From 047cd40313e39b662650bbf6c8059ab0910e5986 Mon Sep 17 00:00:00 2001
From: Paul Sokolovsky <pfalcon@users.sourceforge.net>
Date: Wed, 19 Feb 2014 15:47:59 +0200
Subject: [PATCH] Bytecode int varlen encoding: support arbitrary values for
 signed ints too.

---
 py/emitbc.c | 32 ++++++++++++++++++++++++--------
 py/showbc.c | 15 +++++++++++----
 py/vm.c     | 15 +++++++++++----
 3 files changed, 46 insertions(+), 16 deletions(-)

diff --git a/py/emitbc.c b/py/emitbc.c
index 4fe7ae8cd0..9fab977909 100644
--- a/py/emitbc.c
+++ b/py/emitbc.c
@@ -123,15 +123,31 @@ STATIC void emit_write_byte_code_uint(emit_t* emit, uint num) {
     *c = *p;
 }
 
-// integers (for small ints) are stored as 24 bits, in excess
+// Similar to emit_write_byte_code_uint(), just some extra handling to encode sign
 STATIC void emit_write_byte_code_byte_int(emit_t* emit, byte b1, machine_int_t num) {
-    num += 0x800000;
-    assert(0 <= num && num <= 0xffffff);
-    byte* c = emit_get_cur_to_write_byte_code(emit, 4);
-    c[0] = b1;
-    c[1] = num;
-    c[2] = num >> 8;
-    c[3] = num >> 16;
+    emit_write_byte_code_byte(emit, b1);
+
+    // We store each 7 bits in a separate byte, and that's how many bytes needed
+    byte buf[(BYTES_PER_WORD * 8 + 7) / 7];
+    byte *p = buf + sizeof(buf);
+    // We encode in little-ending order, but store in big-endian, to help decoding
+    do {
+        *--p = num & 0x7f;
+        num >>= 7;
+    } while (num != 0 && num != -1);
+    // Make sure that highest bit we stored (mask 0x40) matches sign
+    // of the number. If not, store extra byte just to encode sign
+    if (num == -1 && (*p & 0x40) == 0) {
+        *--p = 0x7f;
+    } else if (num == 0 && (*p & 0x40) != 0) {
+        *--p = 0;
+    }
+
+    byte* c = emit_get_cur_to_write_byte_code(emit, buf + sizeof(buf) - p);
+    while (p != buf + sizeof(buf) - 1) {
+        *c++ = *p++ | 0x80;
+    }
+    *c = *p;
 }
 
 STATIC void emit_write_byte_code_byte_uint(emit_t* emit, byte b, uint num) {
diff --git a/py/showbc.c b/py/showbc.c
index e3387dbe27..837ee7611f 100644
--- a/py/showbc.c
+++ b/py/showbc.c
@@ -57,11 +57,18 @@ void mp_byte_code_print(const byte *ip, int len) {
                 printf("LOAD_CONST_ELLIPSIS");
                 break;
 
-            case MP_BC_LOAD_CONST_SMALL_INT:
-                unum = (ip[0] | (ip[1] << 8) | (ip[2] << 16)) - 0x800000;
-                ip += 3;
-                printf("LOAD_CONST_SMALL_INT %d", (int)unum);
+            case MP_BC_LOAD_CONST_SMALL_INT: {
+                int num = 0;
+                if ((ip[0] & 0x40) != 0) {
+                    // Number is negative
+                    num--;
+                }
+                do {
+                    num = (num << 7) | (*ip & 0x7f);
+                } while ((*ip++ & 0x80) != 0);
+                printf("LOAD_CONST_SMALL_INT %d", num);
                 break;
+            }
 
             case MP_BC_LOAD_CONST_INT:
                 DECODE_QSTR;
diff --git a/py/vm.c b/py/vm.c
index 0fdf5d4e31..cab340ff38 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -156,11 +156,18 @@ dispatch_loop:
                         PUSH(mp_const_ellipsis);
                         break;
 
-                    case MP_BC_LOAD_CONST_SMALL_INT:
-                        unum = (ip[0] | (ip[1] << 8) | (ip[2] << 16)) - 0x800000;
-                        ip += 3;
-                        PUSH(MP_OBJ_NEW_SMALL_INT(unum));
+                    case MP_BC_LOAD_CONST_SMALL_INT: {
+                        int num = 0;
+                        if ((ip[0] & 0x40) != 0) {
+                            // Number is negative
+                            num--;
+                        }
+                        do {
+                            num = (num << 7) | (*ip & 0x7f);
+                        } while ((*ip++ & 0x80) != 0);
+                        PUSH(MP_OBJ_NEW_SMALL_INT(num));
                         break;
+                    }
 
                     case MP_BC_LOAD_CONST_INT:
                         DECODE_QSTR;