StepTalk patch: argument bytecode sign bug fix, optimization literals, stack overflow fix

[Mateu Batle]

Hi everybody !

I have submitted a patch to savannah web, which fixes following bugs:

- Fixed bug with signed/unsigned bytecode, which made StepTalk to crash 
if has more than 127 literals in the array.

- Possible problem of stack overflow, returned object are never pop from 
stack if nobody uses them. Please check this fix.

- Optimization: literal array usage, don't create a new entry if already 
exists

Some comments also:

- bytecodes seem to be 1 byte long for each, operation and two arguments 
(max 3 bytes).
I think it may fall short in some cases, for example for the literals 
table index. This is the
reason of the optimization submitted in the patch, before all literals 
where always appended
to the array without checking if they could be reused.

- In STBytecode bytecode code and arguments are defined as short, but 
they are always 1 byte.
It does not matter that much, but it can be confusing.

cheers !
 Mateu Batle

Index: STCompiler.m
===================================================================
--- STCompiler.m        (revision 147)
+++ STCompiler.m        (working copy)
@@ -470,8 +470,12 @@
 
 - (unsigned)addLiteral:literal
 {
-    [literals addObject:literal];
-    return [literals count] - 1;
+    unsigned index = [literals indexOfObject: literal];
+    if (index == NSNotFound) {
+        [literals addObject:literal];
+        index = [literals count] - 1;
+    }
+    return index;
 }
 
 - (unsigned)externalVariableIndex:(NSString *)varName
@@ -596,10 +600,17 @@
     
     if(array)
     {
+        BOOL first = YES;
         enumerator = [array objectEnumerator];
         while((expr = [enumerator nextObject]))
         {
-            [self compileExpression:expr];
+            // mateu.batle: ignore returned value on the stack ? please check 
this !!!
+            if (!first) {
+                [self emitPopStack];
+            } else {
+                first = NO;
+            }
+            [self compileExpression:expr];            
         }
     }
 
Index: STBytecodes.m
===================================================================
--- STBytecodes.m       (revision 147)
+++ STBytecodes.m       (working copy)
@@ -206,7 +206,7 @@
 - (STBytecode)fetchNextBytecodeAtPointer:(unsigned *)pointer
 {
     STBytecode  bytecode;
-    const char *bytesPtr = (const char *)[bytes bytes];
+    const unsigned char *bytesPtr = (const unsigned char *)[bytes bytes];
     unsigned    length = [self length];
     
     if(*pointer < length)