Add constant support

doc/ref/language.xml

@@ -539,9 +539,35 @@ Certain special facilities are provided for manipulating global
 variables which are not available for other types of variable (such as 
 local variables or function arguments).
 <P/>
-First, such variables may be marked <E>read-only</E>. In which case
+First, such variables may be marked <E>read-only</E> using
+<Ref Func="MakeReadOnlyGlobal"/>. In which case
 attempts to change them will fail. Most of the global variables
-defined in the &GAP; library are so marked.
+defined in the &GAP; library are so marked. <E>read-only</E> variables
+can be made read-write again by calling <Ref Func="MakeReadWriteGlobal"/>.
+GAP also features <E>constant</E> variables, which are created by calling
+<Ref Func="MakeConstantGlobal"/>. Constant variables can never be changed.
+In some cases, GAP can optimise code which uses <E>constant</E> variables,
+as their value never changes. This can be observed by printing the
+function back out.
+<Example><![CDATA[
+gap> globali := 1 + 2;;
+gap> globalb := true;;
+gap> MakeConstantGlobal("globali");
+gap> MakeConstantGlobal("globalb");
+gap> f := function()
+>     if globalb then
+>         return globali + 1;
+>     else
+>         return globali + 2;
+>     fi;
+> end;;
+gap> Print(f);
+function (  )
+    return 3 + 1;
+end
+]]></Example>
+
+
 <P/>
 Second, a group of functions are supplied for accessing and altering the
 values assigned to global variables. Use of these functions differs

lib/global.g

@@ -70,6 +70,16 @@ UNBIND_GLOBAL := UNB_GVAR;
 
 IS_READ_ONLY_GLOBAL := IsReadOnlyGVar;
 
+#############################################################################
+##
+#F  IS_CONSTANT_GLOBAL ( <name> ) determine if a global variable is constant
+##
+##  IS_CONSTANT_GLOBAL ( <name> ) returns true if the global variable
+##  named by the string <name> is constant and false otherwise (the default)
+##
+
+IS_CONSTANT_GLOBAL := IsConstantGVar;
+
 #############################################################################
 ##
 #F  MAKE_READ_ONLY_GLOBAL ( <name> ) . . . . make a global variable read-only
@@ -90,6 +100,16 @@ MAKE_READ_ONLY_GLOBAL := MakeReadOnlyGVar;
 
 MAKE_READ_WRITE_GLOBAL := MakeReadWriteGVar;
 
+#############################################################################
+##
+#F  MAKE_CONSTANT_GLOBAL ( <name> )
+##
+##  MAKE_CONSTANT_GLOBAL ( <name> ) marks the global variable named
+##  by the string <name> as constant
+##
+
+MAKE_CONSTANT_GLOBAL := MakeConstantGVar;
+
 #############################################################################
 ##
 #V  REREADING                set to true inside a Reread, changes much

lib/global.gd

@@ -115,6 +115,21 @@ DeclareGlobalFunction("UnbindGlobal");
 ##
 DeclareGlobalFunction("IsReadOnlyGlobal");
 
+#############################################################################
+##
+#F  IsConstantGlobal( <name> )  . determine if a global variable is constant
+##
+##  <ManSection>
+##  <Func Name="IsConstantGlobal" Arg='name'/>
+##
+##  <Description>
+##  IsConstantGlobal ( <A>name</A> ) returns true if the global variable
+##  named by the string <A>name</A> is constant and false otherwise (the default).
+##  </Description>
+##  </ManSection>
+##
+DeclareGlobalFunction("IsConstantGlobal");
+
 
 #############################################################################
 ##
@@ -153,6 +168,26 @@ DeclareGlobalFunction("MakeReadOnlyGlobal");
 DeclareGlobalFunction("MakeReadWriteGlobal");
 
 
+#############################################################################
+##
+#F  MakeConstantGlobal( <name> )   . . . . .  make a global variable constant
+##
+##  <ManSection>
+##  <Func Name="MakeConstantGlobal" Arg='name'/>
+##
+##  <Description>
+##  MakeConstantGlobal ( <A>name</A> ) marks the global variable named
+##  by the string <A>name</A> as constant. A constant variable can never
+##  be changed or made read-write. Constant variables can only take an
+##  integer value, <C>true</C> or <C>false</C>.
+##  <P/>
+##  A warning is given if <A>name</A> is already constant.
+##  </Description>
+##  </ManSection>
+##
+DeclareGlobalFunction("MakeConstantGlobal");
+
+
 #############################################################################
 ##
 #F  BindGlobal( <name>, <val> )   . . . . . . sets a global variable `safely'

lib/global.gi

@@ -147,13 +147,31 @@ InstallGlobalFunction( IsReadOnlyGlobal,
     return isro;
 end);
 
+#############################################################################
+##
+#F  IsConstantGlobal ( <name> ) . determine if a global variable is constant
+##
+##  IsConstantGlobal ( <name> ) returns true if the global variable
+##  named by the string <name> is constant and false otherwise (the default)
+##
+
+InstallGlobalFunction( IsConstantGlobal,
+        function (name)
+    local isro;
+    CheckGlobalName( name );
+    isro := IS_CONSTANT_GLOBAL(name);
+    Info( InfoGlobal, 3,
+          "IsConstantGlobal: called for ", name, " returned ", isro);
+    return isro;
+end);
+
 
 #############################################################################
 ##
 #F  MakeReadOnlyGlobal ( <name> ) . . . . .  make a global variable read-only
 ##
-##  MakeReadOnlyGlobal ( <name> ) marks the global variable named
-##  by the string <name> as read-only. 
+##  MakeReadOnlyGlobal ( <name> ) marks the global variable named by the
+##  string <name> as read-only.
 ##
 ##  A warning is given if <name> has no value bound to it or if it is
 ##  already read-only
@@ -182,10 +200,10 @@ end);
 ##
 #F  MakeReadWriteGlobal ( <name> )  . . . . make a global variable read-write
 ##
-##  MakeReadWriteGlobal ( <name> ) marks the global variable named
-##  by the string <name> as read-write
+##  MakeReadWriteGlobal ( <name> ) marks the global variable named by the
+##  string <name> as read-write
 ##
-## A warning is given if <name> is already read-write
+##  A warning is given if <name> is already read-write
 ##
 
 InstallGlobalFunction( MakeReadWriteGlobal, 
@@ -199,6 +217,27 @@ InstallGlobalFunction( MakeReadWriteGlobal,
     MAKE_READ_WRITE_GLOBAL( name );
 end);
 
+#############################################################################
+##
+#F  MakeConstantGlobal ( <name> )  . . . . .  make a global variable constant
+##
+##  MakeConstantGlobal ( <name> ) marks the global variable named by the
+##  string <name> as constant
+##
+##  A warning is given if <name> is already constant
+##
+
+InstallGlobalFunction( MakeConstantGlobal, 
+        function (name)
+    CheckGlobalName( name );
+    if IS_CONSTANT_GLOBAL( name ) then
+        Info( InfoWarning + InfoGlobal, 1, 
+              "MakeConstantGlobal: ", name, " already constant");
+    fi;
+    Info( InfoGlobal, 2, "MakeConstantGlobal: called for ", name);
+    MAKE_CONSTANT_GLOBAL( name );
+end);
+
 
 #############################################################################
 ##

src/code.c

@@ -973,16 +973,54 @@ void CodeIfEnd (
 {
     Stat                stat;           /* if-statement, result            */
     Expr                cond;           /* condition of a branch           */
-    Stat                body;           /* body of a branch                */
     UInt                hase;           /* has else branch                 */
     UInt                i;              /* loop variable                   */
+    Expr                cond1 = 0;      /* first condition                 */
+    Expr                cond2 = 0;      /* second condition                */
+
+    /* peek at the last two conditions                                     */
+    cond1 = PopExpr();
+    hase = (TNUM_EXPR(cond1) == T_TRUE_EXPR);
+    if (nr == 2) {
+        cond2 = PopExpr();
+        PushExpr(cond2);
+    }
+    PushExpr(cond1);
+
+    // Some optimisation cases
+    if (nr == 1) {
+        if (TNUM_EXPR(cond1) == T_TRUE_EXPR) {
+            // Leave statement
+            PopExpr();
+            return;
+        }
+        else if (TNUM_EXPR(cond1) == T_FALSE_EXPR) {
+            // Remove entire if statement
+            PopStat();
+            PopExpr();
+            PushStat(NewStat(T_EMPTY, 0));
+            return;
+        }
+    }
 
-    /* peek at the last condition                                          */
-    body = PopStat();
-    cond = PopExpr();
-    hase = (TNUM_EXPR(cond) == T_TRUE_EXPR);
-    PushExpr( cond );
-    PushStat( body );
+    if (nr == 2 && hase) {
+        if (TNUM_EXPR(cond2) == T_TRUE_EXPR) {
+            // Leave 'true' case
+            PopStat();
+            PopExpr();
+            PopExpr();
+            return;
+        }
+        else if (TNUM_EXPR(cond2) == T_FALSE_EXPR) {
+            // Leave 'false' case
+            Stat body = PopStat();
+            PopExpr();
+            PopStat();
+            PopExpr();
+            PushStat(body);
+            return;
+        }
+    }
 
     /* allocate the if-statement                                           */
     if      ( nr == 1 ) {
@@ -1000,7 +1038,7 @@ void CodeIfEnd (
 
     /* enter the branches                                                  */
     for ( i = nr; 1 <= i; i-- ) {
-        body = PopStat();
+        Stat body = PopStat();
         cond = PopExpr();
         ADDR_STAT(stat)[2*(i-1)] = cond;
         ADDR_STAT(stat)[2*(i-1)+1] = body;
@@ -1616,6 +1654,11 @@ void CodePow ( void )
 }
 
 
+void CodeGAPSmallInt(Obj val)
+{
+    PushExpr(INTEXPR_INT(INT_INTOBJ(val)));
+}
+
 /****************************************************************************
 **
 *F  CodeIntExpr( <str> )  . . . . . . . . . . code literal integer expression

src/code.h

@@ -941,6 +941,7 @@ extern  void            CodePow ( void );
 **  'CodeIntExpr' is the action to code a literal integer expression.  <str>
 **  is the integer as a (null terminated) C character string.
 */
+extern void             CodeGAPSmallInt(Obj obj);
 extern  void            CodeIntExpr (
             Char *              str );
 extern  void            CodeLongIntExpr (