Index: llvm/trunk/lib/Target/X86/X86InstrCompiler.td
===================================================================
--- llvm/trunk/lib/Target/X86/X86InstrCompiler.td
+++ llvm/trunk/lib/Target/X86/X86InstrCompiler.td
@@ -434,37 +434,49 @@
 // FIXME: Should use "(X86rep_stos AL)" as the pattern.
 let Defs = [ECX,EDI], isCodeGenOnly = 1 in {
   let Uses = [AL,ECX,EDI] in
-  def REP_STOSB_32 : I<0xAA, RawFrm, (outs), (ins), "{rep;stosb|rep stosb}",
-                      [(X86rep_stos i8)]>, REP,
-                     Requires<[Not64BitMode]>;
+  def REP_STOSB_32 : I<0xAA, RawFrm, (outs), (ins),
+                       "{rep;stosb %al, %es:(%edi)|rep stosb es:[edi], al}",
+                      [(X86rep_stos i8)]>, REP, AdSize32,
+                     Requires<[NotLP64]>;
   let Uses = [AX,ECX,EDI] in
-  def REP_STOSW_32 : I<0xAB, RawFrm, (outs), (ins), "{rep;stosw|rep stosw}",
-                      [(X86rep_stos i16)]>, REP, OpSize16,
-                     Requires<[Not64BitMode]>;
+  def REP_STOSW_32 : I<0xAB, RawFrm, (outs), (ins),
+                      "{rep;stosw %ax, %es:(%edi)|rep stosw es:[edi], ax}",
+                      [(X86rep_stos i16)]>, REP, AdSize32, OpSize16,
+                     Requires<[NotLP64]>;
   let Uses = [EAX,ECX,EDI] in
-  def REP_STOSD_32 : I<0xAB, RawFrm, (outs), (ins), "{rep;stosl|rep stosd}",
-                      [(X86rep_stos i32)]>, REP, OpSize32,
-                     Requires<[Not64BitMode]>;
+  def REP_STOSD_32 : I<0xAB, RawFrm, (outs), (ins),
+                      "{rep;stosl %eax, %es:(%edi)|rep stosd es:[edi], eax}",
+                      [(X86rep_stos i32)]>, REP, AdSize32, OpSize32,
+                     Requires<[NotLP64]>;
+  let Uses = [RAX,RCX,RDI] in
+  def REP_STOSQ_32 : RI<0xAB, RawFrm, (outs), (ins),
+                        "{rep;stosq %rax, %es:(%edi)|rep stosq es:[edi], rax}",
+                        [(X86rep_stos i64)]>, REP, AdSize32,
+                        Requires<[NotLP64, In64BitMode]>;
 }
 
 let Defs = [RCX,RDI], isCodeGenOnly = 1 in {
   let Uses = [AL,RCX,RDI] in
-  def REP_STOSB_64 : I<0xAA, RawFrm, (outs), (ins), "{rep;stosb|rep stosb}",
-                       [(X86rep_stos i8)]>, REP,
-                       Requires<[In64BitMode]>;
+  def REP_STOSB_64 : I<0xAA, RawFrm, (outs), (ins),
+                       "{rep;stosb %al, %es:(%rdi)|rep stosb es:[rdi], al}",
+                       [(X86rep_stos i8)]>, REP, AdSize64,
+                       Requires<[IsLP64]>;
   let Uses = [AX,RCX,RDI] in
-  def REP_STOSW_64 : I<0xAB, RawFrm, (outs), (ins), "{rep;stosw|rep stosw}",
-                       [(X86rep_stos i16)]>, REP, OpSize16,
-                       Requires<[In64BitMode]>;
+  def REP_STOSW_64 : I<0xAB, RawFrm, (outs), (ins),
+                       "{rep;stosw %ax, %es:(%rdi)|rep stosw es:[rdi], ax}",
+                       [(X86rep_stos i16)]>, REP, AdSize64, OpSize16,
+                       Requires<[IsLP64]>;
   let Uses = [RAX,RCX,RDI] in
-  def REP_STOSD_64 : I<0xAB, RawFrm, (outs), (ins), "{rep;stosl|rep stosd}",
-                       [(X86rep_stos i32)]>, REP, OpSize32,
-                       Requires<[In64BitMode]>;
+  def REP_STOSD_64 : I<0xAB, RawFrm, (outs), (ins),
+                      "{rep;stosl %eax, %es:(%rdi)|rep stosd es:[rdi], eax}",
+                       [(X86rep_stos i32)]>, REP, AdSize64, OpSize32,
+                       Requires<[IsLP64]>;
 
   let Uses = [RAX,RCX,RDI] in
-  def REP_STOSQ_64 : RI<0xAB, RawFrm, (outs), (ins), "{rep;stosq|rep stosq}",
-                        [(X86rep_stos i64)]>, REP,
-                        Requires<[In64BitMode]>;
+  def REP_STOSQ_64 : RI<0xAB, RawFrm, (outs), (ins),
+                        "{rep;stosq %rax, %es:(%rdi)|rep stosq es:[rdi], rax}",
+                        [(X86rep_stos i64)]>, REP, AdSize64,
+                        Requires<[IsLP64]>;
 }
 } // SchedRW
 
Index: llvm/trunk/lib/Target/X86/X86SelectionDAGInfo.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/X86SelectionDAGInfo.cpp
+++ llvm/trunk/lib/Target/X86/X86SelectionDAGInfo.cpp
@@ -170,10 +170,11 @@
     InFlag = Chain.getValue(1);
   }
 
-  Chain = DAG.getCopyToReg(Chain, dl, Subtarget.is64Bit() ? X86::RCX : X86::ECX,
+  bool Use64BitRegs = Subtarget.isTarget64BitLP64();
+  Chain = DAG.getCopyToReg(Chain, dl, Use64BitRegs ? X86::RCX : X86::ECX,
                            Count, InFlag);
   InFlag = Chain.getValue(1);
-  Chain = DAG.getCopyToReg(Chain, dl, Subtarget.is64Bit() ? X86::RDI : X86::EDI,
+  Chain = DAG.getCopyToReg(Chain, dl, Use64BitRegs ? X86::RDI : X86::EDI,
                            Dst, InFlag);
   InFlag = Chain.getValue(1);
 
Index: llvm/trunk/test/CodeGen/X86/pr38865-3.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/pr38865-3.ll
+++ llvm/trunk/test/CodeGen/X86/pr38865-3.ll
@@ -0,0 +1,18 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -show-mc-encoding < %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnux32"
+
+define void @foo(i8* %x) optsize {
+; CHECK-LABEL: foo:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl $707406378, %eax # encoding: [0xb8,0x2a,0x2a,0x2a,0x2a]
+; CHECK-NEXT:    # imm = 0x2A2A2A2A
+; CHECK-NEXT:    movl $32, %ecx # encoding: [0xb9,0x20,0x00,0x00,0x00]
+; CHECK-NEXT:    rep;stosl %eax, %es:(%edi) # encoding: [0xf3,0x67,0xab]
+; CHECK-NEXT:    retq # encoding: [0xc3]
+  call void @llvm.memset.p0i8.i32(i8* align 4 %x, i8 42, i32 128, i1 false)
+  ret void
+}
+declare void @llvm.memset.p0i8.i32(i8*, i8, i32, i1)