fix(qbe): any-child terminator check in QBE_IsTerminator for hBlock — QBE SSA now fully valid

- QBE_IsTerminator(hBlock) checks ALL children, not just last
- Removed legacy c_backend.bux and nim_backend.bux from _selfhost/src/
- QBE self-hosting QBE phase: PASSES (SSA valid, assembled to .s)
- Remaining: linker errors from missing String_Eq/StringBuilder in runtime

vendor/qbe/cfg.c

@@ -0,0 +1,331 @@
+#include "all.h"
+
+Blk *
+newblk()
+{
+	static Blk z;
+	Blk *b;
+
+	b = alloc(sizeof *b);
+	*b = z;
+	return b;
+}
+
+void
+edgedel(Blk *bs, Blk **pbd)
+{
+	Blk *bd;
+	Phi *p;
+	uint a;
+	int mult;
+
+	bd = *pbd;
+	mult = 1 + (bs->s1 == bs->s2);
+	*pbd = 0;
+	if (!bd || mult > 1)
+		return;
+	for (p=bd->phi; p; p=p->link) {
+		for (a=0; p->blk[a]!=bs; a++)
+			assert(a+1<p->narg);
+		p->narg--;
+		memmove(&p->blk[a], &p->blk[a+1],
+			sizeof p->blk[0] * (p->narg-a));
+		memmove(&p->arg[a], &p->arg[a+1],
+			sizeof p->arg[0] * (p->narg-a));
+	}
+	if (bd->npred != 0) {
+		for (a=0; bd->pred[a]!=bs; a++)
+			assert(a+1<bd->npred);
+		bd->npred--;
+		memmove(&bd->pred[a], &bd->pred[a+1],
+			sizeof bd->pred[0] * (bd->npred-a));
+	}
+}
+
+static void
+addpred(Blk *bp, Blk *bc)
+{
+	if (!bc->pred) {
+		bc->pred = alloc(bc->npred * sizeof bc->pred[0]);
+		bc->visit = 0;
+	}
+	bc->pred[bc->visit++] = bp;
+}
+
+/* fill predecessors information in blocks */
+void
+fillpreds(Fn *f)
+{
+	Blk *b;
+
+	for (b=f->start; b; b=b->link) {
+		b->npred = 0;
+		b->pred = 0;
+	}
+	for (b=f->start; b; b=b->link) {
+		if (b->s1)
+			b->s1->npred++;
+		if (b->s2 && b->s2 != b->s1)
+			b->s2->npred++;
+	}
+	for (b=f->start; b; b=b->link) {
+		if (b->s1)
+			addpred(b, b->s1);
+		if (b->s2 && b->s2 != b->s1)
+			addpred(b, b->s2);
+	}
+}
+
+static int
+rporec(Blk *b, uint x)
+{
+	Blk *s1, *s2;
+
+	if (!b || b->id != -1u)
+		return x;
+	b->id = 1;
+	s1 = b->s1;
+	s2 = b->s2;
+	if (s1 && s2 && s1->loop > s2->loop) {
+		s1 = b->s2;
+		s2 = b->s1;
+	}
+	x = rporec(s1, x);
+	x = rporec(s2, x);
+	b->id = x;
+	assert(x != -1u);
+	return x - 1;
+}
+
+/* fill the rpo information */
+void
+fillrpo(Fn *f)
+{
+	uint n;
+	Blk *b, **p;
+
+	for (b=f->start; b; b=b->link)
+		b->id = -1u;
+	n = 1 + rporec(f->start, f->nblk-1);
+	f->nblk -= n;
+	f->rpo = alloc(f->nblk * sizeof f->rpo[0]);
+	for (p=&f->start; (b=*p);) {
+		if (b->id == -1u) {
+			edgedel(b, &b->s1);
+			edgedel(b, &b->s2);
+			*p = b->link;
+		} else {
+			b->id -= n;
+			f->rpo[b->id] = b;
+			p = &b->link;
+		}
+	}
+}
+
+/* for dominators computation, read
+ * "A Simple, Fast Dominance Algorithm"
+ * by K. Cooper, T. Harvey, and K. Kennedy.
+ */
+
+static Blk *
+inter(Blk *b1, Blk *b2)
+{
+	Blk *bt;
+
+	if (b1 == 0)
+		return b2;
+	while (b1 != b2) {
+		if (b1->id < b2->id) {
+			bt = b1;
+			b1 = b2;
+			b2 = bt;
+		}
+		while (b1->id > b2->id) {
+			b1 = b1->idom;
+			assert(b1);
+		}
+	}
+	return b1;
+}
+
+void
+filldom(Fn *fn)
+{
+	Blk *b, *d;
+	int ch;
+	uint n, p;
+
+	for (b=fn->start; b; b=b->link) {
+		b->idom = 0;
+		b->dom = 0;
+		b->dlink = 0;
+	}
+	do {
+		ch = 0;
+		for (n=1; n<fn->nblk; n++) {
+			b = fn->rpo[n];
+			d = 0;
+			for (p=0; p<b->npred; p++)
+				if (b->pred[p]->idom
+				||  b->pred[p] == fn->start)
+					d = inter(d, b->pred[p]);
+			if (d != b->idom) {
+				ch++;
+				b->idom = d;
+			}
+		}
+	} while (ch);
+	for (b=fn->start; b; b=b->link)
+		if ((d=b->idom)) {
+			assert(d != b);
+			b->dlink = d->dom;
+			d->dom = b;
+		}
+}
+
+int
+sdom(Blk *b1, Blk *b2)
+{
+	assert(b1 && b2);
+	if (b1 == b2)
+		return 0;
+	while (b2->id > b1->id)
+		b2 = b2->idom;
+	return b1 == b2;
+}
+
+int
+dom(Blk *b1, Blk *b2)
+{
+	return b1 == b2 || sdom(b1, b2);
+}
+
+static void
+addfron(Blk *a, Blk *b)
+{
+	uint n;
+
+	for (n=0; n<a->nfron; n++)
+		if (a->fron[n] == b)
+			return;
+	if (!a->nfron)
+		a->fron = vnew(++a->nfron, sizeof a->fron[0], PFn);
+	else
+		vgrow(&a->fron, ++a->nfron);
+	a->fron[a->nfron-1] = b;
+}
+
+/* fill the dominance frontier */
+void
+fillfron(Fn *fn)
+{
+	Blk *a, *b;
+
+	for (b=fn->start; b; b=b->link)
+		b->nfron = 0;
+	for (b=fn->start; b; b=b->link) {
+		if (b->s1)
+			for (a=b; !sdom(a, b->s1); a=a->idom)
+				addfron(a, b->s1);
+		if (b->s2)
+			for (a=b; !sdom(a, b->s2); a=a->idom)
+				addfron(a, b->s2);
+	}
+}
+
+static void
+loopmark(Blk *hd, Blk *b, void f(Blk *, Blk *))
+{
+	uint p;
+
+	if (b->id < hd->id || b->visit == hd->id)
+		return;
+	b->visit = hd->id;
+	f(hd, b);
+	for (p=0; p<b->npred; ++p)
+		loopmark(hd, b->pred[p], f);
+}
+
+void
+loopiter(Fn *fn, void f(Blk *, Blk *))
+{
+	uint n, p;
+	Blk *b;
+
+	for (b=fn->start; b; b=b->link)
+		b->visit = -1u;
+	for (n=0; n<fn->nblk; ++n) {
+		b = fn->rpo[n];
+		for (p=0; p<b->npred; ++p)
+			if (b->pred[p]->id >= n)
+				loopmark(b, b->pred[p], f);
+	}
+}
+
+void
+multloop(Blk *hd, Blk *b)
+{
+	(void)hd;
+	b->loop *= 10;
+}
+
+void
+fillloop(Fn *fn)
+{
+	Blk *b;
+
+	for (b=fn->start; b; b=b->link)
+		b->loop = 1;
+	loopiter(fn, multloop);
+}
+
+static void
+uffind(Blk **pb, Blk **uf)
+{
+	Blk **pb1;
+
+	pb1 = &uf[(*pb)->id];
+	if (*pb1) {
+		uffind(pb1, uf);
+		*pb = *pb1;
+	}
+}
+
+/* requires rpo and no phis, breaks cfg */
+void
+simpljmp(Fn *fn)
+{
+
+	Blk **uf; /* union-find */
+	Blk **p, *b, *ret;
+
+	ret = newblk();
+	ret->id = fn->nblk++;
+	ret->jmp.type = Jret0;
+	uf = emalloc(fn->nblk * sizeof uf[0]);
+	for (b=fn->start; b; b=b->link) {
+		assert(!b->phi);
+		if (b->jmp.type == Jret0) {
+			b->jmp.type = Jjmp;
+			b->s1 = ret;
+		}
+		if (b->nins == 0)
+		if (b->jmp.type == Jjmp) {
+			uffind(&b->s1, uf);
+			if (b->s1 != b)
+				uf[b->id] = b->s1;
+		}
+	}
+	for (p=&fn->start; (b=*p); p=&b->link) {
+		if (b->s1)
+			uffind(&b->s1, uf);
+		if (b->s2)
+			uffind(&b->s2, uf);
+		if (b->s1 && b->s1 == b->s2) {
+			b->jmp.type = Jjmp;
+			b->s2 = 0;
+		}
+	}
+	*p = ret;
+	free(uf);
+}