
+  // C-linkage name.
+  const auto getAIXFuncEntryPointSymbol =
+      [&](StringRef FuncName, bool IsDeclaration,
+          const XCOFF::StorageClass &SC) {
+        auto &Context = DAG.getMachineFunction().getMMI().getContext();
+
+        MCSymbolXCOFF *S = cast<MCSymbolXCOFF>(
+            Context.getOrCreateSymbol(Twine(".") + Twine(FuncName)));
+
+        if (IsDeclaration && !S->hasContainingCsect()) {
+          // On AIX, an undefined symbol needs to be associated with a
+          // MCSectionXCOFF to get the correct storage mapping class.
+          // In this case, XCOFF::XMC_PR.
+          MCSectionXCOFF *Sec = Context.getXCOFFSection(
+              S->getName(), XCOFF::XMC_PR, XCOFF::XTY_ER, SC,
+              SectionKind::getMetadata());
+          S->setContainingCsect(Sec);
+        }
+
+        // Replace the callee SDNode with the MCSymbolSDNode.
+        MVT PtrVT =
+            DAG.getTargetLoweringInfo().getPointerTy(DAG.getDataLayout());
+        return DAG.getMCSymbol(S, PtrVT);
+      };
+
   if (isFunctionGlobalAddress(Callee)) {
     const GlobalAddressSDNode *G = cast<GlobalAddressSDNode>(Callee);
+    const GlobalObject *GO = cast<GlobalObject>(G->getGlobal());
+
     if (!Subtarget.isAIXABI())
-      return DAG.getTargetGlobalAddress(G->getGlobal(), dl,
-                                        Callee.getValueType(), 0,
+      return DAG.getTargetGlobalAddress(GO, dl, Callee.getValueType(), 0,
                                         UsePlt ? PPCII::MO_PLT : 0);
 
-    // On AIX, direct function calls reference the symbol for the function's
-    // entry point, which is named by prepending a "." before the function's
-    // C-linkage name.
-    auto &Context = DAG.getMachineFunction().getMMI().getContext();
+    const XCOFF::StorageClass SC =
+        TargetLoweringObjectFileXCOFF::getStorageClassForGlobal(GO);
+    return getAIXFuncEntryPointSymbol(GO->getName(), GO->isDeclaration(), SC);
+  }
 
-    const GlobalObject *GO = cast<GlobalObject>(G->getGlobal());
-    MCSymbolXCOFF *S = cast<MCSymbolXCOFF>(
-        Context.getOrCreateSymbol(Twine(".") + Twine(GO->getName())));
+  if (ExternalSymbolSDNode *S = dyn_cast<ExternalSymbolSDNode>(Callee)) {
+    const char *SymName = S->getSymbol();
+    if (!Subtarget.isAIXABI())
+      return DAG.getTargetExternalSymbol(SymName, Callee.getValueType(),
+                                         UsePlt ? PPCII::MO_PLT : 0);
 
-    if (GO && GO->isDeclaration() && !S->hasContainingCsect()) {
-      // On AIX, an undefined symbol needs to be associated with a
-      // MCSectionXCOFF to get the correct storage mapping class.
-      // In this case, XCOFF::XMC_PR.
-      const XCOFF::StorageClass SC =
-          TargetLoweringObjectFileXCOFF::getStorageClassForGlobal(GO);
-      MCSectionXCOFF *Sec =
-          Context.getXCOFFSection(S->getName(), XCOFF::XMC_PR, XCOFF::XTY_ER,
-                                  SC, SectionKind::getMetadata());
-      S->setContainingCsect(Sec);
+    // If there exists a user-declared function whose name is the same as the
+    // ExternalSymbol's, then we pick up the user-declared version.
+    const Module *Mod = DAG.getMachineFunction().getFunction().getParent();
+    if (const GlobalValue *GV = Mod->getNamedValue(SymName)) 
+      if (const Function *F = dyn_cast<Function>(GV)) {
+        const XCOFF::StorageClass SC =
+            TargetLoweringObjectFileXCOFF::getStorageClassForGlobal(F);
+        return getAIXFuncEntryPointSymbol(F->getName(), F->isDeclaration(), SC);
+      }
+
+    // TODO: Remove this when the support for ExternalSymbolSDNode is complete.
+    if (isValidAIXExternalSymSDNode(SymName)) {
+      return getAIXFuncEntryPointSymbol(SymName, true, XCOFF::C_EXT);
     }
 
-    EVT PtrVT = DAG.getTargetLoweringInfo().getPointerTy(DAG.getDataLayout());
-    return DAG.getMCSymbol(S, PtrVT);
+    report_fatal_error("Unexpected ExternalSymbolSDNode: " + Twine(SymName));
   }
 
-  if (ExternalSymbolSDNode *S = dyn_cast<ExternalSymbolSDNode>(Callee))
-    return DAG.getTargetExternalSymbol(S->getSymbol(), Callee.getValueType(),
-                                       UsePlt ? PPCII::MO_PLT : 0);
-
   // No transformation needed.
   assert(Callee.getNode() && "What no callee?");
   return Callee;
diff --git a/llvm/test/CodeGen/PowerPC/aix-external-sym-sdnode-lowering.ll b/llvm/test/CodeGen/PowerPC/aix-external-sym-sdnode-lowering.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/PowerPC/aix-external-sym-sdnode-lowering.ll
@@ -0,0 +1,128 @@
+; RUN: llc -mcpu=pwr4 -mattr=-altivec -verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff \
+; RUN: -stop-after=machine-cp < %s | FileCheck \
+; RUN: --check-prefix=32BIT %s
+
+; RUN: llc -mcpu=pwr4 -mattr=-altivec -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff \
+; RUN: -stop-after=machine-cp < %s | FileCheck \
+; RUN: --check-prefix=64BIT %s
+
+define i64 @call_divdi3(i64 %p, i64 %num) {
+entry:
+  %div = sdiv i64 %p, %num
+  ret i64 %div
+}
+
+; 32BIT: BL_NOP <mcsymbol .__divdi3>
+
+define i64 @call_fixunsdfdi(double %p) {
+entry:
+  %conv = fptoui double %p to i64
+  ret i64 %conv
+}
+
+; 32BIT: BL_NOP <mcsymbol .__fixunsdfdi>
+
+define double @call_floatundidf(i64 %p) {
+entry:
+  %conv = uitofp i64 %p to double
+  ret double %conv
+}
+
+; 32BIT: BL_NOP <mcsymbol .__floatundidf>
+
+define float @call_floatundisf(i64 %p) {
+entry:
+  %conv = uitofp i64 %p to float
+  ret float %conv
+}
+
+; 32BIT: BL_NOP <mcsymbol .__floatundisf>
+
+define i64 @call_moddi3(i64 %p, i64 %num) {
+entry:
+  %rem = srem i64 %p, %num
+  ret i64 %rem
+}
+
+; 32BIT: BL_NOP <mcsymbol .__moddi3>
+
+define i64 @call_udivdi3(i64 %p, i64 %q) {
+  %1 = udiv i64 %p, %q
+  ret i64 %1
+}
+
+; 32BIT: BL_NOP <mcsymbol .__udivdi3>
+
+define i64 @call_umoddi3(i64 %p, i64 %num) {
+entry:
+  %rem = urem i64 %p, %num
+  ret i64 %rem
+}
+
+; 32BIT: BL_NOP <mcsymbol .__umoddi3>
+
+define double @call_ceil(double %n) {
+entry:
+  %0 = call double @llvm.ceil.f64(double %n)
+  ret double %0
+}
+
+declare double @llvm.ceil.f64(double)
+
+; 32BIT: BL_NOP <mcsymbol .ceil>
+; 64BIT: BL8_NOP <mcsymbol .ceil>
+
+define double @call_floor(double %n) {
+entry:
+  %0 = call double @llvm.floor.f64(double %n)
+  ret double %0
+}
+
+declare double @llvm.floor.f64(double)
+
+; 32BIT: BL_NOP <mcsymbol .floor>
+; 64BIT: BL8_NOP <mcsymbol .floor>
+
+define void @call_memcpy(i8* %p, i8* %q, i32 %n) {
+entry:
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %p, i8* %q, i32 %n, i1 false)
+  ret void
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i32(i8* nocapture writeonly, i8* nocapture readonly, i32, i1)
+
+; 32BIT: BL_NOP <mcsymbol .memcpy>
+; 64BIT: BL8_NOP <mcsymbol .memcpy>
+
+define void @call_memmove(i8* %p, i8* %q, i32 %n) {
+entry:
+  call void @llvm.memmove.p0i8.p0i8.i32(i8* %p, i8* %q, i32 %n, i1 false)
+  ret void
+}
+
+declare void @llvm.memmove.p0i8.p0i8.i32(i8* nocapture, i8* nocapture readonly, i32, i1)
+
+; 32BIT: BL_NOP <mcsymbol .memmove>
+; 64BIT: BL8_NOP <mcsymbol .memmove>
+
+define void @call_memset(i8* %p, i8 %q, i32 %n) #0 {
+entry:
+  call void @llvm.memset.p0i8.i32(i8* %p, i8 %q, i32 %n, i1 false)
+  ret void
+}
+
+declare void @llvm.memset.p0i8.i32(i8* nocapture, i8, i32, i1)
+
+; 32BIT: BL_NOP <mcsymbol .memset>
+; 64BIT: BL8_NOP <mcsymbol .memset>
+
+define double @call_round(double %n) {
+entry:
+  %0 = call double @llvm.round.f64(double %n)
+  ret double %0
+}
+
+declare double @llvm.round.f64(double)
+
+; 32BIT: BL_NOP <mcsymbol .round>
+; 64BIT: BL8_NOP <mcsymbol .round>
diff --git a/llvm/test/CodeGen/PowerPC/aix-user-defined-memcpy.ll b/llvm/test/CodeGen/PowerPC/aix-user-defined-memcpy.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/PowerPC/aix-user-defined-memcpy.ll
@@ -0,0 +1,57 @@
+; RUN: llc -verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff -mcpu=pwr4 \
+; RUN: -mattr=-altivec -filetype=obj -o %t.o < %s | llvm-readobj --syms %t.o | \
+; RUN: FileCheck --check-prefix=32-SYM %s
+
+; RUN: llc -verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff -mcpu=pwr4 \
+; RUN: -mattr=-altivec -filetype=obj -o %t.o < %s | not llvm-readobj --r %t.o \
+; RUN: 2>&1 | FileCheck --check-prefix=32-REL %s
+
+; RUN: not llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff \
+; RUN: -mcpu=pwr4 -mattr=-altivec -filetype=obj < %s 2>&1 | FileCheck \
+; RUN: --check-prefix=64-CHECK %s
+
+; Test verifies:
+; If there exists a user-defined function whose name is the same as the
+; "memcpy" ExternalSymbol's, we pick up the user-defined version, even if this
+; may lead to some undefined behavior.
+
+define dso_local signext i32 @memcpy(i8* %destination, i32 signext %num) {
+entry:
+  ret i32 3
+}
+
+define void @call_memcpy(i8* %p, i8* %q, i32 %n) {
+entry:
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %p, i8* %q, i32 %n, i1 false)
+  ret void
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i32(i8* nocapture writeonly, i8* nocapture readonly, i32, i1)
+
+; TODO: This test should preferably check the symbol table for a.o file and
+;       the relocation associated with the call.
+
+; 32-SYM:       Symbol {
+; 32-SYM:         Index: 2
+; 32-SYM-NEXT:    Name: .memcpy
+; 32-SYM-NEXT:    Value (RelocatableAddress): 0x0
+; 32-SYM-NEXT:    Section: .text
+; 32-SYM-NEXT:    Type: 0x0
+; 32-SYM-NEXT:    StorageClass: C_EXT (0x2)
+; 32-SYM-NEXT:    NumberOfAuxEntries: 1
+; 32-SYM-NEXT:    CSECT Auxiliary Entry {
+; 32-SYM-NEXT:      Index: 3
+; 32-SYM-NEXT:      ContainingCsectSymbolIndex: 0
+; 32-SYM-NEXT:      ParameterHashIndex: 0x0
+; 32-SYM-NEXT:      TypeChkSectNum: 0x0
+; 32-SYM-NEXT:      SymbolAlignmentLog2: 0
+; 32-SYM-NEXT:      SymbolType: XTY_LD (0x2)
+; 32-SYM-NEXT:      StorageMappingClass: XMC_PR (0x0)
+; 32-SYM-NEXT:      StabInfoIndex: 0x0
+; 32-SYM-NEXT:      StabSectNum: 0x0
+; 32-SYM-NEXT:    }
+; 32-SYM-NEXT:  }
+
+; 32-REL: LLVM ERROR: Unexpanded relocation output not implemented.
+
+; 64-CHECK: LLVM ERROR: 64-bit XCOFF object files are not supported yet.