Implement ISCONST, reporting compile-time constness

src/asm/asmy.y

@@ -539,6 +539,7 @@ static void strsubUTF8(char *dest, const char *src, uint32_t pos, uint32_t len)
 %left	T_OP_FLOOR
 
 %token	T_OP_HIGH T_OP_LOW
+%token	T_OP_ISCONST
 
 %left	T_OP_STRCMP
 %left	T_OP_STRIN
@@ -1194,6 +1195,7 @@ relocexpr_no_str : scoped_id	{ rpn_Symbol(&$$, $1); }
 		| T_OP_NOT relocexpr %prec NEG	{ rpn_UNNOT(&$$, &$2); }
 		| T_OP_HIGH '(' relocexpr ')'	{ rpn_HIGH(&$$, &$3); }
 		| T_OP_LOW '(' relocexpr ')'	{ rpn_LOW(&$$, &$3); }
+		| T_OP_ISCONST '(' relocexpr ')'{ rpn_ISCONST(&$$, &$3); }
 		| T_OP_BANK '(' scoped_id ')' {
 			/* '@' is also a T_ID, it is handled here. */
 			rpn_BankSymbol(&$$, $3);

src/asm/globlex.c

@@ -460,6 +460,7 @@ const struct sLexInitString lexer_strings[] = {
 
 	{"high", T_OP_HIGH},
 	{"low", T_OP_LOW},
+	{"isconst", T_OP_ISCONST},
 
 	{"strcmp", T_OP_STRCMP},
 	{"strin", T_OP_STRIN},

src/asm/rgbasm.5

@@ -553,7 +553,7 @@ which references the same macro, which has the same problem.
 .Pp
 One of the best features of an assembler is the ability to write macros for it.
 Macros also provide a method of passing arguments to them and they can then react to the input using
-.Sy IF
+.Ic IF
 constructs.
 .Pp
 .Bd -literal -offset indent
@@ -1261,6 +1261,9 @@ For labels, as the linker has to resolve this, it can't be used when the express
 has been defined.
 .It Fn HIGH arg Ta Returns the top 8 bits of the operand if Ar arg No is a label or constant, or the top 8-bit register if it is a 16-bit register.
 .It Fn LOW arg Ta Returns the bottom 8 bits of the operand if Ar arg No is a label or constant, or the bottom 8-bit register if it is a 16-bit register Pq Cm AF No isn't a valid register for this function .
+.It Fn ISCONST arg Ta Returns 1 if Ar arg Ns No 's value is known by RGBASM (e.g. if it can be an argument to
+.Ic IF ) ,
+or 0 if only RGBLINK can compute its value.
 .El
 .Sh MISCELLANEOUS
 .Ss Changing options while assembling

src/asm/rpn.c

@@ -499,6 +499,14 @@ void rpn_LOW(struct Expression *expr, const struct Expression *src)
 	}
 }
 
+void rpn_ISCONST(struct Expression *expr, const struct Expression *src)
+{
+	rpn_Init(expr);
+	expr->nVal = rpn_isKnown(src);
+	expr->isKnown = true;
+	expr->isSymbol = false;
+}
+
 void rpn_UNNEG(struct Expression *expr, const struct Expression *src)
 {
 	*expr = *src;