How the Grinch stole array.length access
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My favorite way to bootstrap a Christmas dinner conversation is asking people whether they cache array length accesses before their loops or not.

Anybody preferring functional Array.prototype iteration methods to rusty old-school for(;;)-loops gets an additional piece of cake.

Anybody choosing for(let x of y) will find a neatly wrapped ES6-to-ES5-compiler under their Christmas tree.

But the people who cache… But the people who cache tentatively end up on the naughty list. Ho ho ho.

Ok, don’t misunderstand my festive mood. I am not really against the Caching. Caching is one of the most fundamental programming techniques. What makes V8 engine fast? Inline caching.

There is in fact a multitude of valid reasons for wanting to cache a certain property access before the loop:

If these are the reasons my Christmas dinner companions cache length before their loops - they are stricken off the naughty list and pushed into the nice one instead.

Sometimes however this does not happen. Sometimes people try to defend their choice with jsPerf microbenchmarks.

function uncached(arr) {
  for (var i = 0; i < arr.length; i++) {
  }
}

function cached(arr) {
  for (var i = 0, len = arr.length; i < len; i++) {
  }
}

var arr = [];
for (var i = 0; i < 1e4; i++) arr.push(i);

[I am skipping full and proper benchmark.js setup because I am Grinch]

Here the reasoning usually goes as following:

— First loop accesses .length on every iteration and as a result we access the property in total (arr.length + 1)-times. Second loop accesses it 1 time. Obviously better. Look at the jsPerf bars.
— Why do you think arr.length is accessed every time? - asks Grinch eerily smiling.
— Well... I have written it this way. Didn't I?

The Grinch obviously did not ask his question for no reason. Last Christmas he stole himself some IRHydra2, so he can check what kind of code V8 would generate from these loops.

Nothing is surprising in the cached version. Everything we expect to be there is there.This is high-level intermediate representation (IR) used by V8's optimizing compilation pipeline - Crankshaft. Internally it is known as hydrogen. There is no detailed documentation for individual instructions but most of them have self-explanatory mnemonics. It is SSA based.

t3  Parameter 1  // var arr
s11 Constant 0
s39 Constant 1
## for (var i = 0, len = arr.length; i < len; i++) {
##                       ---^------
s16 LoadNamedField t3.%length@12
    Goto B2 Tagged

!1
B2
s21 Phi [ s11  s40 ]  // var i
    ## for (var i = 0, len = arr.length; i < len; i++) {
    ##                                   --^----
    CompareNumericAndBranch LT s21 s16 goto (B3, B5)

B3
  // Loop body would normally be here, but alas it is empty.
  Goto B4

B4
## for (var i = 0, len = arr.length; i < len; i++) {
##                                            ^--
s40 Add s21 s39
    Goto B2

But what happened to uncached version?

t3  Parameter 1  // var arr
s11 Constant 0
s36 Constant 1
## for (var i = 0; i < arr.length; i++) {
##                     ---^------
s24 LoadNamedField t3.%length@12
    Goto B2 Tagged

!1
B2
s16 Phi [ s11  s37 ]  // var i
    ## for (var i = 0; i < arr.length; i++) {
    ##                 --^-----------
    CompareNumericAndBranch LT s16 s24 goto (B3, B5)

B3
  // Loop body would normally be here, but alas it is empty.
  Goto B4

B4
## for (var i = 0; i < arr.length; i++) {
##                                 ^--
s37 Add s16 s36
    Goto B2

Somebody stole our arr.length access from inside the loop and unceremoniously placed it outside of the loop. This somebody is V8’s optimizing compiler, the true Grinch of this story. Grinch that steals all the easy fun from the microbenchmarking and forces you to learn how virtual machines operate.

[Learning VMs is arguably not that bad of kind of fun, so we should even forgive it for not eliminating the do-nothing loop altogether (even though I’m personally looking forward for a day when it will do this)].

If you feel a grudge against the compiler, you might try to put various stuff into the loop in an attempt to make it fail the Loop Invariant Code Motion optimization.

To save you time, something like the code below is not obscure enough to confuse our Grinch-compiler:

function uncached(arr) {
  for (var i = 0; i < arr.length; i++) {
    arr[i]
  }
}

What we need instead is a call. Calls are compiler’s natural enemies, because that’s where intraprocedural analyses fail and a need in interprocedural analyses arises. Compilers partially mitigate this issue by inlining small functions - thus we also need an uninlinable call to really make compiler regret stealing our Christmas arr.length.

function BLACKHOLE(sum, arr) {
  try { } catch (e) { }
}

function uncached(arr) {
  var sum = 0;
  for (var i = 0; i < arr.length; i++) {
    sum += arr[i];
    if (sum < 0) BLACKHOLE(arr, sum);
  }
  return sum;
}

var narr = [];
for (var i = 0; i < 1e4; i++) narr.push(-i);

// PREPARE: make it look like sum can be negative
var r = uncached(narr);
verify(r);

// BENCHMARK: arr contains only positive integers
// so we are not paying for BLACKHOLE call.
var r = uncached(arr);
verify(r);

[I am skipping full and proper benchmark.js setup again]

PREPARE step is actually not needed, but V8 has a tendency to throw away blocks that were never executed if they happen to contain unitialized ICs so I am just guarding against that. In general when benchmarking something it is better to optimistically assume that compiler does a stellar job utilizing profiling information and no worse job discovering dead code. Otherwise compiler might come, passingly stub you in the back and throw away half of your benchmark.

t3  Parameter 1  // var arr
s11 Constant 0
s71 Constant 1

!1
B2
s18 Phi [ s11  s46 ]  // var sum
s19 Phi [ s11  s75 ]  // var i
##  for (var i = 0; i < arr.length; i++) {
##                      ---^------
    CheckMaps t3 [0x5cf0c631]  // check if hidden class changed ①
s27 LoadNamedField t3.%length@12
##  for (var i = 0; i < arr.length; i++) {
##                   -^-
    CompareNumericAndBranch LT s19 s27 goto (B4, B10)

B4
##    sum += arr[i];
##           ---^--
t42 LoadNamedField t3.%elements@8
    BoundsCheck s19 s27  // check if we are reading within bounds ②
s45 LoadKeyed t42[s19] t3
##    sum += arr[i];
##       -^-
s46 Add s18 s45
##    if (sum < 0) BLACKHOLE(arr, sum);
##        ----^--
    CompareNumericAndBranch LT s46 s11 goto (B8, B9)

B8
##    if (sum < 0) BLACKHOLE(arr, sum);
##                 ---------^---------
t85 Change s46 s to t
    PushArguments t62 t3 t85
    CallJSFunction t59 #3 changes[*] // this call changes EVERYTHING!
    Goto B9

B9
##  for (var i = 0; i < arr.length; i++) {
##                                  ^--
s75 Add s19 s74
    Goto B2

It seems our attempt to confuse V8’s optimizing compiler was a huge success: it can no longer even guarantee that array’s hidden class stays the same during iteration so it has to check it every time. Likewise as we desired - the length is reloaded every time too.

[Attentive reader can notice here that something is not working properly in V8’s bounds check elimination pass. Somehow it fails to eliminate an obviously redundant check .]

Hooray! Now the scene has been set for the triumph of the cached version!

t3  Parameter 1  // var arr
s11 Constant 0
s77 Constant 1
s18 LoadNamedField t3.%length@12  // var len 
    Goto B2

!1
B2
s23 Phi [ s11  s49 ]  // var sum
s24 Phi [ s11  s78 ]  // var i
##  for (var i = 0, len = arr.length; i < len; i++) {
##                                    --^----
    CompareNumericAndBranch LT s24 s18 goto (B4, B10)

B4
##    sum += arr[i];
##           ---^--
    CheckMaps t3 [0x28a0c631]
t45 LoadNamedField t3.%elements@8
s46 LoadNamedField t3.%length@12  // 
    BoundsCheck s24 s46
s48 LoadKeyed t45[s24] t3
##    sum += arr[i];
##       -^--
s49 Add s23 s48 !
##    if (sum < 0) BLACKHOLE(arr, sum);
##        ----^--
s54 CompareNumericAndBranch LT s49 s11 goto (B8, B9)

B8
##    if (sum < 0) BLACKHOLE(arr, sum);
##                 ---------^---------
t88 Change s49 s to t
    PushArguments t65 t3 t88
t68 CallJSFunction t62 #3 changes[*]
    Goto B9

B9
##  for (var i = 0, len = arr.length; i < len; i++) {
##                                             ^--
s78 Add s24 s77
    Goto B2

Except not. Now we have two places where we access length inside the function:

Thing to realize here is simple: if compiler is unable to figure out that arr.length does not change within the loop, you can’t help it much by manually caching it. When it needs to access array element, compiler anyway has to emit a bounds check which requires loading length. Could it use len that we cached for it? No. If compiler could figure out that s46 and s18 is the same value it would just hoist s46 out of the loop without our help.

[There is a code optimization available to JIT here that could eliminate the length load: a sophisticated compiler could take this load (and other stuff related to indexed access) and move it to the point after the call because that is the only place where it changes. If calling is infrequent then this could improve the code quality rather drastically.]

Ok. We can’t reason about caching arr.length as an optimization that improves the code by reducing a number of arr.length accesses. How sad! At least we are not making anything worse.

Ho ho ho.

Well, in fact we do make some thing somewhat worse: the life of a poor register allocator. When we manually cache length we give it one more live value to care about and keep around in the loop. Compare these two back edges:

;; uncached version
mov edx, [ebp - 0x18] ;; restore sum
mov eax, [ebp + 0x8]  ;; restore arr
jmp ->B2

;; cached version
mov ebx, [ebp - 0x1c] ;; restore sum
mov eax, [ebp + 0x8]  ;; restore arr
mov ecx, [ebp - 0x14] ;; restore len
jmp ->B2

As you can see in the cached version we end up reloading len again and again on every backedge. Essentially our failed attempt to eliminate a LoadNamedField t3.%length, which itself compiles down to a single mov ebx, [eax + 0xb], ended up adding one more memory move instead of removing one.Even though there are enough free registers to keep all values alive in the loop - register allocator still spills them because there is a call to BLACKHOLE in the loop.

Does this additional move on the back-edge have any performance implications? With our microbenchmark written the way it is this move is probably completely hidden in the shadow of other memory operations on modern desktop CPUs. The story might be different on older CPUs or mobile devices, but I am not sure this a research area we should be digging into here: ultimately the more real work you do in the loop the less our microbenchmark results will matter.

Just for fun I tweaked microbenchmark a bit to increase the pressure:

function uncached(arr, x1, x2, x3) {
  var sum = 0;
  for (var i = 0; i < arr.length; i++) {
    sum += x1;
    sum += x2;
    sum += x3;  // lets hope V8 doesn't reassociate expressions :)
    sum += arr[i];
    if (sum < 0) BLACKHOLE(arr, sum);
  }
  return sum;
  /* ------------------------------------------------------ */
  /* ------------------------------------------------------ */
  /* ------------------------------------------------------ */
  /* ---------- __attribute__((never_inline)) ------------- */
  /* ------------------------------------------------------ */
  /* ------------------------------------------------------ */
  /* ------------------------------------------------------ */
}

function cached(arr, x1, x2, x3) {
  /* similar change */
}

new Benchmark.Suite()
  .add('Cached', function () { cached(arr, 0, 0, 0); })
  .add('Uncached', function () { uncached(arr, 0, 0, 0); })
  .on('cycle', function(event) {
    print(String(event.target));
  })
  .on('complete', function() {
    print('Fastest is ' + this.filter('fastest').pluck('name'));
  })
  .on('error', function (e) {
    print("" + e.target.error);
  })
  .run({'async': false});

Running this on one my boxes yielded the following result:

$ out/ia32.release/d8 test.js
Uncached x 26,435 ops/sec ±2.48% (65 runs sampled)
Cached x 17,922 ops/sec ±1.26% (66 runs sampled)
Fastest is Uncached

Whoa. Uncache all the things! Except I could not repeat this on any other machines available to me. Does it make sense to base your coding style on a flaky microbenchmark?

Yes, we really are treading on a really thin ice of CPU performance effects with these microbenchmarks. I think the Grinch is going to leave you on your own now.

Update 26 December 2014

Every tale has a morale and my story about the Grinch should have one too.

Understand before acting

Yes, surprisingly it has nothing to do with caching or not caching array.length access. Don’t derive your code style from microbenchmarks. Don’t base it on advices from esteemed elders (or mine for that matter). Instead invest into understanding where, how and why your application is spending CPU cycles.

There is another morale here: don’t blindly trust microbenchmarks, they are often not measuring what you think they are measuring.

In my attempt to illustrate this I focused on a single microbenchmark and ignored a lot of other interesting issues:

  1. in the beginning of the post I passingly noted that I favor Array.prototype.* iteration methods to old-school for(;;)-loop. How do I reconcile this with the fact that these methods are still more expensive than a good old for(;;)-loop?
  2. this post deals only with Array objects. What about native DOM objects like HTMLCollection? It’s a fact that accessing HTMLCollection.length is more expensive than array.length.
  3. this posts talks only about V8. What if I have to target other JavaScript VMs? What if I have to target old browsers?

These are all valid questions and unfortunately I simply can’t touch them all in a single post. In some sense mantra “measure, understand, act, measure” is still the best answer to all three even though it might sound disappointingly generic.

Does it matter if you use .map() instead of for(;;)?

Does it matter if you cache HTMLCollection.length?

It depends. Does your application spend considerable time in those methods? Which VMs are you targeting? What’s your coding style?

I don’t think there is a universal answer to these questions.

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