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Rust language want to take into how to#
Rust language want to take into software#
Rust language want to take into code#

Yet, looking at the diagram it seems like that shouldn't be too hard to change, right?Ĭross-language link time optimizationImplementing cross-language LTO is conceptually simple because the feature is built on the shoulders of giants.

Rust language want to take into code#

Therefore, the Rust code is opaque to the optimization taking place at link time.

bc (opt) -+ - +Īs you can see our Rust code is still compiled to a regular object file. With these capabilities in place a new compilation workflow with LTO enabled for C++ code looks like this: the linker, again via the LLVM linker plugin, merges all bitcode modules it encounters and then runs LLVM optimization passes before doing the actual linking.

Rust language want to take into how to#

the linker, via the LLVM linker plugin, knows how to read LLVM bitcode modules like regular object files, and.the compiler translates each compilation unit into LLVM bitcode (i.e.Here is how LTO is concretely implemented: Performing LLVM work at the linking stage is facilitated via a plugin to the linker. the whole set of compilation units) is available at once and thus optimizations across compilation unit boundaries become possible. Why the linking stage? Because that is the point in the pipeline where the entire program (i.e. Link time optimization in LLVMThe basic principle behind LTO is that some of LLVM's optimization passes are pushed back to the linking stage. To enable inlining and optimizations to happen across compilation unit boundaries, LLVM supports link time optimization. The optimizer does not know the definition of functions inside of other compilation units and thus cannot inline them or make other kinds of decisions based on what they actually do. As you can see, each compilation unit is optimized in isolation. This is the regular compilation workflow if no kind of LTO is involved. Finally, the linker will take the set of object files and link them together into a binary:.LLVM then lowers each module into machine code so that we get one object file per module:.

In the next step, LLVM's optimization pipeline will optimize each LLVM module in isolation:.

In Rust each crate is translated into at least one compilation unit. In C and C++ each source file will result in a single compilation unit.

The compiler front-end generates an LLVM bitcode module (.

To explain how cross-language LTO works it is useful to take a step back and review how traditional compilation and "regular" link time optimization work in the LLVM world.īackground - A bird's eye view of the LLVM compilation pipelineClang and the Rust compiler both follow a similar compilation workflow which, to some degree, is prescribed by LLVM: As a consequence, we at Mozilla's Low Level Tools team took it upon ourselves to implement it in the Rust compiler.

Rust language want to take into software#

From a psychological perspective, which arguably is just as important, it helps to alleviate the nagging feeling of inefficiency that many performance conscious developers might have when working on a piece of software that jumps back and forth a lot between functions implemented in different source languages.īecause Firefox is a large, performance sensitive codebase with substantial parts written in Rust, cross-language LTO has been a long-time favorite wish list item among Firefox developers.

From a technical perspective it allows for codebases to be optimized without regard for implementation language boundaries, making it possible for important optimizations, such as function inlining, to be performed across individual compilation units even if, for example, one of the compilation units is written in Rust while the other is written in C++.

So, what does cross-language LTO do? There are two answers to that:

LLVM, as a language agnostic foundation, provides a common ground where the source language a particular piece of code was written in does not matter anymore.

Rust, with its lack of a language runtime and its low-level reach, has an almost unique ability to seamlessly integrate with an existing C/C++ codebase, and.

It is also a feature that beautifully combines two respective strengths of the Rust programming language and the LLVM compiler platform: 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.

Link time optimization (LTO) is LLVM's way of implementing whole-program optimization. LLVM Project News and Details from the TrenchesĬlosing the gap: cross-language LTO between Rust and C/C++