Name: Simplify all libc++ containers without breaking the ABI, with Nikolas Klauser
Start: 2025-06-05T17:00:00.000Z
Location: echtzeit.dev Office

Michel is joining us to talk about his work that he will be presenting at PLDI'25.

As usual there will be pizza, snacks, beer, and soft drinks after the event for participants, sponsored by NextSilicon.

**Astract:** Alias analysis is a fundamental compiler analysis that powers numerous optimizations. While research has focused on deriving more precise alias information assuming that the compiler will optimize better, recent work shows a negligible, or even negative, performance impact of alias information. In this work, we shift the perspective from refining to relaxing alias information, i.e., removing information, to complement existing work and challenge that assumption systematically. Our study on a state-of-the-art compiler, LLVM, running the SPEC CPU 2017 benchmark suite, shows (1) a small overall impact—removing alias analysis entirely has little impact on the final binaries, (2) few influential queries—only a small fraction, namely ∼3%, of the alias information leads to changes in the final binary, and (3) lost potential—random relaxations can reduce execution time by 21% and binary size by 39% for certain cases, suggesting that compilers could better utilize alias information. Through this work, we advocate that it is beneficial for future research to avoid simply refining the general precision of alias analysis, but also to explore how to find and refine the most relevant queries, and how to more effectively utilize alias information.

**Location:** The event is taking place in the room G51 in the CAB building of ETH Zurich at the Zentrum campus. Enter from Universitätstrasse 6.

Compiler Social Zürich

Johannes de Fine Licht

lorenzo chelini

Michel Weber

Tobias Gysi

Christian Ulmann

zero9178

Ioannis Magkanaris

Michal Terepeta

Mehdi Amini

Hans Viessmann

Abhiroop Sarkar

Adam Siemieniuk

Baidyanath Kundu

Philip Metzger

Pengcheng

Relaxing Alias Analysis - Exploring the Unexplored Space

ETH CAB G51

In our June meetup [Nikolas Klauser](https://github.com/philnik777) presents his work on libc++ and how it allowed him to simplify all libc++ containers without breaking the ABI.

In LLVM 20, libc++ replaced __compressed_pair with [[no_unique_address]]. __compressed_pair has been used in many containers, as well as unique_ptr and shared_ptr. Removing __compressed_pair has significantly simplified the library and reduced the size of chromium's debug builds by about 5%. I will talk about why keeping the ABI stable is important, what to look out for when modifying ABI-sensitive code, and how I managed to remove __compressed_pair from libc++.

Small snacks and drinks will be available. There is always space for outside topics and discussions. If you have specific questions, please bring them. Looking forward to meeting everyone again!

LLVM Social Berlin

Stefan Gränitz

AlexDenisov

Morris

Cassio

JulianKuners

Amir Alp

Stefan

Yegor

Omar

Matthias S.

Kornél Lehőcz

Jonas Dohse

Daniil

Alex

Vlad

Bruno

Eugene Zakh

Mark M

Ben Clifford

Simplify all libc++ containers without breaking the ABI, with Nikolas Klauser

echtzeit.dev Office

LLVM Project

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