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Fabian Giesen posts on Mastodon on the use of Intel’s AVX-512. We analyse and explain these one by one:

https://mastodon.gamedev.place/@rygorous/110572829749524388

In computing, an "unsigned integer compare" operation refers to comparing two unsigned integer values. Unsigned integers are numbers that represent non-negative values only, ranging from 0 to a maximum value determined by the number of bits used to store the integer.

On a normal CPU, unsigned integer comparison typically takes 2 or 3 instructions. Here's a general breakdown of the steps involved:

The number of instructions and their specific execution time can vary depending on the processor architecture, instruction set, and optimization techniques used by the compiler. Some CPUs may have dedicated instructions for unsigned integer comparison, while others may rely on more general-purpose instructions.

Now, regarding the faster way to achieve unsigned integer comparison on an Intel CPU with AVX-512, AVX-512 (Advanced Vector Extensions 512-bit) is an extension of the x86 instruction set architecture used in certain Intel processors. AVX-512 provides enhanced vector processing capabilities, including support for wider SIMD (Single Instruction, Multiple Data) operations.

In an Intel CPU with AVX-512, SIMD instructions can be used to perform parallel unsigned integer comparison on multiple elements at once. This can lead to faster execution compared to scalar operations on individual elements.

With AVX-512, unsigned integer comparison can be achieved using SIMD instructions such as vpcmpud (compare packed unsigned doubleword integers) or vpcmpuq (compare packed unsigned quadword integers). These instructions allow for simultaneous comparison of multiple unsigned integers in vector registers, reducing the number of instructions required and improving performance.

An unsigned int to float conversion operation is the conversion of an unsigned integer to a floating-point number. This can be used to store an integer value in a floating-point variable, or to perform mathematical operations on an integer value using floating-point arithmetic.

On a normal CPU, an unsigned int to float conversion operation typically takes 2 or 3 instructions to achieve. This is because the CPU must first load the unsigned integer into a register, and then it must convert the integer to a floating-point number.

However, on an Intel CPU with AVX-512, an unsigned int to float conversion operation can be achieved in a faster way. This is because AVX-512 is a set of instructions that allows the CPU to perform multiple operations simultaneously.

Specifically, AVX-512 has a instruction called vcvttps2uqq that can be used to convert an unsigned integer to a float. This instruction can convert 8 unsigned integers to floats simultaneously, which means that it can perform an unsigned int to float conversion operation in a single instruction.

As a result, AVX-512 can significantly speed up the execution of code that performs unsigned int to float conversions.

Here is an example of how an unsigned int to float conversion operation can be performed using AVX-512:

// Load the unsigned integer into a register. unsigned int a = 10; // Convert the unsigned integer to a float using AVX-512. float b = _mm512_cvttps2uqq_ps(a); // Print the float. printf("%f\n", b); 

In this example, the _mm512_cvttps2uqq_ps instruction converts the unsigned integer to a float and stores the result in the b variable. The printf() function then prints the float value.

Down converts (narrowing) in computing is the process of reducing the precision of a data type. This can be done for a number of reasons, such as to save memory or to improve performance.

For example, a 32-bit floating-point number can be down converted to a 16-bit floating-point number. This would reduce the precision of the number, but it would also reduce the amount of memory required to store the number.

The Intel AVX-512 instruction set helps with down converts by providing a number of instructions that can be used to perform these operations efficiently. For example, the vcvtps2dq instruction can be used to down convert a 32-bit floating-point number to a 32-bit integer.

This can be helpful for a number of applications, such as image processing and video encoding. In these applications, it is often necessary to down convert data types in order to save memory or improve performance.

The VPTERNLOGD instruction of bitwise logic is a bitwise ternary logic instruction that can be used to fuse two or three operations in one using Intel's AVX-512.

The VPTERNLOGD instruction takes three operands: a source operand, a control operand, and a mask operand. The source operand is a 512-bit vector of integers. The control operand is a 3-bit integer that specifies the ternary logic operation to be performed. The mask operand is a 512-bit vector of bits that indicates which elements of the source operand should be used in the operation.

The VPTERNLOGD instruction performs the following steps:

The VPTERNLOGD instruction can be used to fuse two or three operations in one by using the following techniques:

• Combining bitwise operations: The VPTERNLOGD instruction can be used to combine two or three bitwise operations by using the control operand to specify the ternary logic operation to be performed. For example, the VPTERNLOGD instruction can be used to combine a AND operation, a OR operation, and a XOR operation by setting the control operand to 010.

• Combining bitwise operations with memory accesses: The VPTERNLOGD instruction can be used to combine bitwise operations with memory accesses by using the mask operand to specify which elements of the source operand should be used in the operation. For example, the VPTERNLOGD instruction can be used to load a value from memory, perform a bitwise operation on the value, and then store the result back to memory by setting the mask operand to the memory address of the value.

The VPTERNLOGD instruction can be a valuable tool for improving the performance of applications that perform a lot of bitwise operations. By fusing two or three operations in one, the VPTERNLOGD instruction can reduce the number of instructions that need to be executed, which can lead to significant performance improvements.

The VPTEST Intel AVX-512 operation is a logical comparison instruction that can be used to compare two vectors of integers. It is useful for logical comparison instead of arithmetic for vectors because it can be used to perform a variety of logical comparisons, such as equality, inequality, greater than, less than, and so on.

The VPTEST instruction takes two operands: a source operand and a mask operand. The source operand is a 512-bit vector of integers. The mask operand is a 512-bit vector of bits that indicates which elements of the source operand should be used in the comparison.

The VPTEST instruction performs the following steps:

The VPTEST instruction can be used to replace what used to take 2 or 3 instruction sequences because it can perform a logical comparison in a single instruction. For example, the VPTEST instruction can be used to replace the following two instructions:

Code snippet

// Compare two vectors of integers. int a[8]; int b[8]; int result = 0; for (int i = 0; i < 8; i++) { result |= a[i] == b[i]; } 

with the following single instruction:

Code snippet

int result = _mm512_testc_epi32(a, b); 

The VPTEST instruction can be a valuable tool for improving the performance of applications that perform a lot of logical comparisons. By performing a logical comparison in a single instruction, the VPTEST instruction can reduce the number of instructions that need to be executed, which can lead to significant performance improvements.

Here are some of the benefits of using the VPTEST instruction:

• Increased performance: The VPTEST instruction can perform logical comparisons more efficiently than traditional instructions. This can lead to significant performance improvements in applications that perform a lot of logical comparisons.

• Reduced instruction count: The VPTEST instruction can reduce the number of instructions that need to be executed. This can be helpful for applications that are memory-constrained.

• Improved accuracy: The VPTEST instruction can improve the accuracy of logical comparisons. This can be helpful for applications that require high precision.

Overall, the VPTEST instruction can be a valuable tool for performing logical comparisons in computing. It can improve performance, reduce instruction count, and improve accuracy.

The VRANGEPS Intel AVX-512 operation is a range reduction instruction that can be used to find the minimum or maximum value of a vector of floating-point numbers. It is useful not only for range reduction, but also because it can do multiple things including max-abs of multiple operands.

The VRANGEPS instruction takes two operands: a source operand and a mask operand. The source operand is a 512-bit vector of floating-point numbers. The mask operand is a 512-bit vector of bits that indicates which elements of the source operand should be used in the range reduction.

The VRANGEPS instruction performs the following steps:

The VRANGEPS instruction can be used to do multiple things, including:

• Find the minimum or maximum value of a vector of floating-point numbers: The VRANGEPS instruction can be used to find the minimum or maximum value of a vector of floating-point numbers by setting the mask operand to all 1s or all 0s, respectively.

• Find the max-abs of a vector of floating-point numbers: The VRANGEPS instruction can be used to find the max-abs of a vector of floating-point numbers by setting the mask operand to a vector of bits that indicates which elements of the source operand are negative.

The VRANGEPS instruction can be a valuable tool for improving the performance of applications that perform a lot of range reduction operations. By performing a range reduction in a single instruction, the VRANGEPS instruction can reduce the number of instructions that need to be executed, which can lead to significant performance improvements.

Here are some of the benefits of using the VRANGEPS instruction:

• Increased performance: The VRANGEPS instruction can perform range reduction more efficiently than traditional instructions. This can lead to significant performance improvements in applications that perform a lot of range reduction operations.

• Reduced instruction count: The VRANGEPS instruction can reduce the number of instructions that need to be executed. This can be helpful for applications that are memory-constrained.

• Improved accuracy: The VRANGEPS instruction can improve the accuracy of range reduction operations. This can be helpful for applications that require high precision.

Overall, the VRANGEPS instruction can be a valuable tool for performing range reduction in computing. It can improve performance, reduce instruction count, and improve accuracy.

The VRNDSCALE Intel AVX-512 operation is a rounding instruction that can be used to round a floating-point number to a fixed-point number with a specified precision. It is useful for rounding to fixed point with a specified precision because it can be used to perform a round, multiply, and multiply in a single instruction.

The VRNDSCALE instruction takes three operands: a source operand, a scale factor, and a rounding mode. The source operand is a 512-bit vector of floating-point numbers. The scale factor is a 32-bit integer that specifies the precision of the rounding. The rounding mode is a 3-bit integer that specifies the rounding mode to be used.

The VRNDSCALE instruction performs the following steps:

The VRNDSCALE instruction can be used to replace three operations (mul, round, mul) in one because it can perform all three operations in a single instruction. For example, the VRNDSCALE instruction can be used to replace the following three instructions:

Code snippet

// Round to fixed point with precision 8. float a = 1.5; int b = round(a); int c = b * 8; 

with the following single instruction:

Code snippet

int c = _mm512_rndscale_epi32(a, 8, _MM_ROUND_NEAREST); 

The VRNDSCALE instruction can be a valuable tool for improving the performance of applications that perform a lot of rounding operations. By performing a rounding in a single instruction, the VRNDSCALE instruction can reduce the number of instructions that need to be executed, which can lead to significant performance improvements.

Here are some of the benefits of using the VRNDSCALE instruction:

• Increased performance: The VRNDSCALE instruction can perform rounding more efficiently than traditional instructions. This can lead to significant performance improvements in applications that perform a lot of rounding operations.

• Reduced instruction count: The VRNDSCALE instruction can reduce the number of instructions that need to be executed. This can be helpful for applications that are memory-constrained.

• Improved accuracy: The VRNDSCALE instruction can improve the accuracy of rounding operations. This can be helpful for applications that require high precision.

Overall, the VRNDSCALE instruction can be a valuable tool for performing rounding in computing. It can improve performance, reduce instruction count, and improve accuracy.

Beyond the curtain, we list a few Bioinformatics applications that benefit from these type of operations, such as short-read aligners and BAM alignment tools…

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