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-rw-r--r--interpreters/bf-jit.c327
1 files changed, 327 insertions, 0 deletions
diff --git a/interpreters/bf-jit.c b/interpreters/bf-jit.c
new file mode 100644
index 0000000..0be6d94
--- /dev/null
+++ b/interpreters/bf-jit.c
@@ -0,0 +1,327 @@
+// This is an exercise in futility more than anything else
+#define _GNU_SOURCE
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <assert.h>
+#include <errno.h>
+
+#if (defined __x86_64__ || defined __amd64__) && defined __unix__
+ #include <sys/mman.h>
+#else
+ #error Platform not supported
+#endif
+
+#define exit_fatal(...) \
+ do { \
+ fprintf (stderr, "fatal: " __VA_ARGS__); \
+ exit (EXIT_FAILURE); \
+ } while (0)
+
+// --- Safe memory management --------------------------------------------------
+
+static void *
+xcalloc (size_t m, size_t n)
+{
+ void *p = calloc (m, n);
+ if (!p)
+ exit_fatal ("calloc: %s\n", strerror (errno));
+ return p;
+}
+
+static void *
+xrealloc (void *o, size_t n)
+{
+ void *p = realloc (o, n);
+ if (!p && n)
+ exit_fatal ("realloc: %s\n", strerror (errno));
+ return p;
+}
+
+// --- Dynamically allocated strings -------------------------------------------
+
+struct str
+{
+ char *str; ///< String data, null terminated
+ size_t alloc; ///< How many bytes are allocated
+ size_t len; ///< How long the string actually is
+};
+
+static void
+str_init (struct str *self)
+{
+ self->len = 0;
+ self->str = xcalloc (1, (self->alloc = 16));
+}
+
+static void
+str_ensure_space (struct str *self, size_t n)
+{
+ // We allocate at least one more byte for the terminating null character
+ size_t new_alloc = self->alloc;
+ while (new_alloc <= self->len + n)
+ new_alloc <<= 1;
+ if (new_alloc != self->alloc)
+ self->str = xrealloc (self->str, (self->alloc = new_alloc));
+}
+
+static void
+str_append_data (struct str *self, const void *data, size_t n)
+{
+ str_ensure_space (self, n);
+ memcpy (self->str + self->len, data, n);
+ self->str[self->len += n] = '\0';
+}
+
+static void
+str_append_c (struct str *self, char c)
+{
+ str_append_data (self, &c, 1);
+}
+
+// --- Application -------------------------------------------------------------
+
+struct str data; ///< Data tape
+volatile size_t dataptr; ///< Current location on the tape
+FILE *input; ///< User input
+
+enum command { RIGHT, LEFT, INC, DEC, SET, IN, OUT, BEGIN, END };
+bool grouped[] = { 1, 1, 1, 1, 1, 0, 0, 0, 0 };
+struct instruction { enum command cmd; size_t arg; };
+
+// - - Callbacks - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+// Some things I just really don't want to write in assembly even though it
+// is effectively a big performance hit, eliminating the advantage of JIT
+
+static void
+right (size_t arg)
+{
+ assert (SIZE_MAX - dataptr > arg);
+ dataptr += arg;
+
+ while (dataptr >= data.len)
+ str_append_c (&data, 0);
+}
+
+static void
+left (size_t arg)
+{
+ assert (dataptr >= arg);
+ dataptr -= arg;
+}
+
+static int
+cin (void)
+{
+ int c = fgetc (input);
+ assert (c != EOF);
+ return c;
+}
+
+// - - Main - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+int
+main (int argc, char *argv[])
+{
+ (void) argc;
+ (void) argv;
+
+ struct str program;
+ str_init (&program);
+
+ int c;
+ while ((c = fgetc (stdin)) != EOF)
+ str_append_c (&program, c);
+ if (ferror (stdin))
+ exit_fatal ("can't read program\n");
+ if (!(input = fopen ("/dev/tty", "rb")))
+ exit_fatal ("can't open terminal for reading\n");
+
+// - - Decode and group - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+ struct instruction *parsed = xcalloc (sizeof *parsed, program.len);
+ size_t parsed_len = 0;
+
+ for (size_t i = 0; i < program.len; i++)
+ {
+ enum command cmd;
+ switch (program.str[i])
+ {
+ case '>': cmd = RIGHT; break;
+ case '<': cmd = LEFT; break;
+ case '+': cmd = INC; break;
+ case '-': cmd = DEC; break;
+ case '.': cmd = OUT; break;
+ case ',': cmd = IN; break;
+ case '[': cmd = BEGIN; break;
+ case ']': cmd = END; break;
+ default: continue;
+ }
+
+ if (!parsed_len || !grouped[cmd] || parsed[parsed_len - 1].cmd != cmd)
+ parsed_len++;
+
+ parsed[parsed_len - 1].cmd = cmd;
+ parsed[parsed_len - 1].arg++;
+ }
+
+// - - Simple optimization pass - - - - - - - - - - - - - - - - - - - - - - - -
+
+ size_t in = 0, out = 0;
+ for (; in < parsed_len; in++, out++)
+ {
+ if (in + 2 < parsed_len
+ && parsed[in ].cmd == BEGIN
+ && parsed[in + 1].cmd == DEC && parsed[in + 1].arg == 1
+ && parsed[in + 2].cmd == END)
+ {
+ parsed[out].cmd = SET;
+ parsed[out].arg = 0;
+ in += 2;
+ }
+ else if (out && parsed[out - 1].cmd == SET && parsed[in].cmd == INC)
+ parsed[--out].arg += parsed[in].arg;
+ else if (out != in)
+ parsed[out] = parsed[in];
+ }
+
+ parsed_len = out;
+
+// - - Loop pairing - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+ size_t nesting = 0;
+ size_t *stack = xcalloc (sizeof *stack, parsed_len);
+ for (size_t i = 0; i < parsed_len; i++)
+ {
+ switch (parsed[i].cmd)
+ {
+ case BEGIN:
+ stack[nesting++] = i;
+ break;
+ case END:
+ assert (nesting > 0);
+
+ --nesting;
+ parsed[stack[nesting]].arg = i + 1;
+ parsed[i].arg = stack[nesting] + 1;
+ default:
+ break;
+ }
+ }
+ free (stack);
+ assert (nesting == 0);
+
+// - - JIT - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+ // Functions preserve the registers rbx, rsp, rbp, r12, r13, r14, and r15;
+ // while rax, rdi, rsi, rdx, rcx, r8, r9, r10, r11 are scratch registers.
+
+ str_init (&program);
+ size_t *offsets = xcalloc (sizeof *offsets, parsed_len + 1);
+
+#define CODE(x) { char t[] = x; str_append_data (&program, t, sizeof t - 1); }
+#define WORD(x) { size_t t = (size_t)(x); str_append_data (&program, &t, 8); }
+
+ CODE ("\x49\xBD") WORD (&dataptr) // mov r13, qword "&dataptr"
+ CODE ("\x49\xBF") WORD (&data.str) // mov r15, qword "&data.str"
+ CODE ("\x4D\x8B\x37") // mov r14, qword [r15]
+ CODE ("\x30\xDB") // xor bl, bl
+
+ for (size_t i = 0; i < parsed_len; i++)
+ {
+ offsets[i] = program.len;
+
+ size_t arg = parsed[i].arg;
+ switch (parsed[i].cmd)
+ {
+ case RIGHT:
+ CODE ("\x41\x88\x1E") // mov [r14], bl
+ CODE ("\x48\xBF") WORD (arg) // mov rdi, "arg"
+ CODE ("\x48\xB8") WORD (right) // mov rax, "right"
+ CODE ("\xFF\xD0") // call rax
+
+ // The data could get reallocated, so reload the address
+ CODE ("\x4D\x8B\x37") // mov r14, qword [r15]
+ CODE ("\x4D\x03\x75\x00") // add r14, [r13]
+ CODE ("\x41\x8A\x1E") // mov bl, [r14]
+ break;
+ case LEFT:
+ CODE ("\x41\x88\x1E") // mov [r14], bl
+ CODE ("\x48\xBF") WORD (arg) // mov rdi, "arg"
+ CODE ("\x49\x29\xFE") // sub r14, rdi -- optimistic
+ CODE ("\x48\xB8") WORD (left) // mov rax, "left"
+ CODE ("\xFF\xD0") // call rax
+ CODE ("\x41\x8A\x1E") // mov bl, [r14]
+ break;
+
+ case INC:
+ CODE ("\x80\xC3") // add bl, "arg"
+ str_append_c (&program, arg);
+ break;
+ case DEC:
+ CODE ("\x80\xEB") // sub bl, "arg"
+ str_append_c (&program, arg);
+ break;
+ case SET:
+ CODE ("\xB3") // mov bl, "arg"
+ str_append_c (&program, arg);
+ break;
+
+ case OUT:
+ CODE ("\x48\x0F\xB6\xFB") // movzx rdi, bl
+ CODE ("\x48\xBE") WORD (stdout) // mov rsi, "stdout"
+ CODE ("\x48\xB8") WORD (fputc) // mov rax, "fputc"
+ CODE ("\xFF\xD0") // call rax
+ break;
+ case IN:
+ CODE ("\x48\xB8") WORD (cin) // mov rax, "cin"
+ CODE ("\xFF\xD0") // call rax
+ CODE ("\x88\xC3") // mov bl, al
+ break;
+
+ case BEGIN:
+ CODE ("\x84\xDB") // test bl, bl
+ CODE ("\x0F\x84\x00\x00\x00\x00") // jz "offsets[i]"
+ break;
+ case END:
+ CODE ("\x84\xDB") // test bl, bl
+ CODE ("\x0F\x85\x00\x00\x00\x00") // jnz "offsets[i]"
+ break;
+ }
+ }
+ // When there is a loop at the end we need to be able to jump past it
+ offsets[parsed_len] = program.len;
+ str_append_c (&program, '\xC3'); // ret
+
+ // Now that we know where each instruction is, fill in relative jumps
+ for (size_t i = 0; i < parsed_len; i++)
+ {
+ if (parsed[i].cmd != BEGIN && parsed[i].cmd != END)
+ continue;
+ size_t fixup = offsets[i] + 4;
+ *(int32_t *)(program.str + fixup) =
+ ((intptr_t)(offsets[parsed[i].arg]) - (intptr_t)(fixup + 4));
+ }
+ free (offsets);
+
+// - - Runtime - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+ // Some systems may have W^X
+ void *executable = mmap (NULL, program.len, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+ if (!executable)
+ exit_fatal ("mmap: %s\n", strerror (errno));
+
+ memcpy (executable, program.str, program.len);
+ if (mprotect (executable, program.len, PROT_READ | PROT_EXEC))
+ exit_fatal ("mprotect: %s\n", strerror (errno));
+
+ str_init (&data);
+ str_append_c (&data, 0);
+ ((void (*) (void)) executable)();
+ return 0;
+}
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