comparison ext/kissfft/README.simd @ 184:76ec2365b250

Bring in kissfft into this repo (formerly a subrepo, but the remote is not responding)

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