[PATCH v4 16/22] objtool: do not hash DWARF relocations

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

objtool hashes the relocation of each section so it can find them by
destination, but DWARF sections are never looked up this way, so there is
no reason to hash them.

With CONFIG_DEBUG_INFO_DWARF5 set a defconfig vmlinux.o has ~22 million
relocations, 20.5 million of which are .debug_* sections.

Therefore, don't hash these.

With the same configuration the objtool vmlinux.o link-time run takes ~.5s
less to complete (2.63s -> 2.14s) and ~130 MiB memory to do it.

The output remains unchanged.

Assisted-by: LLM
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 tools/objtool/elf.c | 32 +++++++++++++++++++++++++-------
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index a791f4ea6ec1..98a110e3e8f5 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -1071,7 +1071,8 @@ struct reloc *elf_init_reloc(struct elf *elf, struct section *rsec,
 	set_reloc_type(elf, reloc, type);
 	set_reloc_addend(elf, reloc, addend);
 
-	elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));
+	if (!is_dwarf_section(rsec->base))
+		elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));
 	set_sym_next_reloc(reloc, sym->relocs);
 	sym->relocs = reloc;
 
@@ -1125,16 +1126,14 @@ struct reloc *elf_init_reloc_data_sym(struct elf *elf, struct section *sec,
 
 static int read_relocs(struct elf *elf)
 {
-	unsigned long nr_reloc, max_reloc = 0;
+	unsigned long nr_reloc, max_reloc = 0, nr_hashed = 0;
 	struct section *rsec;
 	struct reloc *reloc;
 	unsigned int symndx;
 	struct symbol *sym;
+	bool hashed;
 	int i;
 
-	if (!elf_alloc_hash(reloc, elf->num_relocs))
-		return -1;
-
 	list_for_each_entry(rsec, &elf->sections, list) {
 		if (!is_reloc_sec(rsec))
 			continue;
@@ -1147,6 +1146,23 @@ static int read_relocs(struct elf *elf)
 
 		rsec->base->rsec = rsec;
 
+		/*
+		 * DWARF relocations are never looked up by destination, they
+		 * only need to be on their symbol's list.
+		 */
+		if (!is_dwarf_section(rsec->base))
+			nr_hashed += sec_num_entries(rsec);
+	}
+
+	if (!elf_alloc_hash(reloc, nr_hashed))
+		return -1;
+
+	list_for_each_entry(rsec, &elf->sections, list) {
+		if (!is_reloc_sec(rsec))
+			continue;
+
+		hashed = !is_dwarf_section(rsec->base);
+
 		/* nr_alloc_relocs=0: libelf owns d_buf */
 		rsec->nr_alloc_relocs = 0;
 
@@ -1168,7 +1184,8 @@ static int read_relocs(struct elf *elf)
 				return -1;
 			}
 
-			elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));
+			if (hashed)
+				elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));
 			set_sym_next_reloc(reloc, sym->relocs);
 			sym->relocs = reloc;
 
@@ -1180,6 +1197,7 @@ static int read_relocs(struct elf *elf)
 	if (opts.stats) {
 		printf("max_reloc: %lu\n", max_reloc);
 		printf("num_relocs: %lu\n", elf->num_relocs);
+		printf("num_relocs_hashed: %lu\n", nr_hashed);
 		printf("reloc_bits: %d\n", elf->reloc_bits);
 	}
 
@@ -1634,7 +1652,7 @@ static int elf_alloc_reloc(struct elf *elf, struct section *rsec)
 
 	memcpy(new_relocs, old_relocs, nr_relocs_old * sizeof(struct reloc));
 
-	for (int i = 0; i < nr_relocs_old; i++) {
+	for (int i = 0; i < nr_relocs_old && !is_dwarf_section(rsec->base); i++) {
 		struct reloc *old = &old_relocs[i];
 		struct reloc *new = &new_relocs[i];
 		u32 key = reloc_hash(old);

-- 
2.55.0