use special value of -1 for global value if symbol is undefined

devyn · Aug 17, 2023 · 24fcdc4 · 24fcdc4
1 parent b69e337
commit 24fcdc4
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Showing 3 changed files with 21 additions and 8 deletions.
diff --git a/stage1/symbol.s b/stage1/symbol.s
@@ -108,7 +108,7 @@ symbol_intern:
         mv a2, s4
         call mem_copy
         # create a new object for the symbol
-        mv a3, zero # LISP_SYMBOL_GLOBAL_VALUE
+        li a3, -1 # LISP_SYMBOL_GLOBAL_VALUE = undefined
         mv a2, s2 # LISP_SYMBOL_LEN
         mv a1, s4 # LISP_SYMBOL_BUF
         li a0, LISP_OBJECT_TYPE_SYMBOL

diff --git a/stage1/words.s b/stage1/words.s
@@ -415,23 +415,31 @@ lookup_var:
         mv a0, a1
         mv a1, s1
         call lookup
-        bnez a0, .Llookup_var_ret
+        bnez a0, .Llookup_var_end
         # if not found, return global value of the symbol
         ld a0, LISP_SYMBOL_GLOBAL_VALUE(s1)
+        li t0, -1
+        beq a0, t0, .Llookup_var_undef # undefined
         call acquire_object
         mv a1, a0
-.Llookup_var_ret:
+.Llookup_var_end:
         # clean up the extra symbol ref
         mv t0, a1
         mv a0, s1 # symbol
         mv s1, t0 # s1 now = found return a1
         call release_object
         li a0, 1
         mv a1, s1
+.Llookup_var_ret:
         ld ra, 0x00(sp)
         ld s1, 0x08(sp)
         addi sp, sp, 0x10
         ret
+.Llookup_var_undef:
+        mv a0, s1
+        call release_object
+        li a0, 0
+        j .Llookup_var_ret
 
 # Define a new word
 #
@@ -455,9 +463,12 @@ define:
         # swap the current global value of the symbol
         ld t0, LISP_SYMBOL_GLOBAL_VALUE(a0)
         sd a1, LISP_SYMBOL_GLOBAL_VALUE(a0)
-        # release the old value
+        # release the old value unless it was undefined
+        li t1, -1
+        beq t0, t1, 1f
         mv a0, t0
         call release_object
+1:
         # release the symbol
         ld a0, 0x08(sp)
         call release_object

diff --git a/stage2/22-words-util.lsp b/stage2/22-words-util.lsp
@@ -5,8 +5,10 @@
         (symbol-addr (ref symbol))
         (global-value-addr (peek.d (+ symbol-addr 24)))
       )
-      (seq
-        (call-native acquire-object$ 0 global-value-addr)
-        (call-native release-object$ 0 symbol-addr)
-        (deref global-value-addr)))
+      (if (number-eq? global-value-addr -1)
+        (error (quote undefined:) symbol)
+        (seq
+          (call-native acquire-object$ 0 global-value-addr)
+          (call-native release-object$ 0 symbol-addr)
+          (deref global-value-addr))))
     (error (quote not-a-symbol:) symbol))))