Intel Intrinsics — AVX & AVX2 Learning Notes

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Fundamentals of AVX Programming

Data Types

Data Type	Description
__m128	128-bit vector containing 4 floats
__m128d	128-bit vector containing 2 doubles
__m128i	128-bit vector containing integers
__m256	256-bit vector containing 8 floats
__m256d	256-bit vector containing 4 doubles
__m256i	256-bit vector containing integers

• Each type starts with two underscores, an m, and the width of the vector in bits.
• If a vector type ends in d, it contains doubles, and if it doesn’t have a suffix, it contains floats.
• An integer vector type can contain any type of integer, from chars to shorts to unsigned long longs. That is, an _m256i may contain 32 chars, 16 shorts, 8 ints, or 4 longs. These integers can be signed or unsigned.

Function Naming Conventions

_mm<bit_width>_<name>_<data_type>

• <bit_width> identifies the size of the vector returned by the function. For 128-bit vectors, this is empty. For 256-bit vectors, this is set to 256.
• <name> describes the operation performed by the intrinsic
• <data_type> identifies the data type of the function’s primary arguments

• ps - vectors contain floats (ps stands for packed single-precision)
• pd - vectors contain doubles (pd stands for packed double-precision)
• epi8/epi16/epi32/epi64 - vectors contain 8-bit/16-bit/32-bit/64-bit signed integers
• epu8/epu16/epu32/epu64 - vectors contain 8-bit/16-bit/32-bit/64-bit unsigned integers
• si128/si256 - unspecified 128-bit vector or 256-bit vector
• m128/m128i/m128d/m256/m256i/m256d - identifies input vector types when they’re different than the type of the returned vector

A data type represents memory and a function represents a multimedia operation, so the AVX data types start with two underscores with an m, AVX functions start with an underscore with two ms .

Initialization Intrinsics

Initialization with Scalar Values

Function	Description
_mm256_setzero_ps/pd	Returns a floating-point vector filled with zeros
_mm256_setzero_si256	Returns an integer vector whose bytes are set to zero
_mm256_set1_ps/pd	Fill a vector with a floating-point value
_mm256_set1_epi8/epi16/epi32/epi64x	Fill a vector with an integer
_mm256_set_ps/pd	Initialize a vector with eight floats (ps)or four doubles (pd)
_mm256_set_epi8/epi16/epi32/epi64x	Initialize a vector with integers
_mm256_set_m128/m128d/m128i	Initialize a 256-bit vector with two 128-bit vectors
_mm256_setr_ps/pd	Initialize a vector with eight floats (ps) or four doubles (pd) in reverse order
_mm256_setr_epi8/epi16/epi32/epi64x	Initialize a vector with integers in reverse order

Loading Data from Memory

Data Type	Description
_mm256_load_ps/pd	Loads a floating-point vector from an aligned memory address
_mm256_load_si256	Loads an integer vector from an aligned memory address
_mm256_loadu_ps/pd	Loads a floating-point vector from an unaligned memory address
_mm256_loadu_si256	Loads an integer vector from an unalignedmemory address
_mm_maskload_ps/pd _mm256_maskload_ps/pd	Load portions of a 128-bit/256-bitfloating-point vector according to a mask
(2)_mm_maskload_epi32/64 (2)_mm256_maskload_epi32/64	Load portions of a 128-bit/256-bitinteger vector according to a mask

The last two functions are preceded with (2) because they’re provided by AVX2, not AVX.

Each _mm256_load_*intrinsic accepts a memory address that must be aligned on a 32-byte boundary.

Arithmetic Intrinsics

Addition and Subtraction

Data Type	Description
_mm256_add_ps/pd	Add two floating-point vectors
_mm256_sub_ps/pd	Subtract two floating-point vectors
(2)_mm256_add_epi8/16/32/64	Add two integer vectors
(2)_mm236_sub_epi8/16/32/64	Subtract two integer vectors
(2)_mm256_adds_epi8/16 (2)_mm256_adds_epu8/16	Add two integer vectors with saturation
(2)_mm256_subs_epi8/16 (2)_mm256_subs_epu8/16	Subtract two integer vectors with saturation
_mm256_hadd_ps/pd	Add two floating-point vectors horizontally
_mm256_hsub_ps/pd	Subtract two floating-point vectors horizontally
(2)_mm256_hadd_epi16/32	Add two integer vectors horizontally
(2)_mm256_hsub_epi16/32	Subtract two integer vectors horizontally
(2)_mm256_hadds_epi16	Add two vectors containing shorts horizontally with saturation
(2)_mm256_hsubs_epi16	Subtract two vectors containing shorts horizontally with saturation
_mm256_addsub_ps/pd	Add and subtract two floating-point vectors

Functions that take saturation into account clamp the result to the minimum/maximum value that can be stored. Functions without saturation ignore the memory issue when saturation occurs.

This may seem strange to add and subtract elements horizontally, but these operations are helpful when multiplying complex numbers.

_mm256_addsub_ps/pd, alternately subtracts and adds elements of two floating-point vectors. That is, even elements are subtracted and odd elements are added .

Multiplication and Division

Data Type	Description
_mm256_mul_ps/pd	Multiply two floating-point vectors
(2)_mm256_mul_epi32 (2)_mm256_mul_epu32	Multiply the lowest four elements of vectors containing 32-bit integers
(2)_mm256_mullo_epi16/32	Multiply integers and store low halves
(2)_mm256_mulhi_epi16 (2)_mm256_mulhi_epu16	Multiply integers and store high halves
(2)_mm256_mulhrs_epi16	Multiply 16-bit elements to form 32-bit elements
_mm256_div_ps/pd	Divide two floating-point vectors

This image is WRONG !!!

Please read the reference from this manual.

Only the four low elements of the _mm256_mul_epi32 and _mm256_mul_epu32 intrinsics are multiplied together, and the result is a vector containing four long integers.

They multiply every element of both vectors store only the low half of each product

Fused Multiply and Add (FMA)

Data Type	Description
(2)_mm_fmadd_ps/pd/ (2)_mm256_fmadd_ps/pd	Multiply two vectors and add the product to a third (res = a * b + c)
(2)_mm_fmsub_ps/pd/ (2)_mm256_fmsub_ps/pd	Multiply two vectors and subtract a vector from the product (res = a * b - c)
(2)_mm_fmadd_ss/sd	Multiply and add the lowest element in the vectors (res[0] = a[0] * b[0] + c[0])
(2)_mm_fmsub_ss/sd	Multiply and subtract the lowest element in the vectors (res[0] = a[0] * b[0] - c[0])
(2)_mm_fnmadd_ps/pd (2)_mm256_fnmadd_ps/pd	Multiply two vectors and add the negated product to a third (res = -(a * b) + c)
(2)_mm_fnmsub_ps/pd/ (2)_mm256_fnmsub_ps/pd	Multiply two vectors and add the negated product to a third (res = -(a * b) - c)
(2)_mm_fnmadd_ss/sd	Multiply the two lowest elements and add the negated product to the lowest element of the third vector (res[0] = -(a[0] * b[0]) + c[0])
(2)_mm_fnmsub_ss/sd	Multiply the lowest elements and subtract the lowest element of the third vector from the negated product (res[0] = -(a[0] * b[0]) - c[0])
(2)_mm_fmaddsub_ps/pd/ (2)_mm256_fmaddsub_ps/pd	Multiply two vectors and alternately add and subtract from the product (res = a * b +/- c) (Odd add, even sub)
(2)_mm_fmsubadd_ps/pd/ (2)_mmf256_fmsubadd_ps/pd	Multiply two vectors and alternately subtract and add from the product (res = a * b -/+ c) (Odd sub, even add)

Permuting and Shuffling

Permuting

Data Type	Description
_mm_permute_ps/pd _mm256_permute_ps/pd	Select elements from the input vector based on an 8-bit control value
(2)_mm256_permute4x64_pd/ (2)_mm256_permute4x64_epi64	Select 64-bit elements from the input vector based on an 8-bit control value
_mm256_permute2f128_ps/pd	Select 128-bit chunks from two input vectors based on an 8-bit control value
_mm256_permute2f128_si256	Select 128-bit chunks from two input vectors based on an 8-bit control value
_mm_permutevar_ps/pd _mm256_permutevar_ps/pd	Select elements from the input vector based on bits in an integer vector
(2)_mm256_permutevar8x32_ps (2)_mm256_permutevar8x32_epi32	Select 32-bit elements (floats and ints) using indices in an integer vector

Shuffling

Data Type	Description
_mm256_shuffle_ps/pd	Select floating-point elements according to an 8-bit value
_mm256_shuffle_epi8/ _mm256_shuffle_epi32	Select integer elements according to an8-bit value
(2)_mm256_shufflelo_epi16/ (2)_mm256_shufflehi_epi16	Select 128-bit chunks from two input vectors based on an 8-bit control value

For _mm256_shuffle_pd, only the high four bits of the control value are used. If the input vectors contain ints or floats, all the control bits are used. For _mm256_shuffle_ps, the first two pairs of bits select elements from the first vector and the second two pairs of bits select elements from the second vector.

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