LLMs and the rise of the AI code generators

When I wrote about GitHub Copilot in November 2021, Copilot was one of only a handful of AI code generation technologies available. I tested it as a Visual Studio Code extension. At the time, Copilot didn’t always generate good, correct, or even running code, but it was still somewhat useful. The great promise behind Copilot (and other code generators that use machine learning) is that it was designed to improve over time, both by incorporating user feedback and by ingesting new code samples into its training corpus.

As of May 2023, there are hundreds of “AI” or “code generation” extensions available for Visual Studio Code alone. Several of these might save you some time while coding, but if you believe their generated code without reviewing, testing, and debugging it, I have a bridge to sell you.

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One promising development in this area is that several tools have automatic generation of unit tests. Generating unit tests is a much more tractable problem than generating general-purpose code—in fact, it can be done using simple patterns—but you’ll still have to review and run the generated tests to see whether they make sense.

In the balance of this article, I’ll offer a brief history of language models, before surveying the state-of-the-art large language models (LLMs), such as OpenAI’s GPT family and Google’s LaMDA and PaLM, used for text generation and code generation today. We’ll finish with a quick tour of 10 code generation tools, including Amazon CodeWhisperer, Google Bard, and GitHub Copilot X.

A brief history of AI models for text generation

Language models go back to Andrey Markov in 1913. That area of study is now called Markov chains, a special case of Markov models. Markov showed that in Russian, specifically in Pushkin’s Eugene Onegin, the probability of a letter appearing depends on the previous letter, and that, in general, consonants and vowels tend to alternate. Markov’s methods have since been generalized to words, to other languages, and to other language applications.

Markov’s work was extended by Claude Shannon in 1948 for communications theory, and again by Fred Jelinek and Robert Mercer of IBM in 1985 to produce a language model based on cross-validation (which they called deleted estimates), and applied to real-time, large-vocabulary speech recognition. Essentially, a statistical language model assigns probabilities to sequences of words.

To quickly see a language model in action, type a few words into Google Search, or a text message app on your smartphone, and allow it to offer auto-completion options.

In 2000 Yoshua Bengio et al. published a paper on a neural probabilistic language model in which neural networks replace the probabilities in a statistical language model, bypassing the curse of dimensionality and improving the word predictions (based on previous words) over a smoothed trigram model (then the state of the art) by 20% to 35%. The idea of feed-forward, auto-regressive, neural network models of language is still used today, although the models now have billions of parameters and are trained on extensive corpora, hence the term “large language models.”

As we’ll see, language models have continued to get bigger over time to make them perform better. There are costs to this, however. The 2021 paper On the Dangers of Stochastic Parrots: Can Language Models Be Too Big? by Emily Bender, Timnit Gebru, et al. questions whether we are going too far with this trend. The authors suggest, among other things, that we should weigh the environmental and financial costs first, and invest resources into curating and carefully documenting data sets rather than ingesting everything on the web.

Large language models for text generation

The recent explosion of large language models was triggered by the 2017 paper Attention is All You Need, by Ashish Vaswani et al. of Google Brain and Google Research. That paper introduced “a new simple network architecture, the Transformer, based solely on attention mechanisms, dispensing with recurrence and convolutions entirely.” Transformer models are both simpler than and superior to recurrent and convolutional models. They also require significantly less time to train.

ELMo

ELMo is a 2018 deep contextualized word representation from AllenNLP (see ELMo paper) that models both complex characteristics of word use (e.g., syntax and semantics) and how these uses vary across linguistic contexts (i.e., to model polysemy). The original model has 93.6 million parameters, and was trained on the One Billion Word Benchmark.

BERT

BERT is a 2018 language model from Google AI Language based on the company’s Transformer (2017) neural network architecture (see BERT paper). BERT is designed to pretrain deep bidirectional representations from unlabeled text by jointly conditioning on both left and right context in all layers. The two model sizes used in the original paper were 100 million and 340 million total parameters. BERT uses masked language modeling (MLM) in which ~15% of tokens are “corrupted” for training. It was trained on English Wikipedia plus the Toronto Book Corpus.

T5

The 2020 Text-To-Text Transfer Transformer (T5) model from Google (see T5 paper) synthesizes a new model based on the best transfer learning techniques from GPT, ULMFiT, ELMo, and BERT and their successors, using a new open-source, pre-training data set called the Colossal Clean Crawled Corpus (C4). The standard C4 for English is an 800 GB data set based on the Common Crawl data set. T5 reframes all natural language processing tasks into a unified text-to-text format where the input and output are always text strings, in contrast to BERT-style models that output only a class label or a span of the input. The base T5 model has about 220 million total parameters.

GPT family

OpenAI, an AI research and deployment company, has a mission “to ensure that artificial general intelligence (AGI) benefits all of humanity.” Of course, OpenAI hasn’t achieved AGI yet. And some AI researchers, such as machine learning pioneer Yann LeCun of Meta-FAIR, believe that OpenAI’s current approach to AGI is a dead end.

OpenAI is responsible for the GPT family of language models, which are available via the OpenAI API and Microsoft’s Azure OpenAI Service. Note that the entire GPT family is based on Google’s 2017 Transformer neural network architecture, which is legitimate because Google open-sourced Transformer.

GPT (Generative Pretrained Transformer) is a 2018 model from OpenAI that uses about 117 million parameters (see GPT paper). GPT is a unidirectional transformer that was pre-trained on the Toronto Book Corpus and was trained with a causal language modeling (CLM) objective, meaning that it was trained to predict the next token in a sequence.

GPT-2 is a 2019 direct scale-up of GPT with 1.5 billion parameters, trained on a data set of eight million web pages, or ~40 GB of text data. OpenAI originally restricted access to GPT-2 because it was “too good” and would lead to “fake news.” The company eventually relented, although the potential social problems became even worse with the release of GPT-3.

GPT-3 is a 2020 autoregressive language model with 175 billion parameters, trained on a combination of a filtered version of Common Crawl, WebText2, Books1, Books2, and English Wikipedia (see GPT-3 paper). The neural network used in GPT-3 is similar to that of GPT-2, with a couple of additional blocks.

The biggest downside of GPT-3 is that it tends to “hallucinate,” in other words make up facts with no discernable basis. GPT-3.5 and GPT-4 have the same problem, albeit to a lesser extent.

CODEX is a 2021 descendent of GPT-3 that was fine-tuned for code generation on 54 million open-source GitHub repositories. It is the model used in GitHub Copilot, which I discuss in the next section.

GPT-3.5 is a set of 2022 updates to GPT-3 and CODEX. The gpt-3.5-turbo model is optimized for chat but also works well for traditional completion tasks.

GPT-4 is a 2023 large multimodal model (accepting image and text inputs, emitting text outputs) that OpenAI claims exhibits human-level performance on various professional and academic benchmarks. GPT-4 outperformed GPT-3.5 in a number of simulated exams, including the Uniform Bar Exam, the LSAT, the GRE, and several AP subject exams.

It’s of serious concern that OpenAI hasn’t explained how GPT-4 was trained; the company says it’s for competitive reasons, which makes some sense given the competition between Microsoft (who has been funding OpenAI) and Google. Still, not knowing the biases in the training corpus means that we don’t know the biases in the model. Emily Bender’s take on GPT-4 (posted on Mastodon on March 16, 2023) is that “GPT-4 should be assumed to be toxic trash until and unless #OpenAI is *open* about its training data, model architecture, etc.”

ChatGPT and BingGPT are chatbots that were originally based on gpt-3.5-turbo and in March 2023 upgraded to use GPT-4. Currently, to access the version of ChatGPT based on GPT-4, you need to subscribe to ChatGPT Plus. The standard ChatGPT, based on GPT-3.5, was trained on data that cut off in September 2021. BingGPT, which you can access in the Microsoft Edge browser, was also trained on data that cut off in 2021, but says (when you ask it) that “I am constantly learning and updating my knowledge with new information from the web.”

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In early March 2023, Pascale Fung of the Centre for Artificial Intelligence Research at the Hong Kong University of Science & Technology gave a talk on ChatGPT evaluation. It’s well worth spending the hour to watch it.

LaMDA

LaMDA (Language Model for Dialogue Applications), Google’s 2021 “breakthrough” conversation technology, is a 2017 Transformer model trained on dialogue, and fine-tuned to significantly improve the sensibleness and specificity of its responses. One of LaMDA’s strengths is that it can handle the topic drift that is common in human conversations.

A version of LaMDA powers Bard, Google’s conversational AI service. Bard was released March 21, 2023 and made generally available on May 10, 2023. I discuss its code generation capabilities below.

PaLM

PaLM (Pathways Language Model) is a 2022 dense decoder-only Transformer model from Google Research with 540 billion parameters, trained with the Pathways system (see PaLM paper). PaLM was trained using a combination of English and multilingual data sets that include high-quality web documents, books, Wikipedia, conversations, and GitHub code.

Google also created a “lossless” vocabulary for PaLM that preserves all whitespace (especially important for code), splits out-of-vocabulary Unicode characters into bytes, and splits numbers into individual tokens, one for each digit. PaLM-Coder is a version of PaLM 540B fine-tuned on a Python-only code data set.

PaLM-E

PaLM-E is a 2023 “embodied” (for robotics) multimodal language model from Google. The researchers began with PaLM, a powerful large language model, and embodied it (the “E” in PaLM-E) by complementing it with sensor data from the robotic agent. PaLM-E is also a generally capable vision-and-language model. In addition to PaLM, it incorporates the ViT-22B vision model.

LLaMA

LLaMA (Large Language Model Meta AI) is a 65 billion parameter “raw” large language model released by Meta AI (aka Meta-FAIR) in February 2023. According to Meta, “Training smaller foundation models like LLaMA is desirable in the large language model space because it requires far less computing power and resources to test new approaches, validate others’ work, and explore new use cases. Foundation models train on a large set of unlabeled data, which makes them ideal for fine-tuning for a variety of tasks.”

LLaMA was released at several sizes, along with a model card that details how the model was built. Originally, you had to request the checkpoints and tokenizer, but they are in the wild now, as a downloadable torrent was posted on 4chan by someone who properly obtained the models by filing a request, according to Yann LeCun of Meta-FAIR.

Specialized code generation products

While several large language models including ChatGPT and Bard can be used for code generation as released, it helps if they are fine-tuned on some code, typically from free open-source software to avoid overt copyright violation. That still raises the specter of “open-source software piracy,” which is the claim of a 2022 federal class-action lawsuit against GitHub, Microsoft (owner of GitHub), and OpenAI about the GitHub Copilot product and the OpenAI GPT Codex model.

Note that in addition to using AI models trained largely on publicly available code, some code generation tools rely on searching code-sharing sites such as Stack Overflow.

Amazon CodeWhisperer

Amazon CodeWhisperer integrates with Visual Studio Code and JetBrains IDEs, generates code suggestions in response to comments and code completions based on existing code, and can scan code for security issues. You can also activate CodeWhisperer for use inside AWS Cloud9 and AWS Lambda.

CodeWhisperer supports the Python, Java, JavaScript, TypeScript, and C# programming languages well, and another 10 programming languages to a lesser degree. It’s available free for individual developers and costs $19 per user per month for professional teams.

CodeWhisperer helped me to write the Python code shown below. I reviewed, tested, and debugged it, and it’s fine.

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Bard

Bard support for programming was announced April 21, 2023. The announcement notes support for more than 20 programming languages including C++, Go, Java, JavaScript, TypeScript, and Python. As a quick test, I asked Bard to “write a Go function to return the current date and time.” It did so quickly:

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Not only did Bard write the function, it also explained the function and generated an example of calling the function. 

CodeT5

CodeT5 is a 2021 code-specific unified pre-trained encoder-decoder Transformer model from Salesforce AI Research. It’s based on the 2020 Google T5 model architecture, and fine-tuned on the CodeSearchNet data set plus some C/C# code from BigQuery. The official PyTorch implementation of CodeT5 resides on GitHub, and two checkpoints are available at Hugging Face, with links in the GitHub README.

GitHub Copilot

When I reviewed a pre-release version of GitHub Copilot in November 2021, I found that, while it didn’t always generate good, correct, or even running code, it was still somewhat useful. Copilot is based on OpenAI Codex, which is in turn based on GPT-3, which was fine-tuned for code generation on 54 million open-source GitHub repositories. GitHub Copilot currently costs $10 per month or $100 per year, unless you qualify for a free version. 

I like the way that Copilot works within Visual Studio Code. You basically have to write the first line of a function, or a comment describing the function, and Copilot will generate up to 10 versions of the function that you can use as is, edit, or not use. As I noted above, you should take any code generated by Copilot with a grain of salt, as it does tend to hallucinate, for example in the code comments in lines 8 and 9 of the example shown below.

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GitHub Copilot X

GitHub Copilot X, currently in technical preview, is based on GPT-4. It “levels up” the original Copilot with chat and terminal interfaces, the ability to generate unit tests, the ability to generate pull request descriptions, and the ability to extract explanations from documentation.

GitHub Copilot X is greatly improved over the original GitHub Copilot, and can sometimes generate a correct function and set of tests without much human help. It still makes mistakes and hallucinates, but not nearly as much as its predecessor. For reference, my writeup on the original Copilot is here.

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IntelliSense and IntelliCode

Microsoft IntelliSense is a built-in capability of Visual Studio and Visual Studio Code that uses language semantics to offer a menu of choices for short code completions. It often works well for helping you find the API or method call you want, but tends to offer many choices.

IntelliCode is an add-in enhancement to IntelliSense that uses AI running on your local machine to detect your code context—including variable names, functions, and the type of code you’re writing—to give you the best suggestions, and in some cases give you whole-line completions. IntelliCode can also help you clean up repetitive code and recommend quick actions for common programming tasks.

IntelliCode works with C#, C++, Java, SQL, and XAML in Visual Studio 2022, and with TypeScript, JavaScript, and Python in Visual Studio Code.

Kite

Kite was an early attempt at using AI to help developers write code, operating from 2014 to 2021. While it attracted over 500K developers, it never generated any revenue. The Kiteco repositories contain most of its source code, but there were private bits that have been replaced with XXXXX, so some of the code won’t run.

PolyCoder

PolyCoder is a 2022, 2.7 billion parameter open-source large language model for code generation from Carnegie Mellon University (see paper). It’s based on the GPT-2 model architecture and trained on 249 GB of code across 12 programming languages. In the C programming language, PolyCoder outperforms all models including Codex.

Replit Ghostwriter

Replit Ghostwriter, released on Halloween 2022, offers five functions: code completion, code explanation, code transformation, code generation, and error detection with debugging, for $10 per month (more or less, depending on how many “cycles” you use). It integrates with the Replit online editor (only) and supports Python, Ruby, JavaScript, TypeScript, HTML, CSS, Go, Lisp, Haskell, Bash, C, C++, Rust, Java, and JSON.

According to Replit, Ghostwriter “returns results generated from large language models trained on publicly available code and tuned by Replit.” Replit doesn’t specify either the LLMs or the training corpora it uses for Ghostwriter, which opens it up to the same accusation that Emily Bender made about GPT-4: You should assume Ghostwriter to be toxic trash until and unless Replit is open about its training data, model architecture, etc. It also opens Replit up to the same accusations of “open-source software piracy” making their way through the courts about GitHub Copilot.

Tabnine

Tabnine, from a company of the same name in Tel Aviv, looks like IntelliSense on steroids, and can optionally train on your own code corpus as well as on open-source code. It does whole-line and full-function code completions in your editor or IDE, with support for 20 such tools, from Visual Studio Code and IntelliJ to Emacs and Vim.

Depending on the plan you choose, Tabnine can use a generic AI model trained on open-source code with permissive licenses, or a set of generative AI models optimized for all programming languages, “specialized to match your tech stack,” or a private code model trained on your own repositories.

Tabnine’s free Starter plan only does basic code completion. The Pro plan does whole-line and full-function code completions for $12 per user per month. Tabnine has not disclosed its model architecture or training corpora. So, by the Emily Bender principle, you should assume the worst about any code it generates.

Large language models can sometimes work to generate or complete code, whether or not they’ve been trained on code corpora. Language models that have been trained on code tend to know more about the importance of whitespace. And code generation products such as OpenAI Codex and Tabnine often have better integrations with programming editors than more generic language models.

We should expect AI code generators to improve with time and use. GitHub Copilot X is better than the original Copilot, and I’m confident the next Copilot will be better still. Nevertheless, you can never assume that code generated by AI of any kind is correct or efficient, or even that it will compile and run. You should always treat AI-generated code like a pull request from an unknown programmer, which means reviewing it, testing it, and debugging it before making it part of your application.