Daniel Lemire is a software performance expert. He ranks among the top 2% of scientists globally (Stanford/Elsevier 2025) and is one of GitHub's top 1000 most followed developers.
Visit lemire.me → RSS feedOn Hacker News May 2014 – Sep 2026
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Modern processors are superscalar, meaning that they can execute many instructions at once. For example, some processors can retire four or six instructions per cycle. Furthermore, many of these processors can initiate instructions out-of-order: they can start working on instructions that appear much later in the…
Suppose you want to pick an integer at random in a set of N elements. Your computer has functions to generate random 32-bit integers, how do you transform such numbers into indexes no larger than N? Suppose you have a hash table with a capacity N. Again, you need to transform your hash values (typically 32-bit ……
In most systems, integers are stored using a fixed binary representation. It is common to store integers using 32-bit or 64-bit words. You sometimes need to convert it to a string. For example, the integer 12345 might need to be converted to the five characters '12345'. In an earlier blog post, I presented the simpler…
Most systems today rely on Unicode strings. However, we have two popular Unicode formats: UTF-8 and UTF-16. We often need to convert from one format to the other. For example, you might have a database formatted with UTF-16, but you need to produce JSON documents using UTF-8. This conversion is often called…
In C, the length of a string in marked by a 0 byte at the end of the string. Thus to determine the length of the string, one must scan it, looking for the 0 byte. Recent ARM processors have a powerful instruction set (SVE) that is well suited for such problems. It allows you … Continue reading Implementing ‘strlen’…
The performance of a processor is determined by several factors. For example, processors with a higher frequency tend to do more work per unit of time. Physics makes it difficult to produce processors that have higher frequency. Modern processors can execute many instructions per cycle. Thus a 3.4GHz processor has 3.4…
The C++ library has long been organized around stream classes, at least when it comes to reading and parsing strings. But streams can be surprisingly slow. For example, if you want to parse numbers, then this C++ routine is close to being the worst possible choice for performance: std::stringstream in(mystring)…
Integers in programming languages have a valid range but arithmetic operations can result in values that exceed such ranges. For example, adding two large integers can result in an integer that cannot be represented in the integer type. We often refer to such error conditions as overflows. In a programming languages…
Up to yesterday, my laptop was a large 15-inch MacBook Pro. It contains an Intel Kaby Lake processor (3.8 GHz). I just got a brand-new 13-inch 2020 MacBook Pro with Apple's M1 ARM chip (3.2 GHz). How do they compare? I like precise data points. Recently, I have been busy benchmarking number parsing routines where you…
Suppose that I give you a relatively long string and you want to remove all spaces from it. In ASCII, we can define spaces as the space character (' '), and the line ending characters ('\r' and '\n'). I am mostly interested in algorithmic and performance issues, so we can simplify the problem by removing all ……