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[cc1] Add basic cupport for bit fields

This is a first implementation of bitfields which adds the parsing
of the expression and checks all the errors that can happen in a
bitfield declaration, but does nothing with the bit field definition,
so they will act like usual integers (which is allowed by the
standard).

--- a/cc1/decl.c

+++ b/cc1/decl.c

@@ -609,6 +609,7 @@

 	return sym;

 }

+#define NAME(s) (*(s)->name ? (s)->name : "<anonymous>")

 static Symbol *

 field(struct decl *dcl)

 {

@@ -618,8 +619,30 @@

 	TINT n = structp->n.elem;

 	int err = 0;

-	if (empty(sym, tp, 0))

+	/* TODO: print name of the field in the errors */

+	if (accept(':')) {

+		Node *np;

+		TINT n;

+

+		if ((np = iconstexpr()) == NULL) {

+			unexpected();

+			n = 0;

+		} else {

+			n = np->sym->u.i;

+			freetree(np);

+		}

+		if (n == 0 && *sym->name)

+			errorp("zero width for bit-field '%s'", sym->name);

+		if (tp != booltype && tp != inttype && tp != uinttype)

+			errorp("bit-field '%s' has invalid type", NAME(sym));

+		if (n < 0)

+			errorp("negative width in bit-field '%s'", NAME(sym));

+		else if (n > tp->size*8)

+			errorp("width of '%s' exceeds its type", NAME(sym));

+	} else if (empty(sym, tp, 0)) {

 		return sym;

+	}

+

 	if (tp->op == FTN) {

 		errorp("invalid type in struct/union");

 		err = 1;

@@ -649,6 +672,7 @@

 	return sym;

 }

+#undef NAME

 static void

 bad_storage(Type *tp, char *name)