Daniel Lemire

The simdjson library is a C++ library to parse and generate JSON. It is used in Node.js, ClickHouse, Meta Velox, StarRocks, Apache Doris, the Ladybird browser and many other systems. We released versi...

C# strings are UTF-16. Most characters are one 16-bit code unit. Characters outside the basic multilingual plane, emoji included, take two: a high surrogate (U+D800 to U+DBFF) followed by a low surrog...

To get a PhD, you typically have to enroll in a graduate program. Then you complete a few relatively easy courses.   You might need to pass a comprehensive examination that checks whether you have a b...

We create new strings all the time. How quickly can you produce short strings in your programming languge? To create a meaningful string, we convert an integer to a string. In Python, that is str(i)....

Mainstream economists have been advocating for the marvellous effects of global trade for decades. Who needs these factory jobs anyway? We’ll be designing the robots and the nuclear rockets. Exc...

I maintain and comaintain several open-source libraries. Some of them are widely used: ada parses URLs in Node.js, fast_float parses numbers in GCC’s standard library and in Chromium, simdjson p...

Our processors can execute many instructions per cycle; they are superscalar. But not all instructions are equal. Branches are particularly tricky. A branch occurs often when you use an if-then clause...

If AI is causing mass unemployment among software developers, it is not showing up in the data yet.   The USA had more software developers as a percentage of the population in 2025 than in 2021.   On...

I see the mathematicians panicking at what we can do with agentic AI. Let us be clear on what we are talking about. In a few weeks, someone who is not a high-level mathematician can, with AI, produce...

Recently, I looked at how AMD made its chips 50% faster within a span of two years, in How did AMD Ryzen get 50% faster in two years? Many people asked me to cover Apple processors. So let us go. Cons...

ARM processors, like those in your phone, have instructions capable of processing several elements at once (SIMD). These instructions are called NEON. But many newer processors have a different SIMD e...

People still tell me that CPUs are boring. That nothing much happens anymore. Let us look at AMD Ryzen 7 processors from 2022 to 2024: the 5800X3D (Zen 3), the 7800X3D (Zen 4) and the 9800X3D (Zen 5)....

C++23 added a new type to the standard library: std::flat_map. There is also a std::flat_set and other variants, but let me focus on std::flat_map. A flat map is a sorted vector of keys next to a vect...

We represent floating-point numbers using the IEEE standard. For very small numbers, the standard uses special subnormal numbers. Unfortunately, they have a reputation of making operations slow. Thus...

Mathematicians are unhappy about OpenAI. Several influential mathematicians wrote an open letter. The gist of their argument is that they form a community that trains young people. When AI started pro...

We are told that AI entities much like ChatGPT might soon kill us all. The statement is vague and unfalsifiable. It might be true, it might be false. People with credentials (e.g., Turing Award recipi...

If you write in C, by default, you use a single thread. Extra cores do not help until you create more threads. However, if you include the header <threads.h>, you can pass a function to thrd_c...

In the late 1960s and 1970s, people like David Parnas faced a problem. A decade earlier there were almost no programmers. Suddenly there were hordes of inexperienced ones. What could have been a golde...

In Python, the dict data structure is the conventional key-value structure. E.g., you might store a list of names as keys and have their phone numbers as values. Valentin Ignatev wrote this amusing po...

JSON is a standard format for data interchange. It is effectively a tiny subset of JavaScript made of objects and arrays. It looks as follows {"key":1, "text":[1.0,2.0]}. Many programming languages in...

In Java, you find the location of a substring using indexOf. String haystack = "The quick brown fox jumps over the lazy dog"; String needle = "fox"; int index = haystack.indexOf(needle); Naively, you...

We are all familiar with IP addresses such as 192.168.0.1. They are typically written as four numbers in the range 0 to 255 inclusive, separated by dots. In C#, you can parse them with the standard li...

Like most programming languages, Go has both stack allocations, whose lifetime is limited to the current function, and dynamic (or heap) allocations. The name stack comes from the fact that the memory...

AI-assisted programming is fast evolving and there is a tension between ‘we no longer need to understand the code’ and ‘what is my purpose as a programmer’. I recorded a short...

When a compiler optimizes your program, it has to guess. Which functions are worth inlining? Which side of a branch is the common one? Which method does this interface call actually reach? At compile...

C++26 adds a new container to the standard library: std::hive. It is meant to occupy the ground between std::vector and std::list. Like a vector, it keeps its elements in contiguous blocks of memory,...

When your program asks for memory that is not in cache, the processor has to go to RAM. That trip costs on the order of 100 nanoseconds. On a 3 GHz core, that is about 300 cycles of doing nothing. Mem...

Every week, I discuss with people who want to get a PhD. For years, I have been advising people not to pursue a PhD. It may come as a surprise to some. You would expect people with a PhD to earn more...

In 1945, Vannevar Bush published a report entitled Science: The Endless Frontier. His thesis was that prosperity follows from basic research. The report was highly influential in the United States and...

A few years ago, a friend of mine was stuck. He needed to quickly process over a hundred high-quality images according to a complicated sequence. He was using Photoshop, but it was going to take him d...

I have been on X for a long time. Like most people who post regularly, I have a gut feeling for what might interest people. I post in the morning. Longer posts seem to do better. But gut feelings are...