From d9c8ec257f374172923fd2174364df815c71899a Mon Sep 17 00:00:00 2001
From: jakob <jakob@memeware.net>
Date: Sun, 19 Mar 2017 12:06:21 -0400
Subject: Added basic loop optimizations.

---
 Makefile    |  2 +-
 skullfuck.c | 49 +++++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 42 insertions(+), 9 deletions(-)

diff --git a/Makefile b/Makefile
index ac2a6ff..fdff5d7 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 CC := gcc
 
 CFLAGS = -Wall -Os -std=c99 -pedantic
-MACROS = -D$(shell uname -m) -DAS=\"$(shell which as)\" -DLD=\"$(shell which ld)\"
+MACROS = -D__$(shell uname -m)__ -DAS=\"$(shell which as)\" -DLD=\"$(shell which ld)\"
 
 
 all: skullfuck
diff --git a/skullfuck.c b/skullfuck.c
index 47cb772..a8de668 100644
--- a/skullfuck.c
+++ b/skullfuck.c
@@ -25,7 +25,7 @@
 #include <getopt.h>
 
 
-#if defined(x86_64)
+#if defined(__x86_64__)
 #define ARCH         "x86_64"
 #define OS           "Linux"
 #define INC_DATA_PTR "\tincq %rsi\n"
@@ -36,6 +36,7 @@
 #define ADD_DATA     "\taddb $0x%x, (%%rsi)\n"
 #define DEC_DATA     "\tdecb (%rsi)\n"
 #define SUB_DATA     "\tsubb $0x%x, (%%rsi)\n"
+#define ZERO_DATA    "\tmovb $0x00, (%rsi)\n"
 #define PRINT_DATA   "\tmovq $0x01, %rax\n" \
                      "\tmovq $0x01, %rdi\n" \
                      "\tsyscall\n"
@@ -58,7 +59,7 @@
 #define EXIT         "\tmovq $0x3c, %rax\n" \
                      "\tmovq $0x00, %rdi\n" \
                      "\tsyscall\n"
-#elif defined(i686)
+#elif defined(__i686__)
 #define ARCH         "i686"
 #define OS           "Linux"
 #define INC_DATA_PTR "\tincl %ecx\n"
@@ -69,6 +70,7 @@
 #define ADD_DATA     "\taddb $0x%x, (%%ecx)\n"
 #define DEC_DATA     "\tdecb (%ecx)\n"
 #define SUB_DATA     "\tsubb $0x%x, (%%ecx)\n"
+#define ZERO_DATA    "\tmovb $0x00, (%ecx)\n"
 #define PRINT_DATA   "\tmovl $0x04, %eax\n" \
                      "\tmovl $0x01, %ebx\n" \
                      "\tint $0x80\n"
@@ -116,12 +118,14 @@ struct loop_stack {
     int _stack_index;
 };
 
+
 /* Pushes `n` onto the stack specified by `s`. */
 static void push_loop_index(struct loop_stack *s, int n) {
     s->_mem[s->_stack_index] = n;
     s->_stack_index++;
 }
 
+
 /* Pops the top of `s` off and returns it. */
 static int pop_loop_index(struct loop_stack *s) {
     s->_stack_index--;
@@ -129,6 +133,30 @@ static int pop_loop_index(struct loop_stack *s) {
 }
 
 
+/* Loop optimizations. */
+enum {
+    NONE,
+    LOOP_EMPTY,
+    LOOP_ZERO,
+};
+
+
+/* Optimization subroutine which detects common loop patterns, such as
+   `[-]`, increments the `in` pointer to beyond them, and returns the
+   identity of the pattern. */
+static int optimize_loops(char **in) {
+    if (*(*in + 1) == '-' && *(*in + 2) == ']') {
+        *in += 2;
+        return LOOP_ZERO;
+    }
+    if (*(*in + 1) == ']') {
+        *in += 1;
+        return LOOP_EMPTY;
+    }
+    return NONE;
+}
+
+
 /* Minor optimization subroutine. Increments the `in` pointer to beyond
    the collection of adjacent `op` characters, and returns the number
    of operations counted. */
@@ -144,7 +172,7 @@ static int reduce(char **in, char op) {
    instructions and writes them to `out`. `in` is expected to be
    null-terminated. */
 static void write_instructions(FILE *out, char *in) {
-    int               next_loop = 0, cur_loop = 0, op_count;
+    int               next_loop = 0, cur_loop = 0, op_count, optimization;
     struct loop_stack *loops = calloc(sizeof(struct loop_stack), 1);
 
     for (char *cur = in; *cur != '\0'; cur++) {
@@ -188,11 +216,16 @@ static void write_instructions(FILE *out, char *in) {
                 fputs(READ_DATA, out);
                 break;
             case '[':
-                cur_loop = next_loop;
-                push_loop_index(loops, next_loop++);
-                fprintf(out, "S%d:\n", cur_loop);
-                fputs(DATA_IS_ZERO, out);
-                fprintf(out, JMP_END, cur_loop);
+                optimization = optimize_loops(&cur);
+                if (optimization == LOOP_ZERO) {
+                    fputs(ZERO_DATA, out);
+                } else if (optimization == NONE) {
+                    cur_loop = next_loop;
+                    push_loop_index(loops, next_loop++);
+                    fprintf(out, "S%d:\n", cur_loop);
+                    fputs(DATA_IS_ZERO, out);
+                    fprintf(out, JMP_END, cur_loop);
+                }
                 break;
             case ']':
                 cur_loop = pop_loop_index(loops);
-- 
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