To inspect both LLVM IR and assembly from a standalone Rust source file, run rustc --emit=llvm-ir,asm main.rs. For a Cargo package, select the target and forward the options to rustc: cargo rustc --bin my_program -- --emit=llvm-ir,asm. The output depends on the compiler, target, optimization settings, and other code-generation options, so choose and record those settings before comparing results.
Emit LLVM IR and assembly from a standalone Rust file
Run this from the directory containing the source file:
rustc --emit=llvm-ir,asm main.rsRustc writes textual LLVM IR and target assembly. By default, the filenames are CRATE_NAME.ll and CRATE_NAME.s, where CRATE_NAME is the crate name. To choose predictable filenames, specify a path for each output:
rustc --emit=llvm-ir=out.ll,asm=out.s main.rsThe --emit option accepts comma-separated output kinds. Alongside llvm-ir and asm, the rustc reference lists outputs such as MIR, LLVM bitcode, object files, metadata, and linked artifacts. These are different stages or forms of compiler output, not interchangeable names for assembly.
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Print a text output to standard output
Use - as the output path to send one text output type to stdout:
rustc --emit=llvm-ir=- main.rs
rustc --emit=asm=- main.rsRustc permits only one output type to go to stdout in a single invocation. Run the commands separately, or write both outputs to files, if you need IR and assembly together.
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Inspect output from a Cargo package
Use cargo rustc and put rustc options after Cargo’s -- separator. Select the package target you want to compile:
cargo rustc --bin my_program -- --emit=llvm-ir,asmFor a library target, use:
cargo rustc --lib -- --emit=llvm-ir,asmIf a package has multiple targets, choose the relevant one with an option such as --bin or --lib. Cargo forwards the arguments after -- to the final compiler invocation for that selected package target, not to dependency compiler invocations, as the Cargo cargo rustc reference explains. Cargo may still compile dependencies as part of the build; do not assume these forwarded emit options produce matching output for every dependency.
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Use Cargo’s release profile
To emit output while selecting Cargo’s release profile, add --release:
cargo rustc --release --bin my_program -- --emit=llvm-ir,asmThis is useful when you want to inspect the package under its release-profile settings rather than the default development profile. For standalone rustc, -O selects optimization level 3:
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rustc -O --emit=llvm-ir,asm main.rsRustc also documents optimization levels 0, 1, 2, 3, s, and z; level 0 is the default. Cargo profile settings and direct rustc flags are separate ways to select build settings, so use the one appropriate to the command you are running. See the rustc code-generation options reference for the documented options.
Choose the view you need
| Output | What it shows | Useful for |
|---|---|---|
LLVM IR (llvm-ir) |
The compiler’s LLVM-level representation. | Inspecting operations and lowering before final target instructions. |
Assembly (asm) |
Instructions and conventions for the selected target. | Seeing the target-specific machine-language listing emitted by the compiler. |
These outputs answer different questions. LLVM IR is not a universal, source-level description of a Rust function; assembly is not architecture-neutral. The rustc code-generation options reference documents settings that affect code generation, but there is no single canonical listing that applies to every compiler and target.
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See LLVM IR before LLVM optimization passes
For a specialized view of IR before LLVM’s usual optimization passes, rustc documents this combination:
rustc -O -C no-prepopulate-passes --emit=llvm-ir main.rsThe no-prepopulate-passes option uses an empty pass list instead of the usual pre-populated list. In combination with -O and LLVM IR emission, it can show IR before LLVM applies its usual optimizations. This is not the same view as ordinary optimized backend output; use it when that distinction is relevant to your investigation.
Make comparisons meaningful
A listing describes one compilation under particular conditions, not what Rust always emits. Optimization may inline, simplify, or remove code from a given output. Assembly also varies with the target. Before comparing listings, keep the source and relevant settings controlled, and record:
- The rustc version.
- The target triple.
- The optimization level or Cargo profile.
- Relevant code-generation options, including target CPU or target features where applicable.
- Whether LLVM’s usual optimization passes were enabled.
Use the same values for each compilation in a comparison. Otherwise, a difference may come from a different target or compiler setting rather than the source change you intended to examine.
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The official Rust and Cargo manuals are rolling documentation, and available options can vary by toolchain. For details about the compiler installed on your machine, check rustc --help and rustc -C help. The commands here follow the documented rustc and Cargo interfaces; the exact files and listing contents depend on your compiler and target.
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