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Link time optimization (LTO) is LLVM's way of implementing whole-program optimization. Cross-language LTO is a new feature in the Rust compiler that enables LLVM's link time optimization to be performed across a mixed C/C++/Rust codebase. It is also a feature that beautifully combines two respective strengths of the Rust programming language and the LLVM compiler platform: Rust, with its lack of a

Link time optimization (LTO) is LLVM's way of implementing whole-program optimization. Cross-language LTO is a new feature in the Rust compiler that enables LLVM's link time optimization to be performed across a mixed C/C++/Rust codebase. It is also a feature that beautifully combines two respective strengths of the Rust programming language and the LLVM compiler platform: Rust, with its lack of a

PerformanceWe conducted extensive A/B testing of performance. Performance telemetry numbers are about the same for MSVC-built and clang-built Chrome – some metrics get better, some get worse, but all of them are within 5% of each other. The official MSVC builds used LTCG and PGO, while the Clang builds currently use neither of these. This is potential for improvement that we look forward to explor

This blog post is part of a series of blog posts from students who were funded by the LLVM Foundation to attend the 2016 LLVM Developers' Meeting in San Jose, CA. Please visit the LLVM Foundation's webpage for more information on our Travel Grants program. This post is from Piotr Padlewski on his work that he presented at the meeting: This blogpost will show how C++ devirtualization is performed i

Historically, LLVM's major releases always added "0.1" to the version number, producing major versions like 3.8, 3.9, and 4.0 (expected by March 2017). With our next release though, we're changing this.  The LLVM version number will now increase by "1.0" with every major release, which means that the first major release after LLVM 4.0 will be LLVM 5.0 (expected September 2017). We believe that thi

We have been working hard for a few months now to rewrite the ELF support in lld, the LLVM linker. We are happy to announce that it has reached a significant milestone: it is now able to bootstrap LLVM, Clang, and itself and pass all tests on x86-64 Linux and FreeBSD with the speed expected of an LLVM project. ELF is the standard file format for executables on Unix-like systems, such as Linux and

OpenMP support in Clang compiler is completed! Every pragma and clause from 3.1 version of the standard is supported in full, including combined directives (like ‘#pragma omp parallel for’ and ‘#pragma omp parallel sections’).  In addition, some elements of OpenMP 4.0 are supported as well. This includes “almost complete” support for ‘#pragma omp simd” and full support for ‘#pragma omp atomic’  (c

We've spoken in the past about teaching Clang to fully support Windows and be compatible with MSVC.  Until now, a big missing piece in this story has been debugging the clang-generated executables.  Over the past 6 months, we've started working on making LLDB work well on Windows and support debugging both regular Windows programs and those produced by Clang. Why not use an existing debugger such

Chrome 38 was released early October 2014. It is the first release where the Linux binaries shipped to users are built by clang. Previously, this was done by gcc 4.6. As you can read in the announcement email, the switch happened without many issues. Performance stayed roughly the same, binary size decreased by about 8%. In this post I'd like to discuss the motivation for this switch. MotivationTh

I would like to give a brief update regarding vectorization in LLVM. When LLVM 3.2 was released, it featured a new experimental loop vectorizer that was disabled by default. Since LLVM 3.2 was released, we have continued to work hard on improving vectorization, and we have some news to share. First, the loop vectorizer has new features and is now enabled by default on -O3. Second, we have a new SL

Since the design document for cpp11-migrate, the C++11 migrator tool, was first proposed in early December 2012 development has been making steady progress. In this article I'll talk about what's been implemented in cpp11-migrate so far, what's coming up, and how you can get involved. The purpose of the C++11 Migrator is to do source-to-source translation to migrate existing C++ code to use C++11

One of the most pervasive IR (and thus compiler API) changes in LLVM 3.0 was a complete reimplementation of the LLVM IR type system. This change was long overdue (the original type system lasted from LLVM 1.0!) and made the compiler faster, greatly simplified a critical subsystem of VMCore, and eliminated some design points of IR that were frequently confusing and inconvenient. This post explains

Google has one of the largest monolithic C++ codebases in the world. We have thousands of engineers working on millions of lines of C++ code every day. To help keep the entire thing running and all these engineers fast and productive we have had to build some unique C++ tools, centering around the Clang C++ compiler. These help engineers understand their code and prevent bugs before they get to ou

What Every C Programmer Should Know About Undefined Behavior #3/3 In Part 1 of the series, we took a look at undefined behavior in C and showed some cases where it allows C to be more performant than "safe" languages. In Part 2, we looked at the surprising bugs this causes and some widely held misconceptions that many programmers have about C. In this article, we look at the challenges that compil

What Every C Programmer Should Know About Undefined Behavior #2/3 In Part 1 of our series, we discussed what undefined behavior is, and how it allows C and C++ compilers to produce higher performance applications than "safe" languages. This post talks about how "unsafe" C really is, explaining some of the highly surprising effects that undefined behavior can cause. In Part #3, we talk about what f

What Every C Programmer Should Know About Undefined Behavior #1/3 People occasionally ask why LLVM-compiled code sometimes generates SIGTRAP signals when the optimizer is turned on. After digging in, they find that Clang generated a "ud2" instruction (assuming X86 code) - the same as is generated by __builtin_trap(). There are several issues at work here, all centering around undefined behavior in

The LLVM Machine Code (aka MC) sub-project of LLVM was created to solve a number of problems in the realm of assembly, disassembly, object file format handling, and a number of other related areas that CPU instruction-set level tools work in. It is a sub-project of LLVM which provides it with a number of advantages over other compilers that do not have tightly integrated assembly-level tools. This

In addition to parsing and generating machine code for your source files when valid, a compiler frontend's job is also to detect invalid code and give you a hint that explains what is wrong so you can fix the problem. The bug could either be straight-up invalid (an error) or could just be something that is legal but looks really dubious (a warning). These errors and warnings are known as compiler

Disassemblers make binary analysis work. With a reliable disassembler, you can solve high-level problems like tracing back through a program's call stack or analyzing sample-based profiles to low-level problems like figuring out how your compiler unrolled a tight floating-point loop or what advantages declaring a variable const actually had at the other end of the optimization chain. A reliable di

LLVM Project News and Details from the Trenches LLVM Fortran Levels Up: Goodbye flang-new, Hello flang!LLVM has included a Fortran compiler “Flang” since LLVM 11 in late 2020. However, until recently the Flang binary was not flang (like clang) but instead flang-new. LLVM 20 ends the era of flang-new. Read more…GSoC 2024: Improve Clang DocHi, my name is Peter, and this year I was involved in Google