[RFC][PATCH 08/16] Optimize find_section_by_name()

[Peter Zijlstra]

In order to avoid yet another linear search of (20k) sections, add a
name based hash.

This reduces objtool runtime on vmlinux.o by some 10s to around 35s.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 tools/objtool/elf.c |    9 ++++++++-
 tools/objtool/elf.h |    3 +++
 2 files changed, 11 insertions(+), 1 deletion(-)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -22,11 +22,16 @@
 
 #define MAX_NAME_LEN 128
 
+static inline u32 str_hash(const char *str)
+{
+	return jhash(str, strlen(str), 0);
+}
+
 struct section *find_section_by_name(struct elf *elf, const char *name)
 {
 	struct section *sec;
 
-	list_for_each_entry(sec, &elf->sections, list)
+	hash_for_each_possible(elf->section_name_hash, sec, name_hash, str_hash(name))
 		if (!strcmp(sec->name, name))
 			return sec;
 
@@ -193,6 +198,7 @@ static int read_sections(struct elf *elf
 		sec->len = sec->sh.sh_size;
 
 		hash_add(elf->section_hash, &sec->hash, sec->idx);
+		hash_add(elf->section_name_hash, &sec->name_hash, str_hash(sec->name));
 	}
 
 	if (stats)
@@ -433,6 +439,7 @@ struct elf *elf_read(const char *name, i
 
 	hash_init(elf->symbol_hash);
 	hash_init(elf->section_hash);
+	hash_init(elf->section_name_hash);
 	INIT_LIST_HEAD(&elf->sections);
 
 	elf->fd = open(name, flags);
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -10,6 +10,7 @@
 #include <gelf.h>
 #include <linux/list.h>
 #include <linux/hashtable.h>
+#include <linux/jhash.h>
 
 #ifdef LIBELF_USE_DEPRECATED
 # define elf_getshdrnum    elf_getshnum
@@ -26,6 +27,7 @@
 struct section {
 	struct list_head list;
 	struct hlist_node hash;
+	struct hlist_node name_hash;
 	GElf_Shdr sh;
 	struct list_head symbol_list;
 	struct list_head rela_list;
@@ -73,6 +75,7 @@ struct elf {
 	struct list_head sections;
 	DECLARE_HASHTABLE(symbol_hash, 20);
 	DECLARE_HASHTABLE(section_hash, 16);
+	DECLARE_HASHTABLE(section_name_hash, 16);
 };