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Add used field in Nodes of cc2

This field allows to know when a node was already used and
the register used by it can be reused.

--- a/cc2/cc2.h

+++ b/cc2/cc2.h

@@ -81,6 +81,7 @@

 	uint8_t addable;

 	uint8_t reg;

 	Symbol *sym;

+	bool used : 1;

 	struct node *left, *right;

 };

--- a/cc2/cgen.c

+++ b/cc2/cgen.c

@@ -86,20 +86,22 @@

 {

 	static uint8_t reg8[] = {A, B, C, D, E, H, L, IYL, IY, 0};

 	static uint8_t reg16[] = {BC, DE, IY, 0};

-	uint8_t *bp, c;

+	Node *np;

+	uint8_t *ary, *bp, c;

 	switch (size) {

 	case 1:

-		bp = reg8;

+		ary = reg8;

 		break;

 	case 2:

-		bp = reg16;

+		ary = reg16;

 		break;

 	default:

 		abort();

 	}

-	while (c = *bp++) {

-		if (reguse[c])

+	for (bp = ary; c = *bp; ++bp) {

+		np = reguse[c];

+		if (np && !np->used)

 			continue;

 		return c;

 	}

@@ -299,6 +301,7 @@

 	add1:

 		move(rp, np);

 		code((np->op == OADD) ? ADD : SUB, lp, rp);

+		lp->used = rp->used = 1;

 		np->op = REG;

 		np->reg = A;

 		reguse[A] = np;

@@ -315,7 +318,6 @@

 	Symbol *sym = lp->sym;

 	assert(rp->op == REG);

-	/* TODO: what happens with the previous register? */

 	switch (np->type.size) {

 	case 1:

 		switch (lp->op) {

@@ -337,6 +339,7 @@

 		abort();

 	}

+	lp->used = 1;

 	np->op = REG;

 	np->reg = rp->reg;

 }