Kim Walisch kimwalisch

ARM’s Scalable Vector Extensions: A Critical Look at SVE2 For Integer Workloads

Scalable Vector Extensions (SVE) is ARM’s latest SIMD extension to their instruction set, which was announced back in 2016. A follow-up SVE2 extension was announced in 2019, designed to incorporate all functionality from ARM’s current primary SIMD extension, NEON (aka ASIMD).

Despite being announced 5 years ago, there is currently no generally available CPU which supports any form of SVE (which excludes the [Fugaku supercomputer](https://www.fujitsu.com/global/about/innovation/

What is the Strict Aliasing Rule and Why do we care?

(OR Type Punning, Undefined Behavior and Alignment, Oh My!)

What is strict aliasing? First we will describe what is aliasing and then we can learn what being strict about it means.

In C and C++ aliasing has to do with what expression types we are allowed to access stored values through. In both C and C++ the standard specifies which expression types are allowed to alias which types. The compiler and optimizer are allowed to assume we follow the aliasing rules strictly, hence the term strict aliasing rule. If we attempt to access a value using a type not allowed it is classified as undefined behavior(UB). Once we have undefined behavior all bets are off, the results of our program are no longer reliable.

Unfortunately with strict aliasing violations, we will often obtain the results we expect, leaving the possibility the a future version of a compiler with a new optimization will break code we th

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	/// @file segmented_sieve.cpp
	/// @author Kim Walisch, <kim.walisch@gmail.com>
	/// @brief This is a simple implementation of the segmented sieve of
	/// Eratosthenes with a few optimizations. It generates the
	/// primes below 10^9 in 0.8 seconds (single-threaded) on an
	/// Intel Core i7-6700 3.4 GHz CPU.
	/// @license Public domain.

	#include <iostream>
	#include <algorithm>

	#include <stdio.h>
	#include <string.h>
	#include <stdlib.h>

	#define ARRAY_STRING

	#if defined ARRAY_STRING
	#define ARRAY_PRINT "s"
	typedef char* array_t;
	#else

	# Xcode 4.3.3
	Apple clang version 3.1 (tags/Apple/clang-318.0.61) (based on LLVM 3.1svn)
	Target: x86_64-apple-darwin11.4.0
	Thread model: posix

	# Xcode 4.3.2
	Apple clang version 3.1 (tags/Apple/clang-318.0.58) (based on LLVM 3.1svn)
	Target: x86_64-apple-darwin11.4.0
	Thread model: posix

	/*
	* Copyright (c) 2009-2017, Farooq Mela
	* All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions are met:
	*
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. Redistributions in binary form must reproduce the above copyright