From 7cbef14e8393475b1e569fdbfff2c46db859d43f Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Thu, 4 Nov 2021 11:58:53 +0100
Subject: Add `cons`, `car` and `cdr` primitives

---
 src/compiler.h          | 72 ++++++++++++++++++++++++++++++++++++++++++++-----
 src/parser.c            |  1 +
 src/x86_64/postlude.asm |  4 +++
 src/x86_64/prelude.asm  |  7 ++++-
 4 files changed, 76 insertions(+), 8 deletions(-)

diff --git a/src/compiler.h b/src/compiler.h
index 7323f4d..070f24a 100644
--- a/src/compiler.h
+++ b/src/compiler.h
@@ -5,15 +5,24 @@
 #define PRELUDE_FILE "src/x86_64/prelude.asm"
 #define POSTLUDE_FILE "src/x86_64/postlude.asm"
 
-#define NIL_VAL      1
-#define BOOL_MASK    3
-#define BOOL_TAG     3
-#define BOOL_SHIFT   2
+#define HEAP_SIZE   MB(32)
+
+// Immediate constants.
+#define NIL_VAL      47
+#define BOOL_MASK    127
+#define BOOL_TAG     31
+#define BOOL_SHIFT   7
 #define TRUE_VAL     ((1 << BOOL_SHIFT) | BOOL_TAG)
 #define FALSE_VAL    ((0 << BOOL_SHIFT) | BOOL_TAG)
-#define FIXNUM_MASK  1
+#define FIXNUM_MASK  3
 #define FIXNUM_TAG   0
-#define FIXNUM_SHIFT 1
+#define FIXNUM_SHIFT 2
+
+// Heap allocated objects.
+#define STRING_MASK  7
+#define STRING_TAG   3
+#define PAIR_MASK    7
+#define PAIR_TAG     1
 
 void compile_object(Object *obj);
 void compile_fixnum(Object *obj);
@@ -268,6 +277,49 @@ compile_arithmetic_list(OpType op, Object* args) {
     printf("    ;; <-- compile_arithmetic\n");
 }
 
+void
+compile_cons(Object *obj) {
+    printf("    ;; --> compile_cons\n");
+    // Store objects into the car and cdr.
+    compile_object(obj->head);
+    compile_object(obj->tail->head);
+    printf("    pop     rdx\n");
+    printf("    pop     rax\n");
+    printf("    mov     [rsi], rax\n");
+    printf("    mov     [rsi + 8], rdx\n");
+
+    // Push memory address of cons cell.
+    printf("    mov     rax, rsi\n");
+    printf("    or      rax, %ld\n", PAIR_TAG);
+    printf("    push    rax\n");
+
+    // Bump allocation register.
+    printf("    add     rsi, 16\n");
+    printf("    ;; <-- compile_cons\n");
+}
+
+void
+compile_car(Object *obj) {
+    printf("    ;; <-- compile_car\n");
+    compile_object(obj->head);
+    printf("    pop     rax\n");
+    printf("    and     eax, %ld\n", ~PAIR_MASK);
+    printf("    mov     rdx, [rax]\n");
+    printf("    push    rdx\n");
+    printf("    ;; <-- compile_car\n");
+}
+
+void
+compile_cdr(Object *obj) {
+    printf("    ;; <-- compile_cdr\n");
+    compile_object(obj->head);
+    printf("    pop     rax\n");
+    printf("    and     eax, %ld\n", ~PAIR_MASK);
+    printf("    mov     rdx, [rax + 8]\n");
+    printf("    push    rdx\n");
+    printf("    ;; <-- compile_cdr\n");
+}
+
 void
 compile_proc_call(Object *obj) {
     // TODO: Probably we want to use a hash table for these lookups that is
@@ -310,6 +362,12 @@ compile_proc_call(Object *obj) {
         compile_cmp_list(OP_GREATER_EQ, obj->tail);
     } else if (sv_equal(&obj->head->text, &STRING("<="))) {
         compile_cmp_list(OP_LESS_EQ, obj->tail);
+    } else if (sv_equal(&obj->head->text, &STRING("cons"))) {
+        compile_cons(obj->tail);
+    } else if (sv_equal(&obj->head->text, &STRING("car"))) {
+        compile_car(obj->tail);
+    } else if (sv_equal(&obj->head->text, &STRING("cdr"))) {
+        compile_cdr(obj->tail);
     } else {
         fprintf(stderr, "error: not implemented\n");
         exit(EXIT_FAILURE);
@@ -359,12 +417,12 @@ compile(Root *roots) {
     printf("%%define FIXNUM_MASK  %d\n", FIXNUM_MASK);
     printf("%%define FIXNUM_TAG   %d\n", FIXNUM_TAG);
     printf("%%define FIXNUM_SHIFT %d\n", FIXNUM_SHIFT);
+    printf("%%define HEAP_SIZE    %ld\n", HEAP_SIZE);
     printf("\n");
     emit_file(PRELUDE_FILE);
     for (size_t i = 0; i < array_size(roots); i++) {
         Object *root = roots[i];
         compile_object(root);
-        // OBJ_PRINT(root);
     }
     emit_file(POSTLUDE_FILE);
 }
diff --git a/src/parser.c b/src/parser.c
index 9191aa2..5632e13 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -11,6 +11,7 @@ static char *builtins [] = {
     "not", "and", "or",
     "nil?", "zero?", "fixnum?", "bool?",
     "display",
+    "cons", "car", "cdr",
 };
 
 uint64_t
diff --git a/src/x86_64/postlude.asm b/src/x86_64/postlude.asm
index 3355286..37f9df1 100644
--- a/src/x86_64/postlude.asm
+++ b/src/x86_64/postlude.asm
@@ -9,3 +9,7 @@ exit:
     mov     rax, 60
     xor     rdi, rdi
     syscall
+
+section .bss
+bdl_heap:
+    resb    HEAP_SIZE
diff --git a/src/x86_64/prelude.asm b/src/x86_64/prelude.asm
index 6bb6627..e57e820 100644
--- a/src/x86_64/prelude.asm
+++ b/src/x86_64/prelude.asm
@@ -1,3 +1,4 @@
+section .text
 printdln:
     sar     rdi, FIXNUM_SHIFT
     sub     rsp, 40
@@ -98,4 +99,8 @@ display_end:
 
 global  _start
 _start:
-    push NIL_VAL
+    ;; point `rdi` to the start of the heap.
+    mov     rsi, bdl_heap
+
+    ;; make sure the last element in the stack is the nil value.
+    push    NIL_VAL
-- 
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