objtool: optimize add_dead_ends for split sections

Instead of iterating through all instructions to find the last
instruction each time .rela.discard.(un)reachable points beyond the
section, use find_insn to locate the last instruction by looking at
the last bytes of the section instead.
Suggested-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: NSami Tolvanen <samitolvanen@google.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200421220843.188260-3-samitolvanen@google.com

tools/objtool/check.c

@@ -322,6 +322,19 @@ static int decode_instructions(struct objtool_file *file)
 	return ret;
 }
 
+static struct instruction *find_last_insn(struct objtool_file *file,
+					  struct section *sec)
+{
+	struct instruction *insn = NULL;
+	unsigned int offset;
+	unsigned int end = (sec->len > 10) ? sec->len - 10 : 0;
+
+	for (offset = sec->len - 1; offset >= end && !insn; offset--)
+		insn = find_insn(file, sec, offset);
+
+	return insn;
+}
+
 /*
  * Mark "ud2" instructions and manually annotated dead ends.
  */
@@ -330,7 +343,6 @@ static int add_dead_ends(struct objtool_file *file)
 	struct section *sec;
 	struct rela *rela;
 	struct instruction *insn;
-	bool found;
 
 	/*
 	 * By default, "ud2" is a dead end unless otherwise annotated, because
@@ -356,15 +368,8 @@ static int add_dead_ends(struct objtool_file *file)
 		if (insn)
 			insn = list_prev_entry(insn, list);
 		else if (rela->addend == rela->sym->sec->len) {
-			found = false;
-			list_for_each_entry_reverse(insn, &file->insn_list, list) {
-				if (insn->sec == rela->sym->sec) {
-					found = true;
-					break;
-				}
-			}
-
-			if (!found) {
+			insn = find_last_insn(file, rela->sym->sec);
+			if (!insn) {
 				WARN("can't find unreachable insn at %s+0x%x",
 				     rela->sym->sec->name, rela->addend);
 				return -1;
@@ -398,15 +403,8 @@ static int add_dead_ends(struct objtool_file *file)
 		if (insn)
 			insn = list_prev_entry(insn, list);
 		else if (rela->addend == rela->sym->sec->len) {
-			found = false;
-			list_for_each_entry_reverse(insn, &file->insn_list, list) {
-				if (insn->sec == rela->sym->sec) {
-					found = true;
-					break;
-				}
-			}
-
-			if (!found) {
+			insn = find_last_insn(file, rela->sym->sec);
+			if (!insn) {
 				WARN("can't find reachable insn at %s+0x%x",
 				     rela->sym->sec->name, rela->addend);
 				return -1;