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vl: prevent stack altering instructions to be hoisted above loads

fixes bug with libmemdraw where the linker would hoist
the final ADD $const, SP at the end over loads from the
stack causing the front to fall off once a interrupt hits.

--- a/sys/src/cmd/vl/sched.c

+++ b/sys/src/cmd/vl/sched.c

@@ -85,7 +85,7 @@

 		for(t=s+1; t<=se; t++) {

 			if(!(t->p.mark & LOAD))

 				continue;

-			if(t->p.mark & BRANCH)

+			if(t->p.mark & BRANCH || t->set.ireg & (1<<REGSP))

 				break;

 			if(conflict(s, t))

 				break;

@@ -102,7 +102,7 @@

 		/* put schedule fodder above load */

 		for(t=s+1; t<=se; t++) {

-			if(t->p.mark & BRANCH)

+			if(t->p.mark & BRANCH || t->set.ireg & (1<<REGSP))

 				break;

 			if(s > sch && conflict(s-1, t))

 				continue;

@@ -634,7 +634,6 @@

 int

 conflict(Sch *sa, Sch *sb)

 {

-

 	if(sa->set.ireg & sb->used.ireg)

 		return 1;

 	if(sa->set.freg & sb->used.freg)