SIMD Reference Page

Introduction

SIMD (single instruction multiple data) are sets of instructions that operate on wide registers called vectors. For our assignments, these vectors will generally be 256 bits wide, though you may occasionally use the 128-bit versions. Typically, instructions
that act on these wide registers will treat it as an array of values. They will then perform operations independently on each value in the array. In hardware, this can be implemented by having multiple ALUs that work in parallel. As a result, although these instructions perform many times more arithmetic than “normal” instructions, they can be as fast as the normal instructions.

Generally, we will be accessing these instructions using “intrinsic functions”. These functions usually correspond directly to a particular assembly instruction. This will allow us to write C code that accesses this special functionality consistently without losing all the benefits of having a C compiler.

Intrinsics reference

The intrinsic functions we will be using are an interface defined by Intel. Consequently,
Intel’s documentation, which can be found here is the comprehensive reference for these functions. Note that this documentation includes functions corresponding to instructions that are not supported on lab machines. To avoid seeing these
be sure to check only the boxes labeled “AVX”, “AVX2” and “SSE” through “SSE4.2” on the side.

Intel’s reference generally describes the instructions in pseudocode that uses notation like

a[63:0] := b[127:64]

to represent assigning bits 64 to 127 (inclusive) of a vector b to bits 0 to 63 of a vector a.

Header files

To use the intrinsic functions, you need to include the appropriate header file. For the intrinsics we
will be using this is:

#include <smmintrin.h>
#include <immintrin.h>

Representing vectors in C

To represent 256-bit values that might be stored in one of these registers in C, we will use one of the following types:

  • __m256 (for eight floats)
  • __m256d (for four doubles)
  • __m256i (for integers, no matter the size)

Since each of these is just a 256-bit value, you can cast between these types if a function you want to use expects the “wrong” type of value. For example, you might want to use a function meant to load floating values to load integers. Internally, the functions that expect these types just manipulate 256-bit values in registers or memory.

128-bit versions of types and intrinsics

There are also 128-bit vector types and corresponding instructions. To use this, for the most part, you can replace __m256 with __m128 in the type names and _mm256_ with
_mm_ in the type name.

In some cases, only a 256-bit version of an instruction will exist.

Setting and extracting values

If you want to load a constant in a 128-bit value, you need to use one of the intrinsic functions. You can use one of the functions whose name starts with _mm_setr. For example:

__m256i values = _mm256_setr_epi32(0x1234, 0x2345, 0x3456, 0x4567, 0x5678, 0x6789, 0x789A, 0x89AB);

makes values contain 8 32-bit integers, 0x1234, 0x2345, 0x3456, 0x4567, 0x5678,
0x6789, 0x789A, 0x89AB. We can then extract
each of these integers by doing something like:

int first_value = _mm256_extract_epi32(values, 0);
// first_value == 0x1234
int second_value = _mm256_extract_epi32(values, 1);
// second_value == 0x2345

Note that  it is only possible to pass constant indices to the second argument of
_mm256_extract_epi32 and similar functions.

Loading and storing values

To load an array of values from memory or store an array of values to memory, we can use the intrinsic starting with _mm256_loadu or _mm256_storeu:

int arrayA[8];
_mm256_storeu_si256((__m128i*) arrayA, values);
// arrayA[0] == 0x1234
// arrayA[1] == 0x2345
// ...

int arrayB[8] = {10, 20, 30, 40, 50, 60, 70, 80};
values = _mm256_loadu_si256((__m128i*) arrayB);
// 10 == arrayB[0] == _mm256_extract_epi32(values, 0)
// 20 == arrayB[1] == _mm256_extract_epi32(values, 1)
// ...

Arithmetic

To actually perform arithmetic on values, there are functions for each of the
supported mathematical operations. For example:

__m256i first_values =  _mm256_setr_epi32(10, 20, 30, 40);
__m256i second_values = _mm256_setr_epi32( 5,  6,  7,  8);
__m256i result_values = _mm256_add_epi32(first_values, second_values);
// _mm_extract_epi32(result_values, 0) == 15
// _mm_extract_epi32(result_values, 1) == 26
// _mm_extract_epi32(result_values, 2) == 37
// _mm_extract_epi32(result_values, 3) == 48

Different types of values in vectors

The examples treat the 256-bit values as an array of 8 32-bit integers. There are instructions
that treat in many different types of values, including other sized integers or floating
point numbers. You can usually tell which type is expected by the presence of a something
indicating the type of value in the function names. For example, “epi32” represents
“8 32-bit values” in an __m256 or “4 32-bit values” in an __m128
(The name stands for “extended packed integers, 32-bit”.) Some
other conventions in names you will see:

  • si256 – signed 256-bit integer
  • si128 – signed 128-bit integer
  • epi8, epi32, epi64 — a vector of signed 8-bit integers (32 in a __m256 and 16 in a __m128) or signed 32-bit integers or signed 64-bit integers
  • epu8 — a vector of unsigned 8-bit integers (when there is a difference between what an operation would do with signed and unsigned numbers, such as with conversion to a larger integer or multiplication)
  • epu16, epu32 — an array of unsigned 16-bit integers or 8 unsigned 32-bit integers (when the operation would be different than signed)
  • ps — “packed single” — 8 single-precision floats
  • pd — “packed double” — 4 doubles
  • ss — one float (only 32-bits of a 256-bit or 128-bit value are used)
  • sd — one double (only 64-bits of a 256-bit or 256-bit value are used)

Example (in C)

The following two C functions are equivalent

int add_no_AVX(int size, int *first_array, int *second_array) {
    for (int i = 0; i < size; ++i) {
        first_array[i] += second_array[i];
    }
}

int add_AVX(int size, int *first_array, int *second_array) {
    int i = 0;
    for (; i + 8 <= size; ++i) {
        // load 128-bit chunks of each array
        __m256i first_values = _mm_loadu_si256((__m128i*) &first_array[i]);
        __m256i second_values = _mm_loadu_si256((__m128i*) &second_array[i]);

        // add each pair of 32-bit integers in the 128-bit chunks
        first_values = _mm_add_epi32(first_values, second_values);
        
        // store 128-bit chunk to first array
        _mm_storeu_si128((__m128i*) &first_array[i], first_values);
    }
    // handle left-over
    for (; i < size; ++i) {
        first_array[i] += second_array[i];
    }
}

Selected handy intrinsic functions:

Arithmetic

  • _mm256_add_epi32(a, b) — treats its __m256i arguments as 8 32-bit integers. If a contains the 32-bit integers a0, a1, a2, a3, a4, a5, a6, a7 and b contains
    b0, b1, b2, b3, b4, b5, b6, b7, returns a0 + b0, a1 + b1, a2 + b2, a3 + b3, a4 + b4, a5 + b5, a6 + a6, a7 + b7. (Corresponds to the vpaddd instruction.)
  • _mm256_add_epi16(a, b) — Same as _mm256_add_epi32 but with 16-bit integers. If a contains the 16-bit integers a0, a1, ..., a15 and b contains
    b1, b2, ..., b15, returns a0 + b0, a1 + b1, ..., a15 + b15. (Corresponds to the vpaddw instruction.)
  • _mm256_add_epi8(a, b) — Same as _mm256_add_epi32 but with 8-bit integers.
  • _mm256_mullo_epi16(x, y): treats x and y as a vector of 16-bit signed integers multiplies each pair of integers, and truncates the results to 16 bits.
  • _mm256_mulhi_epi16(x, y): treats x and y as a vector of 16-bit signed integers multiplies each pair of integers to get a 32-bit integer, then returns the top 16 bits of each 32-bit integer result.
  • _mm256_srli_epi16(x, N): treat x and a vector of 16-bit signed integers, and return the result of logically shifting each right by N. (There is also a epi32 and epi64 variant for 32 or 64-bit integers.)
  • _mm256_slli_epi16(x, N): treat x and a vector of 16-bit signed integers, and return the result of shifting each left by N. (There is also a epi32 and epi64 variant for 32 or 64-bit integers.)
  • _mm256_hadd_epi16(a, b) — (“horizontal add”) treats its __m128i arguments as vectors of 16-bit integers. If a contains a0, a1, a2, a3, ..., a15 and b contains b0, b1, b2, b3, ..., b15, returns a0 + a1, a2 + a3, a4 + a5, a6 + a7, b0 + b1, b2 + b3, b4 + b5, b6 + b7, a8 + a9, a10 + a11, a12 + a13, a14 + a15, b8 + b9, b10 + b11, b12 + b13, b14 + b15. Note that this is often substantially slower than _mm_add_epi16. (Corresponds to the vphaddw instruction.)

Load/Store

  • _mm256_loadu_si256, _mm256_storeu_si256 — load or store 256 bits to or from memory. Note that you can use _mm256_storeu_si256 to store into a temporary array as in:
     unsigned short values_as_array[16];
     __m256i values_as_vector;
    
     _mm256_storeu_si128((__m256i*) &values_as_array[0], values_as_vector);
    
  • _mm_loadu_si128, _mm_storeu_si128 — load or store 128 bits to or from memory. (Corresponds to the vmovdqu instruction.) They work exactly like the _mm256_loadu_si256 except that they use the type __m128i instead of __m256i.
  • To store 64 or 32 bits from a vector, one way is to use an extract operation and memcpy:
     unsigned short first_four_values_as_array[4];
     __m256i values_as_vector;
    
     *(long*)(&first_four_values_as_array[0]) = _mm256_extract_epi64(values_as_vetor, 0);
    

    (This code is not actually standard complaint; it violates “strict aliasing” rules. But in the Makefile for the SIMD
    assignments, we’ve disabled this with the compiler option -fno-strict-aliasing. An alternative that wouldn’t violate
    strict-aliasing rules would be to use a union instead of casting a pointer to an int* or to use memcpy, which is
    typically optimized away for small copies.)

  • _mm_cvtsi32_si128: load 32 bits into a 128-bit vector:
     unsigned short values[2];
     __m128i values_as_vector; // only using first 32 bits = 2 shorts
     values_as_vector = _mm_cvtsi32_si128( *(int*) &values[0]);
    

    (This code is not actually standard complaint; it violates “strict aliasing” rules. But in the Makefile for the SIMD
    assignments, we’ve disabled this with the compiler option -fno-strict-aliasing. An alternative that wouldn’t violate
    strict-aliasing rules would be to use a union instead of casting a pointer to an int*.)

  • _mm_cvtsi32_si128: load 64 bits into a 128-bit vector:
     unsigned short values[4];
     __m128i values_as_vector; // only using first 64 bits = 4 shorts
     values_as_vector = _mm_cvtsi64_si128( *(long*) &values[0]);
    

    (This code is not actually standard complaint; see comment above for _mm_cvtsi32_si128)

  • To load 32 or 64 bits in a 256-bit vector, you can use _mm_cvtsi32_si128 or _mm_cvtsi32_si256 together with _mm266_zextsi128_si256 to convert a 128-bit vector to a 256-bit one.
  • _mm256_maskstore_epi32(int *addr, __m256i mask, __m256i a) — store 32-bit values from a at addr, but only the values
    32-bit values that mask specifies. Values are stored if the most significant (i.e. sign) bit of each 32-bit integer in mask
    is set. For example:

     int values[8] = { 0xF, 0xF, 0xF, 0xF, 0xF, 0xF, 0xF, 0xF };
     __m256i a =    __m256_setr_epi32(1,2,3,4,5,6,7,8);
     __m256i mask = __m256_setr_epi32(0,-1,0,0,-1,0,-1,-1);
     _mm256_maskstore_epi32(&values[0], mask, a);
    

    should result in values containing

     { 0xF, 2, 0xF, 0xF, 5, 0xF, 7, 8 }
    
  • For more see the Intel’s reference, under the Load and Store categories

Set constants

  • _mm256_setr_epi32 — returns a __m256i a value containing the specified 32-bit integers. The first integer argument will be in the part of the __m256i that has the lowest address when written to memory. For example:
     __m256i value1 = _mm256_setr_epi16(0, 1, 2, 3, 4, 5, 6, 7);
    

    produces the same result in value1 as in value2 in

     int array[8] = {0, 1, 2, 3, 4, 5, 6, 7};
     __m256i value2 = _mm256_loadu_si256((__m256i*) &array[0]);
    
  • _mm_setr_epi32 — returns a __m128i a value containing the specified 32-bit integers. The first integer argument will be in the part of the __m128i that has the lowest address when written to memory. For example:
     __m128i value1 = _mm_setr_epi32(0, 1, 2, 3);
    

    produces the same result in value1 as in value2 in

     int array[4] = {0, 1, 2, 3, 4, 5, 6, 7};
     __m128i value2 = _mm_loadu_si128((__m256i*) &array[0]);
    
  • _mm256_setr_epi16 — same as _mm256_setr_epi32 but with 16-bit integers. For example:
     __m256i value1 = _mm256_setr_epi16(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15);
    

    produces the same result in value1 as in value2 in

     short array[8] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};
     __m256i value2 = _mm256_loadu_si256((__m256i*) &array[0]);
    
  • _mm256_setr_epi8, _mm_setr_epi8 — same as _mm256_setr_epi32 and _mm_setr_epi32 but with 8-bit integers.
  • _mm_set1_epi32, _mm_set1_epi16, _mm_set1_epi8 — return a __m128i value representing an array of values of the appropriate size, where
    each element of the array has the same value. For example:

     __m128i value = _mm_set1_epi16(42);
    

    has the same effect as:

     __m128i value = _mm_setr_epi16(42, 42, 42, 42,  42, 42, 42, 42);
    
  • _mm256_set_epi8, etc. — same as _mm256_setr_epi8, etc. but takes its arguments in reverse order
  • For more see the Intel’s reference, under the Set category

Extract parts of values

  • _mm256_extract_epi32(a, index) extracts the index‘th 32-bit integer from the 256-bit vector a. The integer with index 0 is the one that will be stored at the lowest memory address if a is copied to memory. index must be a constant.For example
     __m256i a = _mm256_setr_epi32(0, 10, 20, 30, 40, 50, 60, 70);
     int x = _mm256_extract_epi32(a, 2);
    

    assigns 20 to x.

  • _mm_extract_epi32(a, index) extracts the index‘th 32-bit integer from the 128-bit vector a. index must be constant.
  • _mm256_extract_epi16(a, index) is same as _mm256_extract_epi32 but with 16-bit integers
  • _mm256_extracti128_si256(a, index) extract the index 128-bit vector from the 256-bit vector a. index must be constant.For example
     __m256i a = _mm256_setr_epi32(0, 10, 20, 30, 40, 50, 60, 70);
     __m128i result = _mm256_extracti128_si256(a, 1);
    

    is equivalent to

     __m128i result = _mm_setr_epi32(40, 50, 60, 70);
    
  • For more see the Intel’s reference, searching for “extract” or looking under the “Swizzle” and “Cast” categories.

Convert between types of values

  • _mm256_cvtepu8_epi16(eight_bit_numbers): takes a 128-bit vector of sixteen 8-bit numbers, and converts it to a 256-bit vector of sixteen 16-bit signed integers. For example:
     __m128i value1 = _mm_setr_epi8(10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150);
     __m256i value2 = _mm256_cvtepu8_epi16(value1);
    

    results in value2 containing the same value as if we did:

     __m256i value2 = _mm256_setr_epi16(10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150);
    
  • _mm256_packus_epi16(a, b) takes the 16-bit signed integers in the 256-bit vectors a and b and converts
    them to a 256-bit vector of 8-bit unsigned integers.
    The result contains the first 8 integers from a, followed by the first 8 integers from b, followed by
    the last 8 integers from a, followed by the last 8 integers from b.
    Values that are out of range are set to 255 or 0.For example:

     __m256i a = _mm256_setr_epi16(10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160);
     __m256i b = _mm256_setr_epi16(170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 25, 15, 5, -5, -15);
    
     __m256i result = _mm256_packus_epi16(a, b)
    

    sets result the same as if we did:

     __m256i result = _mm256_setr_epu8(
         10, 20, 30, 40, 50, 60, 70, 80, /* first 8 integers from a */
         170, 180, 190, 200, 210, 220, 230, 240, /* first eight integers from b */
         90, 100, 110, 120, 130, 140, 150, /* last 8 integers from a */
         250, 255, 255, 25, 15, 5, 0, 0,  /* last 8 integers from b */
             /* 260, 270 became 255;  -5, -15 became 0 */
     );
    
  • _mm256_zextsi128_si256(a) takes a 128-bit vector a and converts it to a 256-bit vector by adding 0s.
  • For more see the Intel’s reference under the “Swizzle” and “Move” and “Cast” categories.

Rearrange 256-bit values

  • _mm256_permute2x128_si128(a, b, mask) takes two 256-bit vectors a and b and combines the
    128-bit halves of these vectors into a new 256-bit vector according to mask.
    mask is a one-byte integer constant. The least significant nibble specifies the value placed into the lowest
    address of the resulting vector, the most significant nibble specifies the value placed into the highest address
    of the resulting vector.The value chosen by each nibble of the mask is:

    • 0 to select the first 128 bits of a
    • 1 to select the second 128 bits of a
    • 2 to select the first 128 bits of b
    • 3 to select the second 128 bits of b
    • 4 through 15 to select the constant 0 (ignoring the values of a and b)

    For example, to repeat the second 128-bits of a, one would supply a mask of 0x11 like the following
    example:

     __m256i a = _mm256_setr_epi32(0, 1, 2, 3, 4, 5, 6, 7);
     __m256i b = _mm256_setr_epi32(8, 9, 10, 11, 12, 13, 14, 15);
     __m256i result = _mm256_permute2x128_si128(a, b, 0x11);
     // result == _mm256_setr_epi32(4, 5, 6, 7, 4, 5, 6, 7)
    

    To produce a result with the first 128-bits of a followed by the second 128-bits of b one
    would supply a mask like 0x30:

     __m256i a = _mm256_setr_epi32(0, 1, 2, 3, 4, 5, 6, 7);
     __m256i b = _mm256_setr_epi32(8, 9, 10, 11, 12, 13, 14, 15);
     __m256i result = _mm256_permute2x128_si128(a, b, 0x30);
     // result == _mm256_setr_epi32(0, 1, 2, 3, 12, 13, 14, 15)
    
  • _mm256_unpackhi_epi16(a, b) interleaves the 16-bit integers from the top quarter of each 128-bit half of
    the 256-bit vectors a and b. For example:

     __m256i a = _mm256_setr_epi16(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15);
     __m256i b = _mm256_setr_epi16(16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31);
     __m256i result = _mm256_unpackhi_epi16(a, b);
    

    is the same as

     __m256i result = _mm256_setr_epi16(
         /* top quarter of first half of a and b */
         4, 20, 5, 21, 6, 22, 7, 23,
         /* top quarter of second half of a and b */
         12, 28, 13, 29, 14, 30, 15, 31
     )
    
  • _mm256_unpacklo_epi16(a, b) is like _mm256_unpackhi_epi16 but it takes the 16-bit integers from
    the bottom quarter of each half of a and b
  • _mm256_permutevar8x32_epi32(x, indexes) — Produce a vector of 32-bit values by, for each 32-bit index in the vector indexes,
    retrieving the 32-bit value at that index from the vector x and place it in the result. For example:

     __m256i x = _mm256_setr_epi32(10, 20, 30, 40, 50, 60, 70, 80)
     __m256i indexes = _mm256_setr_epi32(3, 3, 0, 1, 2, 3, 6, 7);
     __m256i result = _mm256_permutevar8x32_epi32(x, indexes)
    

    is the same as:

     __m256i reuslt = _mm256_setr_epi32(40, 40, 10, 20, 30, 70, 80);
    
  • For more see the Intel’s reference under the “Swizzle” and “Move” and “Cast” and “Shift” categories.

Rearrange 128-bit values

  • _mm_unpackhi_epi16(a, b) interleaves the 16-bit integers from the top half of the 128-bit vectors
    a and b. For example:

     __m128i a = _mm_setr_epi16(0, 1, 2, 3, 4, 5, 6, 7);
     __m128i b = _mm_setr_epi16(8, 9, 10, 11, 12, 13, 14, 15);
     __m256i result = _mm_unpackhi_epi16(a, b);
    

    is the same as

     __m128i result = _mm_setr_epi16(
         4, 20, 5, 21, 6, 22, 7, 23,
     )
    
  • _mm_shuffle_epi8(a, mask) rearrange the bytes of a according to mask and return the result. mask is a vector of 8-bit integers (type __m128i) that indicates how to rearrange each byte:
    • if a byte in the mask has the high bit set (is greater than 127), then the corresponding byte of the output is 0;
    • otherwise, the byte number specified in the input is copied to the corresponding byte of the output. Bytes are numbered using 0 to represent the byte that would be stored in the lowest address if the vector were copied to memory.

    For example:

     __m128i value1 = _mm_setr_epi8(10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160);
     __m128i mask = _mm_setr_epi8(0x80, 0x80, 0x80, 5, 4, 3, 0x80, 7, 6, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80);
     __m128i value2 = _mm_shuffle_epi8(value1, mask);
    

    should produce the same result as:

     __m128i value2 = _mm_setr_epi8(0, 0, 0, 60, 50, 40, 0, 80, 70, 0, 0, 0, 0, 0, 0, 0, 0);
         /* e.g. since 3rd element of mask is 5, 3rd element of output is 60, element 5 of the input */
    
  • For more see the Intel’s reference under the “Swizzle” and “Move” and “Cast” and “Shift” categories.

Example (assembly instruction)

The instruction

    paddd %xmm0, %xmm1

takes in two 128-bit values, one in the register %xmm0, and another in the register %xmm1. Each
of these registers are treated as an array of two 64-bit values. Each pair of 64-bit values is added
together, and the results are stored in %xmm1.

For example, if %xmm0 contains the 128-bit value (written in hexadecimal):

0x0000 0000 0000 0001 FFFF FFFF FFFF FFFF 

and %xmm1 contains the 128-bit value (written in hexadecimal):

0xFFFF FFFF FFFF FFFE 0000 0000 0000 0003 

Then %xmm0 would be treated as containing the numbers 1 and -1 (or 0xFFFFFFFFFFFFFFFF), and
%xmm1 as containing the numbers -2 and 3. paddd would add 1 and -2 to produce -1 and
-1 and 3 to produce 2, so the final value of %xmm1` would be:

0xFFFF FFFF FFFF FFFF 0000 0000 0000 0002

If we interpret this value as an array of two 64-bit integers, then that would be -1 and 2.