SIMD and difference between packed and scalar double precision
c++, intrinsics, simd, sse, x86
Solution
In SSE, the 128 bits registers can be represented as 4 elements of 32 bits or 2 elements of 64 bits.
SSE defines two types of operations; scalar and packed. Scalar operation only operates on the least-significant data element (bit 0~31 or 0~63), and packed operation computes all elements in parallel.
`_mm_cmpeq_sd` is designed to work with double-precision (64-bit) floating-point elements and would only compare the least-significant data element (first 64 bits) of the two operands (scalar).
`_mm_cmpeq_pd` is designed to work with double-precision (64-bit) floating-point elements as well but would compare each two groups of 64 bits in parallel (packed).
`_mm_cmpeq_ss` is designed to work with single-precision (32-bit) floating-point elements and would only compare the least-significant data element (first 32 bits) of the two operands (scalar).
`_mm_cmpeq_ps` is designed to work with single-precision (32-bit) floating-point elements and would compare each group of 32 bits in parallel (packed).
If you're using 32 bits float, you could pack the float in quadruplet to make use of the 128 bits space. That way, `_mm_cmpeq_ps` would be able to make 4 comparison in parallel.
If you're using 64 bits double, you could pack the double in pair to make use of the 128 bits space. That way, `_mm_cmpeq_pd` would be able to make 2 comparison in parallel.
If you want to make only one comparison at a time, you can use `_mm_cmpeq_sd` to compare two 64 bits double or `_mm_cmpeq_ss` to compare two 32 bits float.
Note that `_mm_cmpeq_sd` and `_mm_cmpeq_pd` are SSE2 while `_mm_cmpeq_ss`and `_mm_cmpeq_ps` are SSE.
Problem
I am reading Intel's intrinsics guide while implementing SIMD support. I have a few confusions and my questions are as below. `__m128 _mm_cmpeq_ps (__m128 a, __m128 b)` documentation says it is used to compare packed single precision floating points. What does "packed" mean? Do I need to pack my float values somehow before I can use them? For double precision there are intrinsics like `_mm_cmpeq_sd` which means compare the "lower" double precision floating point elements. What does lower and upper double precision elemtns mean? Can I use them to compare a vector of C++ `double` type elements or not? Or do I need to process them in some way before I compare them?