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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We sometimes have to look for a value in a sorted array. The simplest algorithm consists in just going through the values We sometimes have to look for a value in a sorted array. The simplest algorithm consists in just going through the values one by one, until we encounter the value, or exhaust the … Continue reading…
Programmers often need to 'filter out' data. Suppose that you are given a database of users where only a small percentage are 'paying customers' (say 5% or less). You can write an SQL query to check whether a given user is indeed a paying customer, but it might require a round trip to your database … Continue reading…
One of the most common "data type" in programming is the text string. When programmers think of a string, they imagine that they are dealing with a list or an array of characters. It is often a "good enough" approximation, but reality is more complex. The characters must be encoded into bits in some way. Most ……
Last year (2019), we released the simjson library. It is a C++ library available under a liberal license (Apache) that can parse JSON documents very fast. How fast? We reach and exceed 3 gigabytes per second in many instances. It can also parse millions of small JSON documents per second. The new version is much ……
Modern processors have instructions to process several bytes at once. Effectively all processors have the capability of processing 16 bytes at once. These instructions are called SIMD, for single instruction, multiple data. It was once an open question whether these instructions could be useful to accelerate common…
When programming, we often need to write 'generic' functions where the exact data type is not important. For example, you might want to write a simple function that sums up numbers. Go lacked this notion until recently, but it was recently added (as of version 1.18). So I took it out for a spin. In … Continue reading…
Let us say that I ask you to find the number I am thinking about between -1000 and 1000, by repeatedly guessing a number. With each guess, I tell you whether your guess is correct, smaller or larger than my number. A binary search algorithm tries to find a value in an interval by repeating … Continue reading Fast…
In software, it is common to represent time as a time-stamp string. It is usually specified by a time format string. Some standards use the format %Y%m%d%H%M%S meaning that we print the year, the month, the day, the hours, the minutes and the seconds. The current time as I write this blog post would be … Continue…
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…
One of the established and most popular programming languages is the C programming language. It is relatively easy to learn, and highly practical. Maybe surprisingly, the C programming language keeps evolving, slowly and carefully. If you have GCC 13 or LLVM (Clang) 16, you already have a compiler with some of the…
Computers store data as streams of bits. Binary files like image, audio or video files are allowed to contain just about any sequence of bits. However, we also often use text formats; for example, web pages and emails are required to be text. So how do we send images by email? How do we embed … Continue reading…
Sometimes programmers want to prune out characters from a string of characters. For example, maybe you want to remove all line-ending characters from a piece of text. Let me consider the problem where I want to remove all spaces (' ') and linefeed characters ('\n' and '\r'). How would you do it efficiently? size_t…
Most commodity processors support single-precision IEEE 754 floating-point numbers. Though they are ubiquitous, they are often misunderstood. One of my readers left a comment suggesting that picking an integer in [0,232) at random and dividing it by 232, was equivalent to picking a number at random in [0,1). I am not…
Almost all our computers are made of several processing cores. Thus it can be efficient to "parallelize" expensive processing in a multicore manner. That is, instead of using a single core to do all of the work, you divide the work among multiple cores. A standard way to approach this problem is to create threads. ……
Developers often believe that software performance follows a Pareto distribution: 80% of the running time is spent in 20% of the code. Using this model, you can write most of your code without any care for performance and focus on the narrow pieces of code that are performance sensitive. Engineers like Casey Muratori…
In the C programming language, we allocate memory dynamically (on the heap) using the malloc function. You pass malloc a size parameter corresponding to the number of bytes you need. The function returns either a pointer to the allocated memory or the NULL pointer if the memory could not be allocated. Or so you may ……
Many recent Intel processors benefit from a new family of instructions called AVX-512. These instructions operate over wide registers (up to 512 bits) and follow the Single instruction, multiple data (SIMD) paradigm. These new AVX-512 instructions allow you to break some speed records, such as decoding base64 data at…
There are various ways in software to handle error conditions. In C or Go, one returns error code. Other programming languages like C++ or Java prefer to throw exceptions. One benefit of using exceptions is that it keeps your code mostly clean since the error-handling code is often separate. It is debatable whether…
If I give a programmer a string such as "9223372036854775808" and I ask them to convert it to an integer, they might do the following in C++: std::string s = .... uint64_t val; auto [ptr, ec] = std::from_chars(s.data(), s.data() + s.size(), val); if (ec != std::errc()) {} // I have an error ! // val … Continue reading…
In my blog post, My programming setup, I stressed how important regular expressions are to my programming activities. Regular expressions can look intimidating and outright ugly. However, they should not be underestimated. Someone asked for examples of regular expressions that I rely upon. Here a few. It is commonly…
One of my favorite languages is the Go language. I love its simplicity. It is popular and useful in a cloud setting. Many popular tools are written in Go, and for good reasons. I gave a talk on Go last year and I was asked for a criticism of Go. I do not mind that … Continue reading The Go compiler needs to be smarter
I know that floating-point arithmetic is a bit crazy on modern computers. For example, floating-point numbers are not associative: 0.1+(0.2+0.3) == 0.599999999999999978 (0.1+0.2)+0.3 == 0.600000000000000089 But, at least, this is fairly consistent in my experience. You should simply not assume fancy properties like…
Modern processors have powerful vector instructions which allow you to load several values at once, and operate (in one instruction) on all these values. Similarly, they allow you to have vector constants. Thus if you wanted to add some integer (say 10001) to all integers in a large array, you might first load a…
At my university, one of our internal software systems allows a professor to submit a revision to a course. The professor might change the content or the objectives of the course. In a university, professors have extensive freedom regarding course content. As long as you reasonably meet the course objectives, you can…
When receiving bytes from the network, we often assume that they are unicode strings, encoded using something called UTF-8. Sadly, not all streams of bytes are valid UTF-8. So we need to check the strings. It is probably a good idea to optimize this problem as much as possible. In earlier work, we showed that ……
I learned to program with BASIC back when I was twelve. I would write elaborate programs and run them. Invariably, they would surprise me by failing to do what I expect. I would struggle for a time, but I'd eventually give up and just accept that whatever "bugs" I had created were there to stay. … Continue reading I…
In software, it is a common problem to want to remove specific characters from a string. To make the problem precise, let us consider the removal of all ASCII control characters and spaces. In practice, it means the removal of all byte values smaller or equal than 32. I covered a related problem before, the … Continue…
I wanted to know how a modern C compiler like clang would process the following C code: #include <stdint.h> int count(uint64_t x) { int v = 0; while(x != 0) { x &= x - 1; v++; } return v; } Can you guess? popcntq %rdi, %rax That is right. A fairly sophisticated C function, … Continue reading The surprising cleverness…
The division instruction is one of the most expensive instruction in your CPU. Thus optimizing compilers often compile divisions by known constants down to a multiplication followed by a shift. However, in some lucky cases, the compiler does not even need a shift. I call the corresponding divisors ideal. For the math…
Most strings found on the Internet are encoded using a particular unicode format called UTF-8. However, not all strings of bytes are valid UTF-8. The rules as to what constitute a valid UTF-8 string are somewhat arcane. Yet it seems important to quickly validate these strings before you consume them. In a previous…