Adds removal of variables with fixed values.

This is disabled by default, enable with "-O +fixed_vars".

README.md

@@ -286,6 +286,15 @@ The optimization sub-options are:
   a variable initialized to the value. The initialization code is added
   before any statement in the program, and tries to use the minimum number
   of bytes posible.
+- `fixed_vars`: This is the complement of the `const_replace` option, tries to
+  identify variables with a fixed value in the whole program and removes the
+  variable. Use this optimization when converting original basic listings, as
+  reversing the constant replacing gives a simpler listing and allows to apply
+  further optimizations.  Note that currently this option can produce bad
+  results, as it does not follows the program flow and can't detect if a
+  variable is used before the first assignment, so it is not enabled by
+  default. You need to check each removed variable, as printed in the output
+  and in the comments in the resulting program.
 
 Note that options can be changed at any place in the file, this is an example
 of changing the parser mode in the middle of the file:

src/main.c

@@ -209,12 +209,12 @@ int main(int argc, char **argv)
                     {
                         // Not an optimize option, go back and
                         // assume "set all optimizations".
-                        do_optimize = -1;
+                        do_optimize = optimize_all();
                         optind --;
                     }
                 }
                 else
-                    do_optimize = -1;
+                    do_optimize = optimize_all();
                 break;
             case 'n':
                 max_line_len = atoi(optarg);

src/optimize.c

@@ -40,10 +40,17 @@ enum optimize_levels optimize_option(const char *opt)
         return OPT_LINE_NUM;
     else if( 0 == strcmp(opt, "const_replace") )
         return OPT_CONST_VARS;
+    else if( 0 == strcmp(opt, "fixed_vars") )
+        return OPT_FIXED_VARS;
     else
         return 0;
 }
 
+enum optimize_levels optimize_all(void)
+{
+    return OPT_CONST_FOLD | OPT_NUMBER_TOK | OPT_COMMUTE | OPT_LINE_NUM | OPT_CONST_VARS;
+}
+
 int optimize_program(program *pgm, int level)
 {
     // Convert program to expression tree
@@ -60,6 +67,17 @@ int optimize_program(program *pgm, int level)
     if( level & OPT_COMMUTE )
         err |= opt_commute(ex);
 
+    if( level & OPT_FIXED_VARS )
+    {
+        err |= opt_replace_fixed_vars(ex);
+        // After fixed variable removal, redo constant propagation and try again
+        if( level & OPT_CONST_FOLD )
+        {
+            err |= opt_constprop(ex);
+            err |= opt_replace_fixed_vars(ex);
+        }
+    }
+
     if( level & OPT_LINE_NUM )
         err |= opt_remove_line_num(ex);
 

src/optimize.h

@@ -24,9 +24,13 @@ enum optimize_levels {
     OPT_NUMBER_TOK = 2,
     OPT_COMMUTE    = 4,
     OPT_LINE_NUM   = 8,
-    OPT_CONST_VARS = 16
+    OPT_CONST_VARS = 16,
+    OPT_FIXED_VARS = 32
 };
 
+// Returns the "standard" optimizations
+enum optimize_levels optimize_all(void);
+
 // Returns the optimization option from the text
 enum optimize_levels optimize_option(const char *txt);