Re: [PATCH v2 15/21] objtool: cache relocations, do less work

[Linus Torvalds]

On Mon, 14 Sept 2026 at 12:44, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> BTW, I found another initialization bug: klp-post-link.c uses
> elf_create_section() to create a reloc section, so it missing the
> initialization of rsec->hashed in elf_create_rela_section().  But that's
> moot if we just get rid of the hashing.

Talking about "getting rid of the hashing": the instruction hashing is
obviously a big deal judging by the hash size expansion in that other
patch.

Do we actually look up all instructions by hash in the first place?
>From a quick glance, it looks like almost every user of "find_insn()"
is looking for just certain *kinds* of instructions: calls, branches,
returns, endbr.  And branch targets.

Maybe stack updates? I didn't check.

Yet we seem to hash them all - even if they are just some random
ALU-only instruction that is in the middle of a function and never a
target of anything and has no impact on any code flow.

So the question becomes: do we actually need to hash those
instructions at all? Are they ever actually looked up?

I don't know this codebase at all, so maybe that's just a really
stupid question. Feel free to look at me condescendinly, shake your
head and say "Poor Linus has lost the plot".

[ And yes, I see that whole "next_insn_same_sec()" thing, which
obviously *does* look up all the instructions when you iterate over
that function, but it strikes me that using a hash table for that is a
bit strange when the natural way to do it would be with juyst a
"struct instruction *next" field instead.. ]

So if hashing was such a big deal that it showed up clearly on
profiles, maybe the answer isn't just a "make the hash table larger".
If a "next" field were to not only make the hash table much smaller,
but also mean that next_insn_same_{func,sec}() would become a "follow
one pointer", mayeb hashing would become rather cheaper?

Stupid? Or maybe "not completely stupid, but too painful to change"?

         Linus