lower: 슬라이싱

x[a..b] 는 인덱스가 아니라 포인터와 길이를 만든다. 양쪽 끝을 -- 서로에 대해,
그리고 있는 것에 대해 -- 검사한 뒤에 포인터를 만든다. 유효한 범위의 빈
슬라이스는 괜찮고 끝보다 늦게 시작하는 것은 아니다.

파서가 x[a] 와 x[a..] 를 구분하지 못했다. 둘 다 b 만 있고 c 가 없어서
모양이 같았다. '..' 가 있었으면 노드에 표시한다.

정수 출력이 된다: fizz 12345 -678

fec/src/ast.h

@@ -44,6 +44,9 @@ struct FeNode {
 #define FE_NODE_PUB    0x8U
 #define FE_NODE_COMPTIME 0x10U
 #define FE_NODE_EXTERN   0x20U
+/* An index expression that had `..` in it, so it makes a slice rather than
+   reaching an element. `x[a]` and `x[a..]` are otherwise the same shape. */
+#define FE_NODE_SLICE    0x40U
 
 typedef struct FeAst {
     FeArena arena;

fec/src/lower.c

@@ -62,6 +62,8 @@ static void lower_stmt(Lower *L, FeNode *n);
 static void store_into(Lower *L, FeIrPlace dst, Slot value, FeNode *n,
                        unsigned long size);
 static void lower_for(Lower *L, FeNode *n);
+static Slot lower_slice(Lower *L, FeNode *n);
+static void guard(Lower *L, unsigned ok, FeIrTrap reason, unsigned long line);
 static void lower_match(Lower *L, FeNode *n);
 static int enum_has_payload(const FeType *t);
 static void lower_global(Lower *L, FeNode *n);
@@ -673,7 +675,7 @@ static Slot lower_expr_core(Lower *L, FeNode *n)
         unsigned scale;
         unsigned offset;
         unsigned addr;
-        if (n->c || !n->b) { fail(L, "a slice expression", n); return slot_void(); }
+        if (n->flags & FE_NODE_SLICE) return lower_slice(L, n);
         base = lower_expr(L, n->a);
         indexable_parts(L, base, bt, &data, &length, n);
         index = as_value(L, lower_expr(L, n->b), n->b);
@@ -973,6 +975,49 @@ static void lower_while(Lower *L, FeNode *n)
     L->b = done;
 }
 
+/* `x[a..b]` makes a pointer and a length out of part of something indexable.
+   Both ends are checked -- against each other and against what is there --
+   before the pointer is formed. An empty slice of a valid range is fine; one
+   that starts past its end is not. */
+static Slot lower_slice(Lower *L, FeNode *n)
+{
+    FeType *bt = n->a ? n->a->sem_type : 0;
+    FeType *elem = bt ? bt->elem : 0;
+    FeType *t = n->sem_type;
+    Slot base = lower_expr(L, n->a);
+    unsigned data;
+    unsigned length;
+    unsigned from;
+    unsigned to;
+    unsigned local;
+    unsigned scale;
+    unsigned off;
+    unsigned at;
+    unsigned count;
+    indexable_parts(L, base, bt, &data, &length, n);
+    from = n->b ? as_value(L, lower_expr(L, n->b), n->b)
+                : fe_ir_const(L->m, L->b, FE_IR_I32, 0);
+    to = n->c ? as_value(L, lower_expr(L, n->c), n->c) : length;
+    if (!L->c->no_checks) {
+        unsigned ordered = fe_ir_binary(L->m, L->b, FE_IR_LE, FE_IR_I32,
+                                        from, to, 1);
+        unsigned within;
+        guard(L, ordered, FE_TRAP_BOUNDS, n->loc.line);
+        within = fe_ir_binary(L->m, L->b, FE_IR_LE, FE_IR_I32, to, length, 1);
+        guard(L, within, FE_TRAP_BOUNDS, n->loc.line);
+    }
+    scale = fe_ir_const(L->m, L->b, FE_IR_I32, (long)ir_size(elem));
+    off = fe_ir_binary(L->m, L->b, FE_IR_MUL, FE_IR_I32, from, scale, 1);
+    at = fe_ir_binary(L->m, L->b, FE_IR_ADD, FE_IR_PTR, data, off, 1);
+    count = fe_ir_binary(L->m, L->b, FE_IR_SUB, FE_IR_I32, to, from, 1);
+    local = scratch(L, t, "slice");
+    fe_ir_store(L->m, L->b, fe_ir_at_local(local, SLICE_PTR_OFFSET), at,
+                FE_IR_PTR);
+    fe_ir_store(L->m, L->b, fe_ir_at_local(local, SLICE_LEN_OFFSET), count,
+                FE_IR_I32);
+    return slot_place(fe_ir_at_local(local, 0), FE_IR_MEM, ir_size(t));
+}
+
 /* Three shapes share the keyword.
 
      for i in a..b { }        counts

fec/src/parser.c

@@ -124,8 +124,8 @@ static FeNode *postfix(FeParser *p)
         if(eat(p,FE_TOK_LPAREN)) { m=toknode(p,FE_N_CALL,t); m->a=n; while(!is(p,FE_TOK_RPAREN)&&!is(p,FE_TOK_EOF)){fe_node_add(m,delimited_expr(p));if(!eat(p,FE_TOK_COMMA))break;} want(p,FE_TOK_RPAREN,"expected ')' after call"); n=m; }
         else if(eat(p,FE_TOK_LBRACKET)) {
             m=toknode(p,FE_N_INDEX,t);m->a=n;
-            if(is(p,FE_TOK_DOTDOT)) m->b=0; else m->b=delimited_expr(p);
-            if(eat(p,FE_TOK_DOTDOT)) { if(!is(p,FE_TOK_RBRACKET)) m->c=delimited_expr(p); }
+            if(is(p,FE_TOK_DOTDOT)) { m->b=0; m->flags|=FE_NODE_SLICE; } else m->b=delimited_expr(p);
+            if(eat(p,FE_TOK_DOTDOT)) { m->flags|=FE_NODE_SLICE; if(!is(p,FE_TOK_RBRACKET)) m->c=delimited_expr(p); }
             want(p,FE_TOK_RBRACKET,"expected ']' after index");n=m;
         }
         else if(eat(p,FE_TOK_DOT)) {

fec/tests/exec/printnum.fe

@@ -0,0 +1,20 @@
+// EXIT:0
+// OUTPUT:fizz 12345 -678
+unit printnum;
+import std.io;
+import std.fmt;
+
+fn show(v: i32) -> void {
+    var buf: [16]u8 = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0];
+    let n: usize = fmt.fmt_i32(buf[..], v);
+    io.print(" ");
+    io.print(buf[0..n]);
+}
+
+fn main() -> i32 {
+    io.print("fizz");
+    show(12345);
+    show(0 - 678);
+    io.print("\n");
+    return 0;
+}