diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -982,6 +982,10 @@
           X86II::isX86_64ExtendedReg(OutMI.getOperand(2).getReg()))
         std::swap(OutMI.getOperand(1), OutMI.getOperand(2));
     }
+    // Add an REP prefix to BSF instructions so that new processors can
+    // recognize as TZCNT, which has better performance than BSF.
+    if (X86::isBSF(OutMI.getOpcode()) && !MF.getFunction().hasOptSize())
+      OutMI.setFlags(X86::IP_HAS_REPEAT);
     break;
   }
   }
diff --git a/llvm/test/CodeGen/X86/clz.ll b/llvm/test/CodeGen/X86/clz.ll
--- a/llvm/test/CodeGen/X86/clz.ll
+++ b/llvm/test/CodeGen/X86/clz.ll
@@ -18,13 +18,13 @@
 define i8 @cttz_i8(i8 %x)  {
 ; X86-LABEL: cttz_i8:
 ; X86:       # %bb.0:
-; X86-NEXT:    bsfl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    rep bsfl {{[0-9]+}}(%esp), %eax
 ; X86-NEXT:    # kill: def $al killed $al killed $eax
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: cttz_i8:
 ; X64:       # %bb.0:
-; X64-NEXT:    bsfl %edi, %eax
+; X64-NEXT:    rep bsfl %edi, %eax
 ; X64-NEXT:    # kill: def $al killed $al killed $eax
 ; X64-NEXT:    retq
 ;
@@ -46,12 +46,12 @@
 define i16 @cttz_i16(i16 %x)  {
 ; X86-LABEL: cttz_i16:
 ; X86:       # %bb.0:
-; X86-NEXT:    bsfw {{[0-9]+}}(%esp), %ax
+; X86-NEXT:    rep bsfw {{[0-9]+}}(%esp), %ax
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: cttz_i16:
 ; X64:       # %bb.0:
-; X64-NEXT:    bsfw %di, %ax
+; X64-NEXT:    rep bsfw %di, %ax
 ; X64-NEXT:    retq
 ;
 ; X86-CLZ-LABEL: cttz_i16:
@@ -72,12 +72,12 @@
 define i32 @cttz_i32(i32 %x)  {
 ; X86-LABEL: cttz_i32:
 ; X86:       # %bb.0:
-; X86-NEXT:    bsfl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    rep bsfl {{[0-9]+}}(%esp), %eax
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: cttz_i32:
 ; X64:       # %bb.0:
-; X64-NEXT:    bsfl %edi, %eax
+; X64-NEXT:    rep bsfl %edi, %eax
 ; X64-NEXT:    retq
 ;
 ; X86-CLZ-LABEL: cttz_i32:
@@ -100,20 +100,20 @@
 ; X86-NOCMOV-NEXT:    testl %eax, %eax
 ; X86-NOCMOV-NEXT:    jne .LBB3_1
 ; X86-NOCMOV-NEXT:  # %bb.2:
-; X86-NOCMOV-NEXT:    bsfl {{[0-9]+}}(%esp), %eax
+; X86-NOCMOV-NEXT:    rep bsfl {{[0-9]+}}(%esp), %eax
 ; X86-NOCMOV-NEXT:    addl $32, %eax
 ; X86-NOCMOV-NEXT:    xorl %edx, %edx
 ; X86-NOCMOV-NEXT:    retl
 ; X86-NOCMOV-NEXT:  .LBB3_1:
-; X86-NOCMOV-NEXT:    bsfl %eax, %eax
+; X86-NOCMOV-NEXT:    rep bsfl %eax, %eax
 ; X86-NOCMOV-NEXT:    xorl %edx, %edx
 ; X86-NOCMOV-NEXT:    retl
 ;
 ; X86-CMOV-LABEL: cttz_i64:
 ; X86-CMOV:       # %bb.0:
 ; X86-CMOV-NEXT:    movl {{[0-9]+}}(%esp), %ecx
-; X86-CMOV-NEXT:    bsfl %ecx, %edx
-; X86-CMOV-NEXT:    bsfl {{[0-9]+}}(%esp), %eax
+; X86-CMOV-NEXT:    rep bsfl %ecx, %edx
+; X86-CMOV-NEXT:    rep bsfl {{[0-9]+}}(%esp), %eax
 ; X86-CMOV-NEXT:    addl $32, %eax
 ; X86-CMOV-NEXT:    testl %ecx, %ecx
 ; X86-CMOV-NEXT:    cmovnel %edx, %eax
@@ -122,7 +122,7 @@
 ;
 ; X64-LABEL: cttz_i64:
 ; X64:       # %bb.0:
-; X64-NEXT:    bsfq %rdi, %rax
+; X64-NEXT:    rep bsfq %rdi, %rax
 ; X64-NEXT:    retq
 ;
 ; X86-CLZ-LABEL: cttz_i64:
@@ -517,7 +517,7 @@
 ; X86-NEXT:    je .LBB12_1
 ; X86-NEXT:  # %bb.2: # %cond.false
 ; X86-NEXT:    movzbl %al, %eax
-; X86-NEXT:    bsfl %eax, %eax
+; X86-NEXT:    rep bsfl %eax, %eax
 ; X86-NEXT:    # kill: def $al killed $al killed $eax
 ; X86-NEXT:    retl
 ; X86-NEXT:  .LBB12_1:
@@ -531,7 +531,7 @@
 ; X64-NEXT:    je .LBB12_1
 ; X64-NEXT:  # %bb.2: # %cond.false
 ; X64-NEXT:    movzbl %dil, %eax
-; X64-NEXT:    bsfl %eax, %eax
+; X64-NEXT:    rep bsfl %eax, %eax
 ; X64-NEXT:    # kill: def $al killed $al killed $eax
 ; X64-NEXT:    retq
 ; X64-NEXT:  .LBB12_1:
@@ -565,7 +565,7 @@
 ; X86-NEXT:    testw %ax, %ax
 ; X86-NEXT:    je .LBB13_1
 ; X86-NEXT:  # %bb.2: # %cond.false
-; X86-NEXT:    bsfw %ax, %ax
+; X86-NEXT:    rep bsfw %ax, %ax
 ; X86-NEXT:    retl
 ; X86-NEXT:  .LBB13_1:
 ; X86-NEXT:    movw $16, %ax
@@ -576,7 +576,7 @@
 ; X64-NEXT:    testw %di, %di
 ; X64-NEXT:    je .LBB13_1
 ; X64-NEXT:  # %bb.2: # %cond.false
-; X64-NEXT:    bsfw %di, %ax
+; X64-NEXT:    rep bsfw %di, %ax
 ; X64-NEXT:    retq
 ; X64-NEXT:  .LBB13_1:
 ; X64-NEXT:    movw $16, %ax
@@ -603,7 +603,7 @@
 ; X86-NEXT:    testl %eax, %eax
 ; X86-NEXT:    je .LBB14_1
 ; X86-NEXT:  # %bb.2: # %cond.false
-; X86-NEXT:    bsfl %eax, %eax
+; X86-NEXT:    rep bsfl %eax, %eax
 ; X86-NEXT:    retl
 ; X86-NEXT:  .LBB14_1:
 ; X86-NEXT:    movl $32, %eax
@@ -614,7 +614,7 @@
 ; X64-NEXT:    testl %edi, %edi
 ; X64-NEXT:    je .LBB14_1
 ; X64-NEXT:  # %bb.2: # %cond.false
-; X64-NEXT:    bsfl %edi, %eax
+; X64-NEXT:    rep bsfl %edi, %eax
 ; X64-NEXT:    retq
 ; X64-NEXT:  .LBB14_1:
 ; X64-NEXT:    movl $32, %eax
@@ -638,7 +638,7 @@
 ; X86-NOCMOV-LABEL: cttz_i64_zero_test:
 ; X86-NOCMOV:       # %bb.0:
 ; X86-NOCMOV-NEXT:    movl {{[0-9]+}}(%esp), %ecx
-; X86-NOCMOV-NEXT:    bsfl {{[0-9]+}}(%esp), %edx
+; X86-NOCMOV-NEXT:    rep bsfl {{[0-9]+}}(%esp), %edx
 ; X86-NOCMOV-NEXT:    movl $32, %eax
 ; X86-NOCMOV-NEXT:    je .LBB15_2
 ; X86-NOCMOV-NEXT:  # %bb.1:
@@ -651,18 +651,18 @@
 ; X86-NOCMOV-NEXT:    xorl %edx, %edx
 ; X86-NOCMOV-NEXT:    retl
 ; X86-NOCMOV-NEXT:  .LBB15_3:
-; X86-NOCMOV-NEXT:    bsfl %ecx, %eax
+; X86-NOCMOV-NEXT:    rep bsfl %ecx, %eax
 ; X86-NOCMOV-NEXT:    xorl %edx, %edx
 ; X86-NOCMOV-NEXT:    retl
 ;
 ; X86-CMOV-LABEL: cttz_i64_zero_test:
 ; X86-CMOV:       # %bb.0:
 ; X86-CMOV-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; X86-CMOV-NEXT:    bsfl {{[0-9]+}}(%esp), %ecx
+; X86-CMOV-NEXT:    rep bsfl {{[0-9]+}}(%esp), %ecx
 ; X86-CMOV-NEXT:    movl $32, %edx
 ; X86-CMOV-NEXT:    cmovnel %ecx, %edx
 ; X86-CMOV-NEXT:    addl $32, %edx
-; X86-CMOV-NEXT:    bsfl %eax, %eax
+; X86-CMOV-NEXT:    rep bsfl %eax, %eax
 ; X86-CMOV-NEXT:    cmovel %edx, %eax
 ; X86-CMOV-NEXT:    xorl %edx, %edx
 ; X86-CMOV-NEXT:    retl
@@ -672,7 +672,7 @@
 ; X64-NEXT:    testq %rdi, %rdi
 ; X64-NEXT:    je .LBB15_1
 ; X64-NEXT:  # %bb.2: # %cond.false
-; X64-NEXT:    bsfq %rdi, %rax
+; X64-NEXT:    rep bsfq %rdi, %rax
 ; X64-NEXT:    retq
 ; X64-NEXT:  .LBB15_1:
 ; X64-NEXT:    movl $64, %eax
@@ -822,7 +822,7 @@
 ; X86-NEXT:    movzbl {{[0-9]+}}(%esp), %eax
 ; X86-NEXT:    orb $2, %al
 ; X86-NEXT:    movzbl %al, %eax
-; X86-NEXT:    bsfl %eax, %eax
+; X86-NEXT:    rep bsfl %eax, %eax
 ; X86-NEXT:    # kill: def $al killed $al killed $eax
 ; X86-NEXT:    retl
 ;
@@ -830,7 +830,7 @@
 ; X64:       # %bb.0:
 ; X64-NEXT:    orb $2, %dil
 ; X64-NEXT:    movzbl %dil, %eax
-; X64-NEXT:    bsfl %eax, %eax
+; X64-NEXT:    rep bsfl %eax, %eax
 ; X64-NEXT:    # kill: def $al killed $al killed $eax
 ; X64-NEXT:    retq
 ;
@@ -983,12 +983,12 @@
 ; X86-NOCMOV-NEXT:  # %bb.2:
 ; X86-NOCMOV-NEXT:    movl $-2147483648, %eax # imm = 0x80000000
 ; X86-NOCMOV-NEXT:    orl {{[0-9]+}}(%esp), %eax
-; X86-NOCMOV-NEXT:    bsfl %eax, %eax
+; X86-NOCMOV-NEXT:    rep bsfl %eax, %eax
 ; X86-NOCMOV-NEXT:    orl $32, %eax
 ; X86-NOCMOV-NEXT:    xorl %edx, %edx
 ; X86-NOCMOV-NEXT:    retl
 ; X86-NOCMOV-NEXT:  .LBB22_1:
-; X86-NOCMOV-NEXT:    bsfl %eax, %eax
+; X86-NOCMOV-NEXT:    rep bsfl %eax, %eax
 ; X86-NOCMOV-NEXT:    xorl %edx, %edx
 ; X86-NOCMOV-NEXT:    retl
 ;
@@ -997,8 +997,8 @@
 ; X86-CMOV-NEXT:    movl {{[0-9]+}}(%esp), %ecx
 ; X86-CMOV-NEXT:    movl $-2147483648, %eax # imm = 0x80000000
 ; X86-CMOV-NEXT:    orl {{[0-9]+}}(%esp), %eax
-; X86-CMOV-NEXT:    bsfl %ecx, %edx
-; X86-CMOV-NEXT:    bsfl %eax, %eax
+; X86-CMOV-NEXT:    rep bsfl %ecx, %edx
+; X86-CMOV-NEXT:    rep bsfl %eax, %eax
 ; X86-CMOV-NEXT:    orl $32, %eax
 ; X86-CMOV-NEXT:    testl %ecx, %ecx
 ; X86-CMOV-NEXT:    cmovnel %edx, %eax
@@ -1009,7 +1009,7 @@
 ; X64:       # %bb.0:
 ; X64-NEXT:    movabsq $-9223372036854775808, %rax # imm = 0x8000000000000000
 ; X64-NEXT:    orq %rdi, %rax
-; X64-NEXT:    bsfq %rax, %rax
+; X64-NEXT:    rep bsfq %rax, %rax
 ; X64-NEXT:    retq
 ;
 ; X86-CLZ-LABEL: cttz_i64_zero_test_knownneverzero:
@@ -1110,3 +1110,29 @@
   %sext = sext i8 %load to i32
   ret i32 %sext
 }
+
+define i32 @cttz_i32_osize(i32 %x) optsize {
+; X86-LABEL: cttz_i32_osize:
+; X86:       # %bb.0:
+; X86-NOT:     rep
+; X86-NEXT:    bsfl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    retl
+;
+; X64-LABEL: cttz_i32_osize:
+; X64:       # %bb.0:
+; X64-NOT:     rep
+; X64-NEXT:    bsfl %edi, %eax
+; X64-NEXT:    retq
+;
+; X86-CLZ-LABEL: cttz_i32_osize:
+; X86-CLZ:       # %bb.0:
+; X86-CLZ-NEXT:    tzcntl {{[0-9]+}}(%esp), %eax
+; X86-CLZ-NEXT:    retl
+;
+; X64-CLZ-LABEL: cttz_i32_osize:
+; X64-CLZ:       # %bb.0:
+; X64-CLZ-NEXT:    tzcntl %edi, %eax
+; X64-CLZ-NEXT:    retq
+  %tmp = call i32 @llvm.cttz.i32( i32 %x, i1 true )
+  ret i32 %tmp
+}
diff --git a/llvm/test/CodeGen/X86/dagcombine-select.ll b/llvm/test/CodeGen/X86/dagcombine-select.ll
--- a/llvm/test/CodeGen/X86/dagcombine-select.ll
+++ b/llvm/test/CodeGen/X86/dagcombine-select.ll
@@ -281,7 +281,7 @@
 define i64 @cttz_64_eq_select(i64 %v) nounwind {
 ; NOBMI-LABEL: cttz_64_eq_select:
 ; NOBMI:       # %bb.0:
-; NOBMI-NEXT:    bsfq %rdi, %rcx
+; NOBMI-NEXT:    rep bsfq %rdi, %rcx
 ; NOBMI-NEXT:    movq $-1, %rax
 ; NOBMI-NEXT:    cmovneq %rcx, %rax
 ; NOBMI-NEXT:    addq $6, %rax
@@ -305,7 +305,7 @@
 define i64 @cttz_64_ne_select(i64 %v) nounwind {
 ; NOBMI-LABEL: cttz_64_ne_select:
 ; NOBMI:       # %bb.0:
-; NOBMI-NEXT:    bsfq %rdi, %rcx
+; NOBMI-NEXT:    rep bsfq %rdi, %rcx
 ; NOBMI-NEXT:    movq $-1, %rax
 ; NOBMI-NEXT:    cmovneq %rcx, %rax
 ; NOBMI-NEXT:    addq $6, %rax
@@ -330,7 +330,7 @@
 define i32 @cttz_32_eq_select(i32 %v) nounwind {
 ; NOBMI-LABEL: cttz_32_eq_select:
 ; NOBMI:       # %bb.0:
-; NOBMI-NEXT:    bsfl %edi, %ecx
+; NOBMI-NEXT:    rep bsfl %edi, %ecx
 ; NOBMI-NEXT:    movl $-1, %eax
 ; NOBMI-NEXT:    cmovnel %ecx, %eax
 ; NOBMI-NEXT:    addl $6, %eax
@@ -354,7 +354,7 @@
 define i32 @cttz_32_ne_select(i32 %v) nounwind {
 ; NOBMI-LABEL: cttz_32_ne_select:
 ; NOBMI:       # %bb.0:
-; NOBMI-NEXT:    bsfl %edi, %ecx
+; NOBMI-NEXT:    rep bsfl %edi, %ecx
 ; NOBMI-NEXT:    movl $-1, %eax
 ; NOBMI-NEXT:    cmovnel %ecx, %eax
 ; NOBMI-NEXT:    addl $6, %eax
@@ -379,7 +379,7 @@
 define i32 @cttz_32_eq_select_ffs(i32 %v) nounwind {
 ; NOBMI-LABEL: cttz_32_eq_select_ffs:
 ; NOBMI:       # %bb.0:
-; NOBMI-NEXT:    bsfl %edi, %ecx
+; NOBMI-NEXT:    rep bsfl %edi, %ecx
 ; NOBMI-NEXT:    movl $-1, %eax
 ; NOBMI-NEXT:    cmovnel %ecx, %eax
 ; NOBMI-NEXT:    incl %eax
@@ -403,7 +403,7 @@
 define i32 @cttz_32_ne_select_ffs(i32 %v) nounwind {
 ; NOBMI-LABEL: cttz_32_ne_select_ffs:
 ; NOBMI:       # %bb.0:
-; NOBMI-NEXT:    bsfl %edi, %ecx
+; NOBMI-NEXT:    rep bsfl %edi, %ecx
 ; NOBMI-NEXT:    movl $-1, %eax
 ; NOBMI-NEXT:    cmovnel %ecx, %eax
 ; NOBMI-NEXT:    incl %eax
@@ -428,7 +428,7 @@
 define i32 @cttz_32_eq_select_ffs_m1(i32 %v) nounwind {
 ; NOBMI-LABEL: cttz_32_eq_select_ffs_m1:
 ; NOBMI:       # %bb.0:
-; NOBMI-NEXT:    bsfl %edi, %ecx
+; NOBMI-NEXT:    rep bsfl %edi, %ecx
 ; NOBMI-NEXT:    movl $-1, %eax
 ; NOBMI-NEXT:    cmovnel %ecx, %eax
 ; NOBMI-NEXT:    retq
@@ -449,7 +449,7 @@
 define i32 @cttz_32_ne_select_ffs_m1(i32 %v) nounwind {
 ; NOBMI-LABEL: cttz_32_ne_select_ffs_m1:
 ; NOBMI:       # %bb.0:
-; NOBMI-NEXT:    bsfl %edi, %ecx
+; NOBMI-NEXT:    rep bsfl %edi, %ecx
 ; NOBMI-NEXT:    movl $-1, %eax
 ; NOBMI-NEXT:    cmovnel %ecx, %eax
 ; NOBMI-NEXT:    retq
diff --git a/llvm/test/CodeGen/X86/peephole-na-phys-copy-folding.ll b/llvm/test/CodeGen/X86/peephole-na-phys-copy-folding.ll
--- a/llvm/test/CodeGen/X86/peephole-na-phys-copy-folding.ll
+++ b/llvm/test/CodeGen/X86/peephole-na-phys-copy-folding.ll
@@ -353,6 +353,7 @@
 ; CHECK32-NEXT:    testl %edx, %edx
 ; CHECK32-NEXT:    setg %al
 ; CHECK32-NEXT:    #APP
+; CHECK32-NOT:     rep
 ; CHECK32-NEXT:    bsfl %edx, %edx
 ; CHECK32-NEXT:    #NO_APP
 ; CHECK32-NEXT:    movl %edx, (%ecx)
@@ -364,6 +365,7 @@
 ; CHECK64-NEXT:    testl %ecx, %ecx
 ; CHECK64-NEXT:    setg %al
 ; CHECK64-NEXT:    #APP
+; CHECK32-NOT:     rep
 ; CHECK64-NEXT:    bsfl %ecx, %ecx
 ; CHECK64-NEXT:    #NO_APP
 ; CHECK64-NEXT:    movl %ecx, (%rdi)
diff --git a/llvm/test/CodeGen/X86/stack-folding-x86_64.ll b/llvm/test/CodeGen/X86/stack-folding-x86_64.ll
--- a/llvm/test/CodeGen/X86/stack-folding-x86_64.ll
+++ b/llvm/test/CodeGen/X86/stack-folding-x86_64.ll
@@ -37,7 +37,7 @@
 ; CHECK-NEXT:    #APP
 ; CHECK-NEXT:    nop
 ; CHECK-NEXT:    #NO_APP
-; CHECK-NEXT:    bsfl {{[-0-9]+}}(%r{{[sb]}}p), %eax # 4-byte Folded Reload
+; CHECK-NEXT:    rep bsfl {{[-0-9]+}}(%r{{[sb]}}p), %eax # 4-byte Folded Reload
 ; CHECK-NEXT:    popq %rbx
 ; CHECK-NEXT:    .cfi_def_cfa_offset 48
 ; CHECK-NEXT:    popq %r12
@@ -82,7 +82,7 @@
 ; CHECK-NEXT:    #APP
 ; CHECK-NEXT:    nop
 ; CHECK-NEXT:    #NO_APP
-; CHECK-NEXT:    bsfq {{[-0-9]+}}(%r{{[sb]}}p), %rax # 8-byte Folded Reload
+; CHECK-NEXT:    rep bsfq {{[-0-9]+}}(%r{{[sb]}}p), %rax # 8-byte Folded Reload
 ; CHECK-NEXT:    popq %rbx
 ; CHECK-NEXT:    .cfi_def_cfa_offset 48
 ; CHECK-NEXT:    popq %r12