Daniel Lemire's blog

How fast is C++26’s std::hive? 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, so scanning it does not require you to chase a pointer for every element. Like a list, it never moves an element once it has been inserted: your pointers, references and iterators stay val…

Memory-level parallelism: AMD is the king 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. Memory latency has not improved in ten years. The 2016 Broadwell answers a random access in 100 ns. The 2025 Turin, with DDR5-6400 and every advantage of a decade of progress, take…

Does a PhD Pay Off? 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 money. Individuals who complete doctorates tend to have higher cognitive abilities and greater motivation. But smarter people tend to earn more, period. So do people with a PhD earn more? Historicall…

Using AI to build your own software 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 days. Initially, he asked for my help, could I do the manual labor? I spent 15 minutes writing a script with ImageMagick that processed all the images in seconds, but in a completely a…

X just gave us an interface that AI agents can use. I pointed it at my own posts. 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 not measurements. And until recently, digging into your own posting data meant either clicking around the web UI or writing custom script…

Chatting with AI Won’t Make You a Top Programmer When I was a kid, most people did not know how to type. We took typing class. The final exam was a speed test: words per minute. Today, you will not impress anyone by saying you can type. In fact, cursive writing is fading. Kids increasingly cannot read or write it. We type constantly. We forget how many skills are learned, and how often some of these skills have fade…

Parsing JSON at compile time with C++26 static reflection Suppose that you have a configuration file in JSON. Something like this: {"width":1920,"height":1080,"fullscreen":true,{:space:}"title":"My{:space:}Game","volume":0.8}{:space:} Normally you ship this file alongside your program, open it at startup, read it, and parse it. That is a lot of work for data that never changes. What if the file is fixed at build tim…

Sovereign The keyword in politics these days is ‘sovereign’. What few will admit is that it is effectively the adoption of the American strategy: Make America Great Again. In other words, reindustrialization of key sectors of the economy. The UK used to be a computing champion. Our chip designs (ARM) originated from the UK. Canada had BlackBerry, everyone was using Canadian phones. Like Canada, many countries have p…

How much do amd64 microarchitecture levels help in Go? Our 64-bit Intel and AMD processors have evolved over decades. When you compile a Go program for a 64-bit Intel or AMD processor, the compiler targets, by default, a nearly 20-year-old instruction set. The binary that comes out runs on essentially any x64 chip, but it also leaves on the table every instruction that was added since 2003. We often refer to microar…

Only 17% of all 64-bit Integers are products of two 32-bit integers In software programming, the product between two integers is often computed to a fixed number of bits with overflow. Consider 8-bit integers. If you multiply 127 by 127, you get back the number 1 as an 8-bit unsigned integer, with an overflow. The actual full product is 16129. To represent 16129, you typically use 16 bits of precision. Thus we have …

SIMD-accelerated integer-to-string conversion Converting a 64-bit integer to its decimal string representation is a mundane task that shows up everywhere: logging, JSON serialization, CSV output, debug prints, etc. In C++, you might use std::to_chars, sprintf, or some library routine. How do these functions work? At a high level, they repeatedly divide by ten. Start with your integer k. Divide it by ten, use the rem…

Checking multiplication overflow Suppose that x is a variable of an unsigned type. In C/C++, it could be of type size_t for example. You have an expression like 6 * x and you want to know whether 6 * x overflows. That is, you want to know if 6 * x exceeds the range of values that can be represented by the type. In most cases, a variable of type size_t will be about to represent all values in the range [0, 2^64-1]. I…