# Loop hoisting optimization technique

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Hi all,

Just thought I'd share a loop hoisting optimization technique I came up with the other day, which greatly simplifies loop hoisting in situations where you have many invariants you would like to hoist. For example if you're hoisting 6 invariants from a single inner loop, that's 64 possible combinations to deal with (assuming you want to handle all of them)... what's a lazy programmer to do??

It's highly unlikely nobody has stumbled across this approach before, I've just not personally come across this particular technique. It would have been handy to know about when I last worked on a particle system. Specifically a variant of this technique was used in said particle system, except the hoisting was done manually with monolithic and ugly switch statements (several pages long *shudder*).

I wasn't sure what performance this would squeeze out when used with modern optimizing compilers, certainly it will be heavily platform dependent - though I was quite surprised at how much a jump in performance I got in my own tests. It generally hovered around 5% improvement (worthwhile IMO considering little sacrifice in terms of functionality/readability), though I contrived an example where I got well over a 10% improvement in performance. This on a clean install of visual studio 2010, release mode (all default settings), on an Intel i5-2500K CPU. I'm interested to see what results others get, so I've pasted some code which people can use to test - but I'll leave people to use their own timing methodology, to see if anyone interested can come up with similar results (in case I've done something to bias results..).

So first off, for those unfamiliar with loop hoisting, it's just the following

 void function(bool invariantCondition) { for(int i = 0; i < 100; ++i) { if(invariantCondition) doSomething; else doSomethingElse; } 

versus

 // hoist condition out if(invariantCondition) { for(int i = 0; i < 100; ++i) doSomething; } else { for(int i = 0; i < 100; ++i) doSomethingElse; } 

All the following technique does is allow you to automatically generate a flag set parameter to pass into a function, which can be used to replace all the contained if statements. So say I have the following function (thrown together as a simple test case), where flags defines what properties of the particle that need to be updated. Ignore the use of globals and the fact you're going to get a pretty crap looking particle simulation, my aim was to keep it simple. Think of 'flags' as parameters artists have configured in the editor, defining the behavior of the particle system.

 void UpdateParticles(unsigned int systemIndex, unsigned int flags) { const float dt = 1.0f / 60.0f; const float accel[3] = {-1.0f, -9.8f, -1.0f}; const float NOISE_SCALE = 2.0f * dt; VertexPCT* buffer = g_vertices[systemIndex]; for(int i = 0; i < PARTICLE_COUNT; ++i) { VertexPCT& vert = buffer; if(flags & F_UPDATE_VEL) { vert.vel[0] += accel[0] * dt; vert.vel[1] += accel[1] * dt; vert.vel[2] += accel[2] * dt; } if(flags & F_VEL_NOISE) { vert.vel[0] += NOISE_SCALE * (float(rand()) / RAND_MAX); vert.vel[1] += NOISE_SCALE * (float(rand()) / RAND_MAX); vert.vel[2] += NOISE_SCALE * (float(rand()) / RAND_MAX); } vert.pos[0] += vert.vel[0] * dt; vert.pos[1] += vert.vel[1] * dt; vert.pos[2] += vert.vel[2] * dt; if(flags & F_UPDATE_COLOR) { vert.color = 0xffffff00 | (i % 0xff); } if(flags & F_UPDATE_U) { vert.uv[0] = fmodf(vert.pos[0], 10.0f); } if(flags & F_UPDATE_V) { vert.uv[1] = fmodf(vert.pos[2], 10.0f); } if(flags & F_UPDATE_SCALE) { vert.scale[0] = std::min<float>(vert.scale[0] + dt, 1.0f); vert.scale[1] = std::min<float>(vert.scale[1] + dt, 1.0f); } } } 

Above you can see there are six invariants, for the various things that we update in the loop. We can rewrite this method as:

 template <int flags> void UpdateParticles2(unsigned int systemIndex) { const float dt = 1.0f / 60.0f; const float accel[3] = {-1.0f, -9.8f, -1.0f}; const float NOISE_SCALE = 2.0f * dt; VertexPCT* buffer = g_vertices[systemIndex]; for(int i = 0; i < PARTICLE_COUNT; ++i) { VertexPCT& vert = buffer; if(flags & F_UPDATE_VEL) { vert.vel[0] += accel[0] * dt; vert.vel[1] += accel[1] * dt; vert.vel[2] += accel[2] * dt; } if(flags & F_VEL_NOISE) { vert.vel[0] += NOISE_SCALE * (float(rand()) / RAND_MAX); vert.vel[1] += NOISE_SCALE * (float(rand()) / RAND_MAX); vert.vel[2] += NOISE_SCALE * (float(rand()) / RAND_MAX); } vert.pos[0] += vert.vel[0] * dt; vert.pos[1] += vert.vel[1] * dt; vert.pos[2] += vert.vel[2] * dt; if(flags & F_UPDATE_COLOR) { vert.color = 0xffffff00 | (i % 0xff); } if(flags & F_UPDATE_U) { vert.uv[0] = fmodf(vert.pos[0], 10.0f); } if(flags & F_UPDATE_V) { vert.uv[1] = fmodf(vert.pos[2], 10.0f); } if(flags & F_UPDATE_SCALE) { vert.scale[0] = std::min<float>(vert.scale[0] + dt, 1.0f); vert.scale[1] = std::min<float>(vert.scale[1] + dt, 1.0f); } } } 

Now the flags set is defined at compile time, effectively eliminating those branches (effectively hoisting them out) of the loop. By passing in flags as a template parameter, the conditionals become known at compile time, so it's no different from a set of if(true) or if(false) statements. How do we generate the template parameter 'flags' to pass in? A basic implementation would be a huge switch statement that invokes every required variant of UpdateParticles2, such as.

 switch(flags) { case F_UPDATE_VEL: UpdateParticles2<F_UPDATE_VEL>(systemIndex); break; case F_UPDATE_VEL | F_UPDATE_COLOR: UpdateParticles2<F_UPDATE_VEL | F_UPDATE_COLOR>(systemIndex); break; ..etc 

Instead we can use the following template class to generate the flags for us (for any given number of flags)

 template <int STATE_COUNT, int FLAGS> class GenerateFlags { public: static const int CHECK_BIT = (1 << (STATE_COUNT - 1)); static void Invoke(unsigned int systemIndex, unsigned int flags) { if(flags & CHECK_BIT) { GenerateFlags<STATE_COUNT - 1, FLAGS | CHECK_BIT>::Invoke(systemIndex, flags); } else { GenerateFlags<STATE_COUNT - 1, FLAGS>::Invoke(systemIndex, flags); } } }; template <int FLAGS> class GenerateFlags<0, FLAGS> { public: static void Invoke(unsigned int systemIndex, unsigned int flags) { assert(flags == FLAGS); UpdateParticles2<FLAGS>(systemIndex); } }; 

and call

 GenerateFlags<FLAG_COUNT, 0>::Invoke(flags); 

Where FLAG_COUNT is the maximum number of bit flags, in this case, 6. The compiler will now generate 64 specializations of the UpdateParticles2 method, each with the invariants as constant conditionals. As such if you have the relevant compile warnings enabled, you'll get a heap of warnings such as this: 'warning C4127: conditional expression is constant' .
In terms of performance you have to be mindful that you could easily wreak havoc with the instruction cache by bloating the code footprint (lowering performance), but the nice thing is you can easily test functions to see if there is any performance benefit.

Anyway that's about it, interested to hear feedback from others. I've attached the remainder of code that will allow the above to compile, for people who want to test the performance themself. For reference, the non hoisted version (single call to TestFunction) took a pretty consistent 137ms in my tests.

 #include <conio.h> #include <windows.h> #include <math.h> #include <numeric> #include <assert.h> void TestFunction(unsigned int flags); void TestFunction2(unsigned int flags); void UpdateParticles(unsigned int systemIndex, unsigned int flags); template <int flags> void UpdateParticles2(unsigned int systemIndex); enum UpdateFlags { F_UPDATE_VEL = (1 << 0), F_UPDATE_COLOR = (1 << 1), F_UPDATE_U = (1 << 2), F_UPDATE_V = (1 << 3), F_UPDATE_SCALE = (1 << 4), F_VEL_NOISE = (1 << 5), FLAG_COUNT = 6, F_UPDATE_ALL = 0xffffffff, }; static const int NUM_ITERATIONS = 8; static const int NUM_PARTICLE_SYSTEMS = 50; static const int PARTICLE_COUNT = 50000; struct VertexPCT { float pos[3]; float vel[3]; unsigned int color; float uv[2]; float scale[2]; }; VertexPCT** g_vertices = 0; //#pragma warning(1: 4127) int _tmain(int argc, _TCHAR* argv[]) { // this particular combination of flags demonstrates significant speedup when using loop hoisting unsigned int flags = F_UPDATE_COLOR | F_UPDATE_VEL; if(argc == 4) { // just in case the compiler magically optimizes our non hoisted version // using the knowledge 'flags' is in fact constant (as it happens the compiler does, at least a little..) flags |= 0xff000000; } // initialise our particle system memory once off g_vertices = new VertexPCT*[NUM_PARTICLE_SYSTEMS]; for(int i = 0; i < NUM_PARTICLE_SYSTEMS; ++i) { g_vertices = new VertexPCT[PARTICLE_COUNT]; memset(g_vertices, 0, sizeof(VertexPCT) * PARTICLE_COUNT); } /////////////////////////////// // [timer code here, for timing TestFunction and TestFunction2] /////////////////////////////// return 0; } void TestFunction(unsigned int flags) { const unsigned int mask = (1 << FLAG_COUNT) - 1; for(int i = 0; i < NUM_ITERATIONS; ++i) { for(int j = 0; j < NUM_PARTICLE_SYSTEMS; ++j) { UpdateParticles(j, flags & mask); } } } void TestFunction2(unsigned int flags) { const unsigned int mask = (1 << FLAG_COUNT) - 1; for(int i = 0; i < NUM_ITERATIONS; ++i) { for(int j = 0; j < NUM_PARTICLE_SYSTEMS; ++j) { GenerateFlags<FLAG_COUNT, 0>::Invoke(j, flags & mask); } } } 

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Techniques like this exist and are quite common in certain types of processing. But several considerations that don't make them a clear win in general case.

Unrolling all possible cases may cause considerable code bloat or multiple function calls, something that compilers have difficulty balancing and use heuristics.

Here's an example of similar optimization. Shader generator tools also apply similar techniques when merging effects.

Templated solution is mostly personal design choice. It may be flexible and mimic common design, or it might be worth going one extra step and simply applying processing functions directly. Neither is ideal as far as programmer efficiency goes.

In some cases, especially if dealing with GPU, simply performing all operations on all elements might win out through brute force. If an element isn't to be processed, merely specify identity transform, such as multiply by 1 or add 0, ignoring the decision stage altogether.

Organization of code will be mostly a stylistic choice, linear data layout and SIMD friendly processing is the big win here. It might not be worth applying these techniques to algorithms which aren't perfect fit since they'll limit flexibility, especially if there are many diverse states applied to a proportionally small number of elements.

It would be possible to mechanically transform more familiar OO or procedural design into such layout, but it's dubious whether world needs yet another language. Concurrency libraries almost force such approach, it's also a good fit for functional languages.

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Techniques like this exist and are quite common in certain types of processing. But several considerations that don't make them a clear win in general case.

Unrolling all possible cases may cause considerable code bloat or multiple function calls, something that compilers have difficulty balancing and use heuristics.

It was more the how of the technique, rather than the end result, that I thought was handy - people can and do do this loop unrolling manually. I would argue the majority of low level optimizations are not a clear win in the general case, you really must profile, even if you're 99% certain it's a win.

That said, using the test case above, through profiling it's demonstrably a clear win on my specific hardware, and by quite a large margin. In practice you would need to group by type to ensure you don't thrash the instruction cache, and quite possibly only generate a subset of combinations (hoisting out paths shared by many systems).

Templated solution is mostly personal design choice. It may be flexible and mimic common design, or it might be worth going one extra step and simply applying processing functions directly. Neither is ideal as far as programmer efficiency goes.

I don't quite follow, what do you mean by 'applying processing functions directly'? The time complexity of actuallying implementing this type of loop hoisting is O(n) with respect to the number of variables to be hoisted, versus O(2^n) for the naive implementation. This translates to real world benefits in terms of efficiency, because if you suspect branches are hurting performance, you can try try this out quickly.

In some cases, especially if dealing with GPU, simply performing all operations on all elements might win out through brute force. If an element isn't to be processed, merely specify identity transform, such as multiply by 1 or add 0, ignoring the decision stage altogether.

Absolutely, and quite likely the case in the sample code I provided. In practice the conditional blocks branch quite a bit larger, and you might well hurt your data cache by touching more than you need.

Organization of code will be mostly a stylistic choice, linear data layout and SIMD friendly processing is the big win here.

This, and using a struct of arrays format, can be a big win, though that's a whole topic in itself.

It might not be worth applying these techniques to algorithms which aren't perfect fit since they'll limit flexibility, especially if there are many diverse states applied to a proportionally small number of elements.

Agreed, use sparingly, and most definitely profile. It would be very easy to reduce performance applying this in an indiscriminate way.

It would be possible to mechanically transform more familiar OO or procedural design into such layout, but it's dubious whether world needs yet another language. Concurrency libraries almost force such approach, it's also a good fit for functional languages.

I'm not sure this matters - you shouldn't be applying this optimization to a function that's available in a public interface. It should happen behind the scenes, so what happens within the function call is nobody's business, and it's not a big deal whether it's OO or hand rolled assembler. If it helps the design, you might consider passing in a boost::function to the Invoke call, or something like that.