diff --git a/py/lexer.c b/py/lexer.c
index 6a3fa65..5c942f9 100644
--- a/py/lexer.c
+++ b/py/lexer.c
@@ -25,6 +25,7 @@
  */
 
 #include <stdio.h>
+#include <string.h>
 #include <assert.h>
 
 #include "py/mpstate.h"
@@ -39,19 +40,6 @@
 // TODO seems that CPython allows NULL byte in the input stream
 // don't know if that's intentional or not, but we don't allow it
 
-// TODO replace with a call to a standard function
-STATIC bool str_strn_equal(const char *str, const char *strn, mp_uint_t len) {
-    mp_uint_t i = 0;
-
-    while (i < len && *str == *strn) {
-        ++i;
-        ++str;
-        ++strn;
-    }
-
-    return i == len && *str == 0;
-}
-
 #define MP_LEXER_EOF ((unichar)MP_READER_EOF)
 #define CUR_CHAR(lex) ((lex)->chr0)
 
@@ -225,10 +213,12 @@
 };
 
 // must have the same order as enum in lexer.h
+// must be sorted according to strcmp
 STATIC const char *const tok_kw[] = {
     "False",
     "None",
     "True",
+    "__debug__",
     "and",
     "as",
     "assert",
@@ -263,7 +253,6 @@
     "while",
     "with",
     "yield",
-    "__debug__",
 };
 
 // This is called with CUR_CHAR() before first hex digit, and should return with
@@ -531,16 +520,18 @@
         // We also check for __debug__ here and convert it to its value.  This is
         // so the parser gives a syntax error on, eg, x.__debug__.  Otherwise, we
         // need to check for this special token in many places in the compiler.
-        // TODO improve speed of these string comparisons
+        const char *s = vstr_null_terminated_str(&lex->vstr);
         for (size_t i = 0; i < MP_ARRAY_SIZE(tok_kw); i++) {
-            if (str_strn_equal(tok_kw[i], lex->vstr.buf, lex->vstr.len)) {
-                if (i == MP_ARRAY_SIZE(tok_kw) - 1) {
-                    // tok_kw[MP_ARRAY_SIZE(tok_kw) - 1] == "__debug__"
+            int cmp = strcmp(s, tok_kw[i]);
+            if (cmp == 0) {
+                lex->tok_kind = MP_TOKEN_KW_FALSE + i;
+                if (lex->tok_kind == MP_TOKEN_KW___DEBUG__) {
                     lex->tok_kind = (MP_STATE_VM(mp_optimise_value) == 0 ? MP_TOKEN_KW_TRUE : MP_TOKEN_KW_FALSE);
-                } else {
-                    lex->tok_kind = MP_TOKEN_KW_FALSE + i;
                 }
                 break;
+            } else if (cmp < 0) {
+                // Table is sorted and comparison was less-than, so stop searching
+                break;
             }
         }
 
diff --git a/py/lexer.h b/py/lexer.h
index 32aef96..d407192 100644
--- a/py/lexer.h
+++ b/py/lexer.h
@@ -61,6 +61,7 @@
     MP_TOKEN_KW_FALSE,              // 14
     MP_TOKEN_KW_NONE,
     MP_TOKEN_KW_TRUE,
+    MP_TOKEN_KW___DEBUG__,
     MP_TOKEN_KW_AND,
     MP_TOKEN_KW_AS,
     MP_TOKEN_KW_ASSERT,
@@ -71,7 +72,7 @@
     MP_TOKEN_KW_BREAK,
     MP_TOKEN_KW_CLASS,
     MP_TOKEN_KW_CONTINUE,
-    MP_TOKEN_KW_DEF,                // 23
+    MP_TOKEN_KW_DEF,
     MP_TOKEN_KW_DEL,
     MP_TOKEN_KW_ELIF,
     MP_TOKEN_KW_ELSE,
@@ -81,7 +82,7 @@
     MP_TOKEN_KW_FROM,
     MP_TOKEN_KW_GLOBAL,
     MP_TOKEN_KW_IF,
-    MP_TOKEN_KW_IMPORT,             // 33
+    MP_TOKEN_KW_IMPORT,
     MP_TOKEN_KW_IN,
     MP_TOKEN_KW_IS,
     MP_TOKEN_KW_LAMBDA,
@@ -91,12 +92,12 @@
     MP_TOKEN_KW_PASS,
     MP_TOKEN_KW_RAISE,
     MP_TOKEN_KW_RETURN,
-    MP_TOKEN_KW_TRY,                // 43
+    MP_TOKEN_KW_TRY,
     MP_TOKEN_KW_WHILE,
     MP_TOKEN_KW_WITH,
     MP_TOKEN_KW_YIELD,
 
-    MP_TOKEN_OP_PLUS,               // 47
+    MP_TOKEN_OP_PLUS,
     MP_TOKEN_OP_MINUS,
     MP_TOKEN_OP_STAR,
     MP_TOKEN_OP_DBL_STAR,
@@ -106,7 +107,7 @@
     MP_TOKEN_OP_LESS,
     MP_TOKEN_OP_DBL_LESS,
     MP_TOKEN_OP_MORE,
-    MP_TOKEN_OP_DBL_MORE,           // 57
+    MP_TOKEN_OP_DBL_MORE,
     MP_TOKEN_OP_AMPERSAND,
     MP_TOKEN_OP_PIPE,
     MP_TOKEN_OP_CARET,
@@ -116,7 +117,7 @@
     MP_TOKEN_OP_DBL_EQUAL,
     MP_TOKEN_OP_NOT_EQUAL,
 
-    MP_TOKEN_DEL_PAREN_OPEN,        // 66
+    MP_TOKEN_DEL_PAREN_OPEN,
     MP_TOKEN_DEL_PAREN_CLOSE,
     MP_TOKEN_DEL_BRACKET_OPEN,
     MP_TOKEN_DEL_BRACKET_CLOSE,
@@ -126,7 +127,7 @@
     MP_TOKEN_DEL_COLON,
     MP_TOKEN_DEL_PERIOD,
     MP_TOKEN_DEL_SEMICOLON,
-    MP_TOKEN_DEL_AT,                // 76
+    MP_TOKEN_DEL_AT,
     MP_TOKEN_DEL_EQUAL,
     MP_TOKEN_DEL_PLUS_EQUAL,
     MP_TOKEN_DEL_MINUS_EQUAL,
@@ -136,7 +137,7 @@
     MP_TOKEN_DEL_PERCENT_EQUAL,
     MP_TOKEN_DEL_AMPERSAND_EQUAL,
     MP_TOKEN_DEL_PIPE_EQUAL,
-    MP_TOKEN_DEL_CARET_EQUAL,       // 86
+    MP_TOKEN_DEL_CARET_EQUAL,
     MP_TOKEN_DEL_DBL_MORE_EQUAL,
     MP_TOKEN_DEL_DBL_LESS_EQUAL,
     MP_TOKEN_DEL_DBL_STAR_EQUAL,