cjameshuff / vectests

Some simple benchmarks comparing the Clang vector extension, SSE intrinsics, and a plain vector class for single-precision float vectors.

Some simple benchmarks comparing the Clang vector extension, SSE intrinsics, and a plain vector class for single-precision float vectors.

Summary

The extension and intrinsics tests use 4 component vectors for better mapping to 128-bit SSE registers (loading/storing 4 floats requires one instruction, 3 requires two separate loads and some shuffling).

The structs, and arrays where implemented, have 3D and 4D versions selected by a preprocessor define. The 4D version more accurately represents the relative capabilities in general computation, the 3D version is more representative for 3D geometry, physics, and graphics.

Vector class: class/struct of scalars.

• Slowest. Even when wrapping a primitive vector and using intrinsics, there is a performance penalty, probably because primitive vectors can be passed through SSE registers.
• Platform and compiler independent.
• Can use 3-component vectors.

Array of scalars:

• Basically indistinguishable in performance from the vector class.
• Less convenient to work with.

Vector extension: OpenCL-style vector types in C/C++.

• Non-standard and compiler dependent (though Clang and GCC have similar extensions).
• Processor-independent.
• Convenient member access and composition syntax.
• Mixes well with intrinsics.
• Often comes close to or equals code using intrinsics in performance.

Intrinsics:

• Generally fastest, but sometimes by little or no margin over the vector extension.
• Use more limited non-standard language extensions: just 128-bit types.
• Processor-specific.
• Ugly code.

Timings:

On my dual quad-core Mac Pro, clang version 3.2 (trunk 167613):

Math tests

Struct: 0.734957 s
Array: 0.734050 s
Extension: 0.479301 s
Intrinsics: 0.480404 s


Ray tracing tests

Structs: 0.989610 s
Extension: 0.756928 s
Intrinsics: 0.677218 s
Rays: 0.846294 s
Ray+obj: 0.802400 s


Particle system tests

Structs: 0.865972 s
extension: 0.570777 s
intrinsics: 0.570669 s

Notes:

SSE4.1 is supported by Harpertown (2008 MacPro3,1) and Q6600. It seems a reasonable minimum requirement for modern high-performance code.

http://sseplus.sourceforge.net/fntable.html

Header files for intrinsics:

• mmintrin.h: MMX
• xmmintrin.h: SSE
• emmintrin.h: SSE2
• pmmintrin.h: SSE3
• tmmintrin.h: SSSE3
• smmintrin.h: SSE4.1
• nmmintrin.h: SSE4.2
• ammintrin.h: SSE4A
• wmmintrin.h: AES
• immintrin.h: AVX