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Experimental TPDE backend

Spice ships with the LLVM code generator as its default and stable compilation backend. Alongside it, an experimental alternative backend is available: TPDE, a fast compiler back-end framework that consumes LLVM IR directly and emits ELF objects at roughly -O0 code quality but 10-20× faster than clang -O0 — useful for tight edit/compile iteration loops.

This backend is not a drop-in replacement for LLVM CodeGen. It performs no optimization and supports only a narrow slice of the targets LLVM does. It is opt-in at both build time and runtime.

Experimental

The TPDE backend is under active development and disabled by default. Expect rough edges, unsupported language features, and possible codegen bugs. Do not use for production builds. Bug reports welcome.

Supported configurations

Property Value
Host operating systems Linux (ELF only)
Target architectures x86_64, aarch64
Code model small only
Relocation model PIC only
Optimization none — -O flags are accepted but ignored
Link-time optimization not supported (-lto is rejected)

Building Spice with TPDE support

TPDE is pulled in as a git submodule at deps/tpde/. To include it in your Spice build, configure CMake with the SPICE_ENABLE_TPDE option turned on:

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cmake -S . -B cmake-build-debug -DSPICE_ENABLE_TPDE=ON
cmake --build cmake-build-debug --target spice

Requirements:

  • The same LLVM version Spice is built against must be one of TPDE's supported versions (currently 19.1, 20.1, 21.1, 22.1). Spice's regular LLVM build already satisfies this on modern systems.
  • A matching clang binary must be discoverable at configure time; TPDE uses it to compile its encoding templates. dev-setup.py installs a suitable one alongside LLVM.

If either requirement is missing, the CMake configuration will fail with a clear error — no partial build.

Selecting the backend

Once built with -DSPICE_ENABLE_TPDE=ON, choose the backend per compilation with --backend:

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# Default — LLVM CodeGen
spice build main.spice
spice build --backend=llvm main.spice

# Experimental TPDE backend
spice build --backend=tpde main.spice
spice run   --backend=tpde main.spice

If the compiler was built without SPICE_ENABLE_TPDE, passing --backend=tpde produces a CLI error directing you to rebuild with the option turned on.

Limitations you may hit

  • --dump-assembly and --dump-object-file are not meaningful under TPDE; they return placeholder text.
  • Any Spice code that requires wide vectors, exception handling, or non-small code model will fail to compile.
  • Debug info emission (-g) works but produces -O0-quality info regardless of the --build-mode.