cj

is a small JIT framework written in C, with x86 and arm64 backends.

what

• multi-architecture support: generates native code for:
  • x86-64
  • ARM64 (except for 26 SIMD different-size instructions)
• low-level API: direct instruction emission, no high level constructs
• no dependencies: pure C with clib, nothing else

how

the backends are autogenerated (check out codegen). x86 uses asmdb as a data source, arm64 uses a handgenerated file from mra_tools. register definitions are hand-crafted.

the rest is handwritten and basically trivial in the grand scheme of jit compilation. examples, tests, and codegen docs contain some llm-generation, so peruse at your own peril.

why

because i wanted to understand the isa for both processor architectures and it seemed like a fun project.

quick start

build

# dev build
make dev

# "prod" build
make all

# install (don't)
make install

basic usage

#include "ctx.h"
#include "op.h"

int main(void) {
  // Create JIT context
  cj_ctx* cj = create_cj_ctx();

  // Emit instructions
  cj_nop(cj);  // NOP
  cj_ret(cj);  // RET

  // Create executable function
  cj_fn f = create_cj_fn(cj);

  // Execute JIT-compiled code!
  f();

  // Cleanup
  destroy_cj_fn(cj, f);
  destroy_cj_ctx(cj);

  return 0;
}

you can find some more examples in the examples directory.

builder helpers

For reusable building blocks, the optional builder helpers provide prologue/epilogue setup and structured loops:

#include <stdio.h>
#include "builder.h"

typedef int (*sum_fn)(int);

int main(void) {
  cj_ctx* cj = create_cj_ctx();
  cj_builder_frame frame;
  cj_builder_fn_prologue(cj, 0, &frame);

  cj_operand n = cj_builder_arg_int(cj, 0);
  cj_operand sum = cj_builder_scratch_reg(0);
  cj_operand i = cj_builder_scratch_reg(1);
  cj_operand one = cj_make_constant(1);

  cj_builder_assign(cj, sum, cj_builder_zero_operand());

  cj_builder_for_loop loop = cj_builder_for_begin(cj, i, one, n, one, CJ_COND_GE);
  cj_builder_add_assign(cj, sum, i);
  cj_builder_for_end(cj, &loop);

  cj_builder_return_value(cj, &frame, sum);

  sum_fn fn = (sum_fn)create_cj_fn(cj);
  printf("triangular(5) = %d\n", fn ? fn(5) : -1);

  destroy_cj_fn(cj, (cj_fn)fn);
  destroy_cj_ctx(cj);
  return 0;
}

see also docs/builder.md.

requirements

• c11 compiler (gcc, clang)
• POSIX-compliant OS (for mmap)
• supported architecture (x86-64 or ARM64)

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Have fun!