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[as-powerpc] Check correct limits in i_form()

--- a/src/cmd/as/target/powerpc/ins.c

+++ b/src/cmd/as/target/powerpc/ins.c

@@ -135,7 +135,9 @@

 i_form(Op *op, Node **args)

 {

 	unsigned long ins, opcd, li, aa, lk;

-	unsigned long long dst;

+	long long dst;

+	long long max = 1l << 23;

+	long long min = -(1l << 23);

 	opcd = op->bytes[0];

 	aa = op->bytes[1];

@@ -146,8 +148,11 @@

 		error("unaligned branch");

 	if (aa)

 		dst -= cursec->curpc - 4;

-	li = dst >> 2;

+	if (dst < min || dst > max)

+		error("out of range branch");

+	li = dst;

+	li >>= 2;

 	ins = opcd<<26 | li<<2 | aa<<1 | lk;

 	emit_packed(ins);

 }