diff --git a/llvm/lib/Target/X86/X86InstrCompiler.td b/llvm/lib/Target/X86/X86InstrCompiler.td
--- a/llvm/lib/Target/X86/X86InstrCompiler.td
+++ b/llvm/lib/Target/X86/X86InstrCompiler.td
@@ -2247,3 +2247,13 @@
 let Predicates = [HasMOVBE] in {
  def : Pat<(bswap GR16:$src), (ROL16ri GR16:$src, (i8 8))>;
 }
+
+
+// Match idiomatic (sizeof(x) - 1 - clz(x)) with bsr instead of
+// emitting extra xor + lzcnt.
+def : Pat<(xor (ctlz GR16:$src), 15), (BSR16rr GR16:$src)>;
+def : Pat<(xor (ctlz GR32:$src), 31), (BSR32rr GR32:$src)>;
+def : Pat<(xor (ctlz GR64:$src), 63), (BSR64rr GR64:$src)>;
+def : Pat<(xor (ctlz_zero_undef GR16:$src), 15), (BSR16rr GR16:$src)>;
+def : Pat<(xor (ctlz_zero_undef GR32:$src), 31), (BSR32rr GR32:$src)>;
+def : Pat<(xor (ctlz_zero_undef GR64:$src), 63), (BSR64rr GR64:$src)>;
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
@@ -741,14 +741,12 @@
 ;
 ; X86-CLZ-LABEL: ctlz_bsr:
 ; X86-CLZ:       # %bb.0:
-; X86-CLZ-NEXT:    lzcntl {{[0-9]+}}(%esp), %eax
-; X86-CLZ-NEXT:    xorl $31, %eax
+; X86-CLZ-NEXT:    bsrl {{[0-9]+}}(%esp), %eax
 ; X86-CLZ-NEXT:    retl
 ;
 ; X64-CLZ-LABEL: ctlz_bsr:
 ; X64-CLZ:       # %bb.0:
-; X64-CLZ-NEXT:    lzcntl %edi, %eax
-; X64-CLZ-NEXT:    xorl $31, %eax
+; X64-CLZ-NEXT:    bsrl %edi, %eax
 ; X64-CLZ-NEXT:    retq
   %ctlz = call i32 @llvm.ctlz.i32(i32 %n, i1 true)
   %bsr = xor i32 %ctlz, 31
@@ -790,14 +788,12 @@
 ;
 ; X86-CLZ-LABEL: ctlz_bsr_zero_test:
 ; X86-CLZ:       # %bb.0:
-; X86-CLZ-NEXT:    lzcntl {{[0-9]+}}(%esp), %eax
-; X86-CLZ-NEXT:    xorl $31, %eax
+; X86-CLZ-NEXT:    bsrl {{[0-9]+}}(%esp), %eax
 ; X86-CLZ-NEXT:    retl
 ;
 ; X64-CLZ-LABEL: ctlz_bsr_zero_test:
 ; X64-CLZ:       # %bb.0:
-; X64-CLZ-NEXT:    lzcntl %edi, %eax
-; X64-CLZ-NEXT:    xorl $31, %eax
+; X64-CLZ-NEXT:    bsrl %edi, %eax
 ; X64-CLZ-NEXT:    retq
   %ctlz = call i32 @llvm.ctlz.i32(i32 %n, i1 false)
   %bsr = xor i32 %ctlz, 31
@@ -1079,8 +1075,7 @@
 ; X86-CLZ-LABEL: PR47603_zext:
 ; X86-CLZ:       # %bb.0:
 ; X86-CLZ-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; X86-CLZ-NEXT:    lzcntl {{[0-9]+}}(%esp), %ecx
-; X86-CLZ-NEXT:    xorl $31, %ecx
+; X86-CLZ-NEXT:    bsrl {{[0-9]+}}(%esp), %ecx
 ; X86-CLZ-NEXT:    movsbl (%eax,%ecx), %eax
 ; X86-CLZ-NEXT:    retl
 ;