Archives for: May 2007


Overview of project

Permalink 22:02:44, Categories: Optimisation Framework  

Hey, my name is Mike Gist (aka Xordan). Currently I'm a first year student studying Computing at Imperial College London. I'll be working on the optimisation framework project for the next few months and I'll be keeping note of my progress here and explain a bit about what I'll be doing now.

At the moment there are various optimisations that could be done using SIMD instructions, but there is no way to properly detect support and use the correct code path. Obviously an Athlon XP won't be able to use SSE3 instructions, and a PPC processor will be able to use AltiVec only.. assuming it supports it. My job is to add runtime and compile time detection of the supported instruction sets (both for the processor and the OS), add a method for the correct code path to be selected and used, and then to make use of this in various places in the existing CS code.

Within the scope of this project I'll be concentrating on MMX, SSE, SSE2, SSE3 and AltiVec. Later I will expand and include SSSE3 and SSE4, but those aren't priorities for now.

Currently there is a class called csProcessorCapability which contains some MMX detection code. I plan to rewrite all of this, but keep the name :) There are different ways of detecting supported instructions (asm or inbuilt compiler functions). VC has a function called isProcessorFeaturePresent() which makes detection very simple. For gcc it's a bit harder, we will need to use cpuid to get the info. We will know the minimum instruction set supported by an architecture so we can short cut some checks as well (amd64 all support from MMX through to SSE2 for example).

The csProcessorCapability class will look something like this:

class csProcessorCapability

csProcessorCapability() {}

~csProcessorCapability() {}

static inline bool HasMMX() {}

static inline bool HasSSE() {}

static inline bool HasSSE2() {}

static inline bool HasSSE3() {}

static inline bool HasAltiVec() {}


static inline void Initialise() {}
static bool isInitialised;
static bool supportsMMX;
static bool supportsSSE;
static bool supportsSSE2;
static bool supportsSSE3;
static bool supportsAltiVec;

static inline void CheckSupportedInstructions(bool MMX, bool SSE, bool SSE2, bool SSE3, bool altiVec) {}

The CheckSupportedInstructions function is run once by the Initialise() function, and contains all the detection code. If it's already known that an instruction is supported or isn't supported then this will be passed to that function and it won't be checked. I'll go into more detail as I progress.

Compile time checks will be for __m128 support (checking for xmmintrin.h), so we know if the compiler actually supports this stuff :) No compiler support, no extra cpu juice. Some other things will be used, like target arch, which will allow us to rule out some instruction sets. I'll add an entry just on this when I get to it. First thing is getting that class written. I'm giving myself two weeks to finish the detection, then I'll move on to making use of it. I'm hoping to get a good chunk of that done by the mid-term evaluation, leaving me about a month to finish it and optimise some areas (to be chosen) of code. I'll detail that a bit more very soon.

May 2007
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