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79 lines
2.4 KiB
Plaintext
79 lines
2.4 KiB
Plaintext
If you are reading this, it means you think you may be interested in using the SIMD extensions in kissfft
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to do 4 *separate* FFTs at once.
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Beware! Beyond here there be dragons!
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This API is not easy to use, is not well documented, and breaks the KISS principle.
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Still reading? Okay, you may get rewarded for your patience with a considerable speedup
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(2-3x) on intel x86 machines with SSE if you are willing to jump through some hoops.
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The basic idea is to use the packed 4 float __m128 data type as a scalar element.
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This means that the format is pretty convoluted. It performs 4 FFTs per fft call on signals A,B,C,D.
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For complex data, the data is interlaced as follows:
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rA0,rB0,rC0,rD0, iA0,iB0,iC0,iD0, rA1,rB1,rC1,rD1, iA1,iB1,iC1,iD1 ...
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where "rA0" is the real part of the zeroth sample for signal A
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Real-only data is laid out:
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rA0,rB0,rC0,rD0, rA1,rB1,rC1,rD1, ...
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Compile with gcc flags something like
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-O3 -mpreferred-stack-boundary=4 -DUSE_SIMD=1 -msse
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Be aware of SIMD alignment. This is the most likely cause of segfaults.
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The code within kissfft uses scratch variables on the stack.
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With SIMD, these must have addresses on 16 byte boundaries.
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Search on "SIMD alignment" for more info.
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Robin at Divide Concept was kind enough to share his code for formatting to/from the SIMD kissfft.
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I have not run it -- use it at your own risk. It appears to do 4xN and Nx4 transpositions
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(out of place).
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void SSETools::pack128(float* target, float* source, unsigned long size128)
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{
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__m128* pDest = (__m128*)target;
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__m128* pDestEnd = pDest+size128;
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float* source0=source;
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float* source1=source0+size128;
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float* source2=source1+size128;
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float* source3=source2+size128;
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while(pDest<pDestEnd)
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{
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*pDest=_mm_set_ps(*source3,*source2,*source1,*source0);
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source0++;
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source1++;
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source2++;
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source3++;
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pDest++;
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}
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}
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void SSETools::unpack128(float* target, float* source, unsigned long size128)
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{
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float* pSrc = source;
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float* pSrcEnd = pSrc+size128*4;
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float* target0=target;
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float* target1=target0+size128;
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float* target2=target1+size128;
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float* target3=target2+size128;
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while(pSrc<pSrcEnd)
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{
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*target0=pSrc[0];
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*target1=pSrc[1];
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*target2=pSrc[2];
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*target3=pSrc[3];
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target0++;
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target1++;
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target2++;
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target3++;
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pSrc+=4;
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}
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}
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