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+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;
+ }
+}