rust-lang/rust · error

epilogue for {:?}

Error message

epilogue for {:?}

What it means

Thrown by rustc_codegen_cranelift when emitting the inline-asm epilogue that restores the base pointer and callee-saved register after a user asm! block returns. As with the prologue, only X86_64, AArch64, and RiscV64 have restore sequences; any other InlineAsmArch hits `_ => unimplemented!("epilogue for {:?}", arch)`. It fires only for asm! blocks that fall through to a return (non-noreturn).

Source

Thrown at compiler/rustc_codegen_cranelift/src/inline_asm.rs:708

    fn epilogue(generated_asm: &mut String, arch: InlineAsmArch) {
        match arch {
            InlineAsmArch::X86_64 => {
                generated_asm.push_str("    pop rbx\n");
                generated_asm.push_str("    pop rbp\n");
                generated_asm.push_str("    ret\n");
            }
            InlineAsmArch::AArch64 => {
                generated_asm.push_str("    ldr x19, [sp, #24]\n");
                generated_asm.push_str("    ldp fp, lr, [sp], #32\n");
                generated_asm.push_str("    ret\n");
            }
            InlineAsmArch::RiscV64 => {
                generated_asm.push_str("    ld s1, 0(sp)\n");
                generated_asm.push_str("    ld ra, 8(sp)\n");
                generated_asm.push_str("    addi sp, sp, 16\n");
                generated_asm.push_str("    ret\n");
            }
            _ => unimplemented!("epilogue for {:?}", arch),
        }
    }

    fn epilogue_noreturn(generated_asm: &mut String, arch: InlineAsmArch) {
        match arch {
            InlineAsmArch::X86_64 => {
                generated_asm.push_str("    ud2\n");
            }
            InlineAsmArch::AArch64 => {
                generated_asm.push_str("    brk     #0x1\n");
            }
            InlineAsmArch::RiscV64 => {
                generated_asm.push_str("    ebreak\n");
            }
            _ => unimplemented!("epilogue_noreturn for {:?}", arch),
        }
    }

View on GitHub (pinned to 22057b88b0)

Solutions

1. Switch the codegen backend to LLVM (default) for this build target.
2. Constrain the crate/arch to x86_64, aarch64, or riscv64 when using Cranelift.
3. Add #[cfg(not(target_arch = "..."))] gates around the offending asm! so it is skipped on unsupported arches.
4. Implement the arch arm in epilogue() (mirror the corresponding prologue() restore order) and patch upstream.

Example fix

// before
unsafe fn pause() { asm!("nop", out("eax") _, options(nostack)); }

// after - only build the asm on backends/arches Cranelift supports
#[cfg(any(target_arch = "x86_64", target_arch = "aarch64", target_arch = "riscv64"))]
unsafe fn pause() { asm!("nop", out("eax") _, options(nostack)); }

// Reject asm! macros that contain prologue/epilogue-style register clobbers
// before invoking the cranelift backend.
let src = std::fs::read_to_string("src/lib.rs")?;
if src.contains("asm!") && (src.contains("in(") || src.contains("out(") || src.contains("clobber_abi")) {
    eprintln!("cg_clif epilogue path for inline asm is unimplemented; switch to LLVM.");
}

// Unreachable at runtime: panics inside rustc_codegen_cranelift abort the
// compiler process. Guard at build time instead.
// build.rs:
fn main() {
    let backend = std::env::var("RUSTC_CODEGEN").unwrap_or_default();
    if backend == "cranelift" { println!("cargo:rustc-cfg=needs_llvm_codegen"); }
}

When it happens

Trigger: Compiling asm! with register operands that need an epilogue restore, on a target other than X86_64/AArch64/RiscV64, using the Cranelift backend. Specifically asm! blocks that are not marked noreturn and therefore require restoring the saved base register (e.g. s1 on RiscV, rbx on x86_64) before ret.

Common situations: Using cg_clif on a tier-3 target (LoongArch64, PowerPC, S390x, Arm32) where the crate uses inline asm for perf counters, spinlocks, or CPUID-like probes; nightly toolchain with the cranelift codegen backend forced on.

Related errors

• prologue for {:?}
• epilogue_noreturn for {:?}
• save_register for {:?}
• restore_register for {:?}
• {:?}

AI-assisted analysis of rust-lang/rust@22057b88b0 (2026-08-03). Data as JSON: /data/errors/b5a1a57bbb0475f9.json. Report an issue: GitHub.