Index: include/llvm/Transforms/Utils/SimplifyLibCalls.h
===================================================================
--- include/llvm/Transforms/Utils/SimplifyLibCalls.h
+++ include/llvm/Transforms/Utils/SimplifyLibCalls.h
@@ -138,6 +138,7 @@
 
   // Integer Library Call Optimizations
   Value *optimizeFFS(CallInst *CI, IRBuilder<> &B);
+  Value *optimizeFls(CallInst *CI, IRBuilder<> &B);
   Value *optimizeAbs(CallInst *CI, IRBuilder<> &B);
   Value *optimizeIsDigit(CallInst *CI, IRBuilder<> &B);
   Value *optimizeIsAscii(CallInst *CI, IRBuilder<> &B);
Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1572,6 +1572,22 @@
   return B.CreateSelect(Cond, V, B.getInt32(0));
 }
 
+Value *LibCallSimplifier::optimizeFls(CallInst *CI, IRBuilder<> &B) {
+  Function *Callee = CI->getCalledFunction();
+  if (!checkIntUnaryReturnAndParam(Callee))
+    return nullptr;
+  Value *Op = CI->getArgOperand(0);
+
+  // Constant fold.
+  if (ConstantInt *CI = dyn_cast<ConstantInt>(Op)) {
+    if (CI->isZero()) // fls(0) -> 0.
+      return B.getInt32(0);
+    // fls(c) -> 32 - ctlz(c)
+    return B.getInt32(32 - CI->getValue().countLeadingZeros());
+  }
+  return nullptr;
+}
+
 Value *LibCallSimplifier::optimizeAbs(CallInst *CI, IRBuilder<> &B) {
   Function *Callee = CI->getCalledFunction();
   FunctionType *FT = Callee->getFunctionType();
@@ -2157,6 +2173,10 @@
     case LibFunc::ffsl:
     case LibFunc::ffsll:
       return optimizeFFS(CI, Builder);
+    case LibFunc::fls:
+    case LibFunc::flsl:
+    case LibFunc::flsll:
+      return optimizeFls(CI, Builder);
     case LibFunc::abs:
     case LibFunc::labs:
     case LibFunc::llabs:
Index: test/Transforms/InstCombine/fls.ll
===================================================================
--- test/Transforms/InstCombine/fls.ll
+++ test/Transforms/InstCombine/fls.ll
@@ -0,0 +1,15 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+target triple = "x86_64-unknown-freebsd11.0"
+
+define i32 @myfls() {
+entry:
+  %call = call i32 @fls(i32 42) #2
+  ret i32 %call
+}
+
+declare i32 @fls(i32)
+
+; CHECK-LABEL: define i32 @myfls(
+; CHECK: ret i32 6
+; CHECK: }