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08 May 2017

Curious case of branch performance

Branching, performance

When doing my last performance tests for bool packing, I got strange results sometimes. It appeared that one constant generated different results than the other. Why was that? Let’s have a quick look at branching performance.

The problem

Just to recall (first part, second part) I wanted to pack eight booleans (results of a condition) into one byte, 1 bit per condition result. The problem is relatively simple, but depending on the solution you might write code that’s 5x…8x times slower than the other version.

Let’s take a simple version that uses std::vector<bool>:

static const int ThresholdValue = X;
std::unique_ptr<int[]> inputValues = PrepareInputValues();
std::vector<bool> outputValues;

outputValues.resize(experimentValue);

// start timer
{
    for (size_t i = 0; i < experimentValue; ++i)
        outputValues[i] = inputValues[i] > ThresholdValue;
}
// end timer

And see the results:

std::vector<bool> performance

The chart shows timings for 100 samples taken from running the code, vector size (experimentValue) is 1mln.

Do you know what the difference between the above results is?

It’s only X - the value of ThresholdValue!

If it’s 254 then you got the yellow performance, if it’s 127, then you got those green, blue squares. The generated code is the same, so why we see the difference? The same code can run eve 4x slower!

So maybe vector implementation is wrong?

Let’s use a (not optimal) manual version:

uint8_t OutByte = 0;
int shiftCounter = 0;

for (int i = 0; i < experimentValue; ++i)
{
    if (*pInputData > Threshold)
        OutByte |= (1 << shiftCounter);

    pInputData++;
    shiftCounter++;

    if (shiftCounter > 7)
    {
        *pOutputByte++ = OutByte;
        OutByte = 0;
        shiftCounter = 0;
    }
}

And the results:

Again, when running with Threshold=127, you get the top output, while Threshold=254 returns the bottom one.

OK, but also some of the versions of the algorithm didn’t expose this problem.

For example, the optimized version. That packed 8 values at ‘once’.

uint8_t Bits[8] = { 0 };
const int64_t lenDivBy8 = (experimentValue / 8) * 8;

for (int64_t j = 0; j < lenDivBy8; j += 8)
{
    Bits[0] = pInputData[0] > Threshold ? 0x01 : 0;
    Bits[1] = pInputData[1] > Threshold ? 0x02 : 0;
    Bits[2] = pInputData[2] > Threshold ? 0x04 : 0;
    Bits[3] = pInputData[3] > Threshold ? 0x08 : 0;
    Bits[4] = pInputData[4] > Threshold ? 0x10 : 0;
    Bits[5] = pInputData[5] > Threshold ? 0x20 : 0;
    Bits[6] = pInputData[6] > Threshold ? 0x40 : 0;
    Bits[7] = pInputData[7] > Threshold ? 0x80 : 0;

    *pOutputByte++ = Bits[0] | Bits[1] | Bits[2] | Bits[3] | 
                     Bits[4] | Bits[5] | Bits[6] | Bits[7];
    pInputData += 8;
}

The samples are not lining up perfectly, and there are some outliers, but still, the two runs are very similar.

And also the baseline (no packing at all, just saving into bool array)

std::unique_ptr<uint8_t[]> outputValues(new uint8_t[experimentValue]);

// start timer
{
    for (size_t i = 0; i < experimentValue; ++i)
        outputValues[i] = inputValues[i] > ThresholdValue;
});
// end timer

This time, Threshold=254 is slower… but still not that much, only few percents. Not 3x…4x as with the first two cases.

What’s the reason for those results?

The test data

So far I didn’t explain how my input data are even generated. Let’s unveil that.

The input values simulate greyscale values, and they are ranging from 0 up to 255. The threshold is also in the same range.

The data is generated randomly:

std::mt19937 gen(0);
std::uniform_int_distribution<> dist(0, 255);

for (size_t i = 0; i < experimentValue; ++i)
    inputValues[i] = dist(gen);

Branching

As you might already discover, the problem lies in the branching (mis)predictions. When the Threshold value is large, there’s little chance input values will generate TRUE. While for Threshold = 127 we get 50% chances (still it’s a random pattern).

Here’s a great experiment that shows some problems with branching: Fast and slow if-statements: branch prediction in modern processors @igoro.com. And also Branch predictor - Wikipedia.

Plus read more in The Software Optimization Cookbook: High Performance Recipes for IA-32 Platforms, 2nd Edition

For a large threshold value, most of my code falls into FALSE cases, and thus no additional instructions are executed. CPU sees this in it’s branch history and can predict the next operations. When we have random 50% pattern, the CPU cannot choose the road effectively, so there are many mispredictions.

Unfortunately, I don’t have tools to measure those exact numbers, but for me, it’s a rather clear situation. Maybe you can measure the data? Let me know!

But why the other code - the optimized version didn’t show the effect? Why it runs similarly, no matter what the constant is?


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Details

Let’s look at the generated assembly: play @ godbolt.org.

Optimized version (From MSVC)

$LL4@Foo:
        cmp      DWORD PTR [ecx-8], 128   ; 00000080H
        lea      edi, DWORD PTR [edi+1]
        lea      ecx, DWORD PTR [ecx+32]
        setg     BYTE PTR _Bits$2$[esp+8]
        cmp      DWORD PTR [ecx-36], 128  ; 00000080H
        setle    al
        dec      al
        and      al, 2
        cmp      DWORD PTR [ecx-32], 128  ; 00000080H
        mov      BYTE PTR _Bits$1$[esp+8], al
        setle    bh
        dec      bh
        and      bh, 4
        cmp      DWORD PTR [ecx-28], 128  ; 00000080H
        setle    dh
        dec      dh
        and      dh, 8
        cmp      DWORD PTR [ecx-24], 128  ; 00000080H
        setle    ah
        dec      ah
        and      ah, 16             ; 00000010H
        cmp      DWORD PTR [ecx-20], 128  ; 00000080H
        setle    bl
        dec      bl
        and      bl, 32             ; 00000020H
        cmp      DWORD PTR [ecx-16], 128  ; 00000080H
        setle    al
        dec      al
        and      al, 64             ; 00000040H
        cmp      DWORD PTR [ecx-12], 128  ; 00000080H
        setle    dl
        dec      dl
        and      dl, 128              ; 00000080H
        or       dl, al
        or       dl, bl
        or       dl, ah
        or       dl, dh
        or       dl, bh
        or       dl, BYTE PTR _Bits$2$[esp+8]
        or       dl, BYTE PTR _Bits$1$[esp+8]
        mov      BYTE PTR [edi-1], dl
        sub      esi, 1
        jne      $LL4@Foo
        pop      esi
        pop      ebx

And for first manual version: https://godbolt.org/g/csLeHe

        mov      edi, DWORD PTR _len$[esp+4]
        test     edi, edi
        jle      SHORT $LN3@Foo
$LL4@Foo:
        cmp      DWORD PTR [edx], 128     ; 00000080H
        jle      SHORT $LN5@Foo
        movzx    ecx, cl
        bts      ecx, eax
$LN5@Foo:
        inc      eax
        add      edx, 4
        cmp      eax, 7
        jle      SHORT $LN2@Foo
        mov      BYTE PTR [esi], cl
        inc      esi
        xor      cl, cl
        xor      eax, eax
$LN2@Foo:
        sub      edi, 1
        jne      SHORT $LL4@Foo
$LN3@Foo:
        pop      edi
        pop      esi
        ret      0

As we can see the optimized version doesn’t use branching. It uses setCC instruction, but this is not a real branch. Strangely GCC doesn’t use this approach and uses branches so that the code could be possibly slower.

SETcc – sets the destination register to 0 if the condition is not met and to 1 if the condition is met.

See Branch and Loop Reorganization to Prevent Mispredicts | Intel® Software

Great book about perf:Branch and Loop Reorganization to Prevent Mispredicts | Intel® Software

See also this explanation for avoiding branches: x86 Disassembly/Branches wikibooks

So, if I am correct, this is why the optimized version doesn’t show any effects of branch misprediction.

The first, non-optimal version of the code contains two jumps in the loop, so that’s why we can experience the drop in performance.

Still, bear in mind that conditional moves are not always better than branches. For example read more details at Krister Walfridsson’s blog: like The cost of conditional moves and branches.

Summary

Things to remember:

  • Doing performance benchmarks is a really delicate thing.
  • Look not only at the code but also on the test data used - as different distribution might give completely different results.
  • Eliminate branches as it might give a huge performance boost!

Charts made with Nonius library, see more about in my micro-benchmarking library blog post.

A question to you:

  • How do you reduce branches in your perf critical code?

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© 2017, Bartlomiej Filipek, Blogger platform
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