GitHub - forksnd/jit: A header-only, cross-platform JIT compiler library in C. Targets x86-32, x86-64, ARM32 and ARM64

jit

A header-only, cross-platform JIT compiler library in C. Targets x86-32, x86-64, ARM32, ARM64, RISC-V 64.

Features

Targets x86-32, x86-64, ARM32, ARM64, RISC-V 64 (auto-detected or set JIT_ARCH)
Works on Windows, Linux, macOS and any POSIX system
Works with any C89+ compiler (GCC, Clang, MSVC, TCC, etc)
Full instruction set: arithmetic, logic, shifts, memory, branches, calls, stack frames
Label + fixup system for forward/backward jumps and loops
Buffer auto-grow which starts small and doubles on overflow
Zero external dependencies, only requires libc

Usage

#include "jit.h"

typedef long long (*fn2)(long long, long long);

int main(void) {
  jit_buf j;
  jit_init(&j, 0);           // 0 → use default capacity (4096 bytes)

  jit_prolog(&j);            // push rbp; mov rbp, rsp
  jit_mov_rr64(&j, RAX, RDI);
  jit_add_rr64(&j, RAX, RSI);
  jit_epilog(&j);            // mov rsp, rbp; pop rbp; ret

  fn2 add = (fn2)jit_compile(&j);
  printf("%lld\n", add(3, 5));  // 8

  jit_free(&j);
}

Compile and run (after including jit.h):

gcc -O2 -o prog prog.c && ./prog

API

Lifecycle

Function	Description
`jit_init(j, cap)`	Allocate RWX buffer. `cap=0` → 4096 bytes
`jit_free(j)`	Free buffer
`jit_compile(j)`	Patch all labels, flush icache, return `void*` to code
`jit_fn(j)`	Return raw pointer without patching

Labels & branches

Labels emit forward/backward jumps without knowing the target address up front.

int lbl_loop = jit_label(&j);  // allocate label id
int lbl_end  = jit_label(&j);

jit_bind(&j, lbl_loop);        // mark current position as lbl_loop

jit_cmp_ri64(&j, RCX, 0);
jit_jcc_lbl(&j, JIT_CC_EQ, lbl_end);  // jump to lbl_end if RCX == 0

// ... loop body ...
jit_jmp_lbl(&j, lbl_loop);

jit_bind(&j, lbl_end);

All fixups are resolved when you call jit_compile().

Condition codes (`jit_cc`)

Code	Meaning
`JIT_CC_EQ`	Equal / zero
`JIT_CC_NE`	Not equal
`JIT_CC_LT`	Signed less than
`JIT_CC_LE`	Signed less or equal
`JIT_CC_GT`	Signed greater than
`JIT_CC_GE`	Signed greater or equal
`JIT_CC_ULT`	Unsigned less than
`JIT_CC_ULE`	Unsigned less or equal
`JIT_CC_UGT`	Unsigned greater than
`JIT_CC_UGE`	Unsigned greater or equal

Used by: jit_jcc_lbl, jit_setcc, jit_cmov_rr64.

x86-64 Instruction Reference

Register enum

RAX RCX RDX RBX RSP RBP RSI RDI R8–R15
EAX ECX EDX EBX ESP EBP ESI EDI

Stack frames

jit_prolog(j)                // push rbp; mov rbp, rsp
jit_epilog(j)                // mov rsp, rbp; pop rbp; ret
jit_prolog_frame(j, n)       // prolog + sub rsp, n (aligned to 16)
jit_epilog_frame(j)          // same as epilog

Move

jit_mov_rr64(j, dst, src)          // dst = src  (64-bit)
jit_mov_rr32(j, dst, src)          // dst = src  (32-bit)
jit_mov_ri64(j, dst, imm64)        // dst = imm64
jit_mov_ri32(j, dst, imm32)        // dst = imm32 (zero extends)
jit_mov_rm64(j, dst, base, disp)   // dst = [base+disp]
jit_mov_mr64(j, base, disp, src)   // [base+disp] = src
jit_mov_rm32(j, dst, base, disp)
jit_mov_mr32(j, base, disp, src)
jit_mov_mr8 (j, base, disp, src)
jit_movzx_rm8 (j, dst, base, disp) // zero-extend byte  → 64-bit
jit_movzx_rm16(j, dst, base, disp) // zero-extend word  → 64-bit
jit_movsx_r32_r8(j, dst, src)      // sign-extend byte  → 32-bit
jit_movsx_r64_r32(j, dst, src)     // sign-extend dword → 64-bit
jit_movzx_r32_r8(j, dst, src)
jit_movzx_r64_r32(j, dst, src)
jit_lea_rm(j, dst, base, disp)     // dst = base+disp (LEA)

Arithmetic

jit_add_rr64(j, dst, src)
jit_add_ri64(j, dst, imm32)
jit_add_rr32(j, dst, src)
jit_add_ri32(j, dst, imm32)
jit_add_rm64(j, dst, base, disp)   // dst += [base+disp]
jit_sub_rr64(j, dst, src)
jit_sub_ri64(j, dst, imm32)
jit_sub_rr32(j, dst, src)
jit_sub_ri32(j, dst, imm32)
jit_imul_rr64(j, dst, src)         // dst *= src (signed)
jit_imul_rr32(j, dst, src)
jit_neg_r64(j, r)                  // r = -r
jit_neg_r32(j, r)
jit_idiv_r64(j, src)               // RDX:RAX / src → RAX (quot), RDX (rem)
jit_idiv_r32(j, src)               // use jit_cqo / jit_cdq first
jit_div_r64(j, src)                // unsigned
jit_div_r32(j, src)
jit_cqo(j)                         // sign-extend RAX → RDX:RAX
jit_cdq(j)                         // sign-extend EAX → EDX:EAX

Logic

jit_and_rr64(j, dst, src)   jit_and_ri64(j, dst, imm32)
jit_or_rr64(j, dst, src)    jit_or_ri64(j, dst, imm32)
jit_xor_rr64(j, dst, src)   jit_xor_ri64(j, dst, imm32)
jit_not_r64(j, r)
jit_and_rr32 / jit_or_rr32 / jit_xor_rr32 / jit_not_r32  (same pattern)

Shifts

jit_shl_ri64(j, dst, src, shift)   // dst = src << shift
jit_shr_ri64(j, dst, src, shift)   // dst = src >> shift  (logical)
jit_sar_ri64(j, dst, src, shift)   // dst = src >> shift  (arithmetic)
jit_shl_rr64(j, r)   // shift r left  by CL
jit_shr_rr64(j, r)   // shift r right by CL (logical)
jit_sar_rr64(j, r)   // shift r right by CL (arithmetic)
// _32 variants exist for all of the above

Compare & conditional

jit_cmp_rr64(j, a, b)
jit_cmp_ri64(j, a, imm32)
jit_cmp_rr32(j, a, b)
jit_cmp_ri32(j, a, imm32)
jit_test_rr64(j, a, b)        // sets flags on a & b, discards result
jit_test_rr32(j, a, b)
jit_setcc(j, cc, dst)         // dst = (condition ? 1 : 0) — 8-bit
jit_cmov_rr64(j, cc, dst, src) // if (cc) dst = src  (no branch)
jit_cmov_rr32(j, cc, dst, src)

Jumps & calls

jit_jmp_lbl(j, lbl)           // unconditional jump to label
jit_jmp_r64(j, r)             // jmp *r
jit_jmp_rel32(j, rel)         // jmp rel32
jit_jcc_lbl(j, cc, lbl)       // conditional jump to label
jit_call_abs(j, ptr)          // call absolute address (via RAX)
jit_call_r64(j, r)            // call *r
jit_call_rel32(j, rel)        // call rel32
jit_ret(j)                    // ret

Stack

jit_push_r64(j, r)
jit_pop_r64(j, r)
jit_sub_rsp(j, n)             // sub rsp, n
jit_add_rsp(j, n)             // add rsp, n
jit_xchg_rr64(j, a, b)

Bit operations

jit_bswap_r64(j, r)
jit_bswap_r32(j, r)
jit_popcnt_r64(j, dst, src)
jit_popcnt_r32(j, dst, src)
jit_lzcnt_r32(j, dst, src)
jit_tzcnt_r32(j, dst, src)

Misc

jit_nop(j)

x86-32 details

Same patterns as x86-64 but without REX prefixes and only 8 registers (EAX–EDI). The _64 suffix functions are not available. Calling convention on Linux is cdecl (args on stack), on Windows stdcall or cdecl depending on target.

ARM64 details

Instructions use a 3-operand form: jit_add_rr64(j, dst, a, b). Registers are X0–X30, XZR/SP. jit_prolog saves FP/LR and sets up the frame pointer. Call external functions with jit_bl_abs(j, tmp_reg, fn_ptr).

ARM32 details

Same 3-operand form. Registers R0–R15 with aliases SP=13, LR=14, PC=15. jit_prolog saves FP/LR via PUSH. Call externals with jit_bl_abs(j, tmp_reg, fn_ptr).

RISC-V 64 details

RV64GC (base integer + M extension for mul/div). Instructions use a 3-operand form: jit_add_rr64(j, dst, a, b).

Register enum

ZERO  RA    SP    GP    TP
T0–T2       (temporaries)
S0/FP S1    (saved / frame pointer)
A0–A7       (args / return values: A0=return)
S2–S11      (saved)
T3–T6       (temporaries)

Arithmetic & logic

jit_add_rr64(j, d, a, b)     jit_add_ri64(j, d, s, imm12)
jit_sub_rr64(j, d, a, b)
jit_mul_rr64(j, d, a, b)
jit_div_rr64(j, d, a, b)     // signed (requires M ext)
jit_divu_rr64(j, d, a, b)    // unsigned
jit_rem_rr64(j, d, a, b)     // signed remainder
jit_remu_rr64(j, d, a, b)    // unsigned remainder
jit_neg_r64(j, d, s)
jit_not_r64(j, d, s)
jit_and_rr64(j, d, a, b)     jit_and_ri64(j, d, s, imm12)
jit_or_rr64(j, d, a, b)      jit_or_ri64(j, d, s, imm12)
jit_xor_rr64(j, d, a, b)     jit_xor_ri64(j, d, s, imm12)
jit_shl_ri64(j, d, s, sh)    jit_shl_rr64(j, d, a, b)
jit_shr_ri64(j, d, s, sh)    jit_shr_rr64(j, d, a, b)   // logical
jit_sar_ri64(j, d, s, sh)    jit_sar_rr64(j, d, a, b)   // arithmetic

Word (32-bit) ops

jit_add_rr32(j, d, a, b)     // ADDW - sign-extends to 64-bit
jit_sub_rr32(j, d, a, b)     // SUBW
jit_mul_rr32(j, d, a, b)     // MULW
jit_div_rr32(j, d, a, b)     // DIVW
jit_rem_rr32(j, d, a, b)     // REMW
jit_shl_ri32(j, d, s, sh)    // SLLIW
jit_shr_ri32(j, d, s, sh)    // SRLIW
jit_sar_ri32(j, d, s, sh)    // SRAIW

Memory

jit_ld64(j, dst, base, off)    // LD  — load 64-bit
jit_ld32(j, dst, base, off)    // LW  — sign-extend
jit_ld32u(j, dst, base, off)   // LWU — zero-extend
jit_ld16(j, dst, base, off)    // LH
jit_ld16u(j, dst, base, off)   // LHU
jit_ld8(j, dst, base, off)     // LB
jit_ld8u(j, dst, base, off)    // LBU
jit_sd64(j, src, base, off)    // SD
jit_sd32(j, src, base, off)    // SW
jit_sd16(j, src, base, off)    // SH
jit_sd8(j, src, base, off)     // SB

Compare & set

jit_slt_rr(j, d, a, b)          // d = (a < b) signed
jit_sltu_rr(j, d, a, b)         // d = (a < b) unsigned
jit_slt_ri(j, d, s, imm12)
jit_sltu_ri(j, d, s, imm12)
jit_seqz(j, d, s)               // d = (s == 0)
jit_snez(j, d, s)               // d = (s != 0)
jit_sltz(j, d, s)               // d = (s < 0)
jit_sgtz(j, d, s)               // d = (s > 0)

Branches

On RV64, jit_jcc_lbl takes two source registers to compare directly (no prior cmp):

jit_jcc_lbl(j, cc, rs1, rs2, lbl)

jit_jcc_lbl(&j, JIT_CC_EQ, A0, A1, lbl)  // branch if A0 == A1
jit_jcc_lbl(&j, JIT_CC_LT, A0, ZERO, lbl) // branch if A0 < 0
jit_jmp_lbl(&j, lbl)
jit_jmp_r64(&j, r)                         // jalr zero, 0(r)
jit_call_abs(&j, T0, fn_ptr)               // load address into T0, jalr ra, 0(T0)

Stack & frames

jit_prolog(j)            // addi sp,-16; sd ra,8(sp); sd fp,0(sp); addi fp,sp,16
jit_epilog(j)            // ld ra,8(sp); ld fp,0(sp); addi sp,16; ret
jit_prolog_frame(j, n)   // same but allocates n extra bytes (16-byte aligned)
jit_epilog_frame(j)      // same as epilog

Building & testing

# native x86-64
make test

# RISC-V 64 via QEMU (requires riscv64-linux-gnu-gcc and qemu-riscv64)
make test-rv64

# all targets
make

Install the RV64 toolchain on Debian/Ubuntu:

sudo apt install gcc-riscv64-linux-gnu qemu-user-static

Then:

# x86-64 native
gcc -O2 -o test.x86-64 test.x86-64.c && ./test.x86-64

# RISC-V 64 (requires riscv64-linux-gnu-gcc + qemu-riscv64)
riscv64-linux-gnu-gcc -O2 -static -o test.rv64 test.rv64.c
qemu-riscv64 ./test.rv64

If QEMU can't find the libc for the binary, you may also need libc6-riscv64-cross, but the -static flag should make that a non-issue.

JIT_ARCH is auto-detected from compiler predefined macros. Override it manually if cross-compiling:

#define JIT_ARCH JIT_ARCH_ARM64
#include "jit.h"

Available values: JIT_ARCH_X86_32, JIT_ARCH_X86_64, JIT_ARCH_ARM32, JIT_ARCH_ARM64, JIT_ARCH_RV64.

The x86-64 test suite covers: constants, arithmetic (add/sub/mul/div), bitwise ops, shifts, negation, sign extension, branches, loops, stack frames, local variables, C function calls, conditional moves, setcc, LEA, bswap, popcnt, buffer grow, factorial, fibonacci and multi-label dispatch.

The RV64 suite covers: constants (including large 48-bit), all ALU ops (add/sub/mul/div/rem), bitwise, shifts (imm/reg), slt/sltu, branches (eq/ne/lt/le/gt/ge), loops, stack locals, memory load/store, immediate arithmetic, C function calls, multi-label dispatch, W (32-bit) ops and buffer grow.

Examples

Loop: sum 0..n

jit_buf j;
jit_init(&j, 0);
jit_prolog(&j);
jit_mov_ri64(&j, RAX, 0);    // acc = 0
jit_mov_ri64(&j, RCX, 0);    // i   = 0
int lbl_loop = jit_label(&j);
int lbl_end  = jit_label(&j);
jit_bind(&j, lbl_loop);
jit_cmp_rr64(&j, RCX, RDI);  // cmp i, n
jit_jcc_lbl(&j, JIT_CC_GE, lbl_end);
jit_add_rr64(&j, RAX, RCX);  // acc += i
jit_add_ri64(&j, RCX, 1);    // i++
jit_jmp_lbl(&j, lbl_loop);
jit_bind(&j, lbl_end);
jit_epilog(&j);
long long (*sum)(long long) = jit_compile(&j);
printf("%lld\n", sum(10));    // 45

Conditional: max(a, b)

jit_buf j;
jit_init(&j, 0);
jit_prolog(&j);
jit_mov_rr64(&j, RAX, RDI);
jit_cmp_rr64(&j, RDI, RSI);
jit_cmov_rr64(&j, JIT_CC_LT, RAX, RSI);   // if a < b: RAX = b
jit_epilog(&j);
long long (*maxfn)(long long,long long) = jit_compile(&j);
printf("%lld\n", maxfn(3, 7));  // 7

Stack frame with local variable

jit_buf j;
jit_init(&j, 0);
jit_prolog_frame(&j, 16);     // allocate 16 bytes on stack
jit_mov_mr64(&j, RBP, -8, RDI);   // [rbp-8] = arg0
jit_mov_rm64(&j, RAX, RBP, -8);   // RAX = [rbp-8]
jit_add_ri64(&j, RAX, 1);
jit_epilog_frame(&j);
long long (*inc)(long long) = jit_compile(&j);
printf("%lld\n", inc(41));     // 42

Calling a C function from JIT code

jit_buf j;
jit_init(&j, 0);
jit_prolog_frame(&j, 0);
jit_sub_rsp(&j, 8);               // align stack to 16 bytes before call
jit_mov_ri64(&j, RDI, (long long)(uintptr_t)"hello\n");
jit_call_abs(&j, (void*)puts);
jit_add_rsp(&j, 8);
jit_mov_ri64(&j, RAX, 0);
jit_epilog_frame(&j);
((void(*)(void))jit_compile(&j))();

Missing features

Encode more JIT intrinsics/ops, e.g. vectorization
Additional architectures (ppc, loongarch, MIPS, SPARC, etc)
Heuristics for multithreaded JIT, probably not necessary

License

Apache v2.0 License

Name		Name	Last commit message	Last commit date
Latest commit History 10 Commits
.clang-format		.clang-format
.gitignore		.gitignore
LICENSE		LICENSE
Makefile		Makefile
README.md		README.md
jit.h		jit.h
test.rv64.c		test.rv64.c
test.x86-64.c		test.x86-64.c

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jit

Features

Usage

API

Lifecycle

Labels & branches

Condition codes (jit_cc)

x86-64 Instruction Reference

Register enum

Stack frames

Move

Arithmetic

Logic

Shifts

Compare & conditional

Jumps & calls

Stack

Bit operations

Misc

x86-32 details

ARM64 details

ARM32 details

RISC-V 64 details

Register enum

Arithmetic & logic

Word (32-bit) ops

Memory

Compare & set

Branches

Stack & frames

Building & testing

Examples

Loop: sum 0..n

Conditional: max(a, b)

Stack frame with local variable

Calling a C function from JIT code

Missing features

License

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