Why is String.IsNullOrEmpty faster than String.Length?

Question

ILSpy shows that String.IsNullOrEmpty is implemented in terms of String.Length. But then why is String.IsNullOrEmpty(s) faster than s.Length == 0?

For example, it's 5% faster in this benchmark:

var stopwatches = Enumerable.Range(0, 4).Select(_ => new Stopwatch()).ToArray();
var strings = "A,B,,C,DE,F,,G,H,,,,I,J,,K,L,MN,OP,Q,R,STU,V,W,X,Y,Z,".Split(',');
var testers = new Func<string, bool>[] { s => s == String.Empty, s => s.Length == 0, s => String.IsNullOrEmpty(s), s => s == "" };
int count = 0;
for (int i = 0; i < 10000; ++i) {
    stopwatches[i % 4].Start();
    for (int j = 0; j < 1000; ++j)
        count += strings.Count(testers[i % 4]);
    stopwatches[i % 4].Stop();
}

(Other benchmarks show similar results. This one minimized the effect of cruft running on my computer. Also, as an aside, the tests comparing to empty strings came out the same at about 13% slower than IsNullOrEmpty.)

Additionally, why is IsNullOrEmpty only faster on x86, whereas on x64 String.Length is about 9% faster?

Update: Test setup details: .NET 4.0 running on 64-bit Windows 7, Intel Core i5 processor, console project compiled with "Optimize code" enabled. However, "Suppress JIT optimization on module load" was also enabled (see accepted answer and comments).

With optimization fully enabled, Length is about 14% faster than IsNullOrEmpty with the delegate and other overhead removed, as in this test:

var strings = "A,B,,C,DE,F,,G,H,,,,I,J,,K,L,MN,OP,Q,R,,STU,V,,W,,X,,,Y,,Z,".Split(',');
int count = 0;
for (uint i = 0; i < 100000000; ++i)
    count += strings[i % 32].Length == 0 ? 1 : 0; // Replace Length test with String.IsNullOrEmpty

Answer 1

It's because you ran your benchmark from within Visual Studio which prevents JIT compiler from optimizing code. Without optimizations, this code is produced for String.IsNullOrEmpty

00000000   push        ebp 
00000001   mov         ebp,esp 
00000003   sub         esp,8 
00000006   mov         dword ptr [ebp-8],ecx 
00000009   cmp         dword ptr ds:[00153144h],0 
00000010   je          00000017 
00000012   call        64D85BDF 
00000017   mov         ecx,dword ptr [ebp-8] 
0000001a   call        63EF7C0C 
0000001f   mov         dword ptr [ebp-4],eax 
00000022   movzx       eax,byte ptr [ebp-4] 
00000026   mov         esp,ebp 
00000028   pop         ebp 
00000029   ret 

and now compare it to code produced for Length == 0

00000000   push   ebp 
00000001   mov    ebp,esp 
00000003   sub    esp,8 
00000006   mov    dword ptr [ebp-8],ecx 
00000009   cmp    dword ptr ds:[001E3144h],0 
00000010   je     00000017 
00000012   call   64C95BDF 
00000017   mov    ecx,dword ptr [ebp-8] 
0000001a   cmp    dword ptr [ecx],ecx 
0000001c   call   64EAA65B 
00000021   mov    dword ptr [ebp-4],eax 
00000024   cmp    dword ptr [ebp-4],0 
00000028   sete   al 
0000002b   movzx  eax,al 
0000002e   mov    esp,ebp 
00000030   pop    ebp 
00000031   ret 

You can see, that code for Length == 0 does everything that does code for String.IsNullOrEmpty, but additionally it tries something like foolishly convert boolean value (returned from length comparison) again to boolean and this makes it slower than String.IsNullOrEmpty.

If you compile program with optimizations enabled (Release mode) and run .exe file directly from Windows, code generated by JIT compiler is much better. For String.IsNullOrEmpty it is:

001f0650   push    ebp
001f0651   mov     ebp,esp
001f0653   test    ecx,ecx
001f0655   je      001f0663
001f0657   cmp     dword ptr [ecx+4],0
001f065b   sete    al
001f065e   movzx   eax,al
001f0661   jmp     001f0668
001f0663   mov     eax,1
001f0668   and     eax,0FFh
001f066d   pop     ebp
001f066e   ret

and for Length == 0:

001406f0   cmp     dword ptr [ecx+4],0
001406f4   sete    al
001406f7   movzx   eax,al
001406fa   ret

With this code, result are as expected, i.e. Length == 0 is slightly faster than String.IsNullOrEmpty.

It's also worth mentioning, that using Linq, lambda expressions and computing modulo in your benchmark is not such a good idea, because these operations are slow (relatively to string comparison) and make result of benchmark inaccurate.