[RFC][PATCH 07/16] objtool: Optimize find_section_by_index()

[Peter Zijlstra]

In order to avoid a linear search (over 20k entries), add an
section_hash to the elf object.

This reduces objtool on vmlinux.o from a few minutes to around 45
seconds.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 tools/objtool/elf.c |    7 ++++++-
 tools/objtool/elf.h |    2 ++
 2 files changed, 8 insertions(+), 1 deletion(-)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -38,7 +38,7 @@ static struct section *find_section_by_i
 {
 	struct section *sec;
 
-	list_for_each_entry(sec, &elf->sections, list)
+	hash_for_each_possible(elf->section_hash, sec, hash, idx)
 		if (sec->idx == idx)
 			return sec;
 
@@ -191,6 +191,8 @@ static int read_sections(struct elf *elf
 			}
 		}
 		sec->len = sec->sh.sh_size;
+
+		hash_add(elf->section_hash, &sec->hash, sec->idx);
 	}
 
 	if (stats)
@@ -430,6 +432,7 @@ struct elf *elf_read(const char *name, i
 	memset(elf, 0, sizeof(*elf));
 
 	hash_init(elf->symbol_hash);
+	hash_init(elf->section_hash);
 	INIT_LIST_HEAD(&elf->sections);
 
 	elf->fd = open(name, flags);
@@ -570,6 +573,8 @@ struct section *elf_create_section(struc
 	shstrtab->len += strlen(name) + 1;
 	shstrtab->changed = true;
 
+	hash_add(elf->section_hash, &sec->hash, sec->idx);
+
 	return sec;
 }
 
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -25,6 +25,7 @@
 
 struct section {
 	struct list_head list;
+	struct hlist_node hash;
 	GElf_Shdr sh;
 	struct list_head symbol_list;
 	struct list_head rela_list;
@@ -71,6 +72,7 @@ struct elf {
 	char *name;
 	struct list_head sections;
 	DECLARE_HASHTABLE(symbol_hash, 20);
+	DECLARE_HASHTABLE(section_hash, 16);
 };