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7l: throw an error if CASE uses the same register for base and offset (CASE R1, R1)

--- a/sys/src/cmd/7l/asmout.c

+++ b/sys/src/cmd/7l/asmout.c

@@ -818,6 +818,8 @@

 		break;

 	case 62:	/* case Rv, Rt -> adr tab, Rt; movw Rt[R<<2], Rl; add Rt, Rl; br (Rl) */

+		if(p->from.reg == p->to.reg)

+			diag("invalid SWITCH\n%P", p);

 		o1 = ADR(0, 4*4, p->to.reg);	/* adr 4(pc), Rt */

 		o2 = (2<<30)|(7<<27)|(2<<22)|(1<<21)|(3<<13)|(1<<12)|(2<<10)|(p->from.reg<<16)|(p->to.reg<<5)|REGTMP;	/* movw Rt[Rv<<2], REGTMP */

 		o3 = oprrr(AADD) | (p->to.reg<<16) | (REGTMP<<5) | REGTMP;	/* add Rt, REGTMP */