Can I compare different compilers or language versions on Compiler Explorer?

Updated

Yes. Compiler Explorer (rust.godbolt.org) is built around exactly that use case: you can add multiple compilers or language versions in the same session and view their outputs side by side. The comparison works best when you keep the source code identical and change only one variable at a time — the compiler, the version, the optimization level, or the target platform.

How the comparison model works

The interface is organized as a set of panes. On the left is a source editor; on the right is one or more output panes, each tied to a specific compiler. When you add another compiler, you get another output pane, and both compile the same source. This is the core mechanism that makes side-by-side comparison possible.

The site describes itself as "an interactive online compiler which shows the assembly output of compiled C++, Rust, Go (and many more) code." The same source-to-output pipeline applies regardless of which language you select.

Adding compilers and language versions

Use the Add new... control to create additional compiler entries. Each entry has its own compiler selection, so you can:

  • Pick two different compilers for the same language (for example, different Rust toolchain versions, or Rust versus another language).
  • Pick the same compiler with different settings.
  • Mix languages entirely, since the site supports C++, Rust, Go, and many others.

Once added, each compiler gets its own output pane, and all panes recompile when you edit the source.

What you can compare

Dimension How to change it What you observe
Compiler / toolchain Select a different compiler in a pane Assembly or IR differences between implementations
Language version Choose a different compiler version Codegen changes across releases
Optimization level Set compiler flags / optimization options How much the optimizer transforms the code
Target platform Change the target in compiler options Architecture-specific instruction selection
Output type Use the output selector Assembly, binary object, LLVM IR, Rust MIR, Rust HIR, and more

The output selector is worth noting for Rust work specifically: the page evidence lists Rust MIR, Rust HIR, and Rust Macro Expansion as available outputs, alongside the standard assembly and LLVM IR views. That means you can compare not just final assembly but intermediate representations across compilers or versions.

A practical comparison workflow

  1. Write or paste your source into the editor.
  2. Add a second compiler with Add new....
  3. Set each pane's compiler and version to the two you want to compare.
  4. Set the same optimization flags on both so the only difference is the compiler.
  5. Read the two output panes side by side.

Expected result: two independent outputs for identical input, so any difference you see comes from the compiler, version, or flags — not from the code.

Common sticking points

  • Different flags on each pane. If you change optimization on one side only, you are comparing flags, not compilers. Keep flags matched unless flags are the variable under test.
  • Output type mismatch. Comparing one pane's assembly against another pane's LLVM IR is not a like-for-like comparison. Match the output type first.
  • Target differences. A different target architecture changes instruction selection, so pin the target when the compiler is the variable.
  • JavaScript requirement. The page states that "without Javascript the regular website is not functional" and points to a separate noscript version. The interactive multi-pane comparison depends on the regular site.

When this is the right tool

Compiler Explorer fits when you want to see what the compiler produced — assembly, IR, or macro expansion — for a small, self-contained piece of code, and you want that view for more than one compiler or version at once. It is a codegen inspection and comparison tool, not a benchmark harness: the page evidence shows execution and runtime tooling options, but the primary value for comparison is the static output panes.