RE: [PATCH] objtool,x86: Teach decode about LOOP* instructions

David Laight
2022-09-07T09:06:22Z

From: Peter Zijlstra
> Sent: 07 September 2022 10:01
>
> On Wed, Sep 07, 2022 at 09:06:45AM +0200, Peter Zijlstra wrote:
> > On Wed, Sep 07, 2022 at 09:55:21AM +0900, Masami Hiramatsu (Google) wrote:
> >
> > > +/* Return the jump target address or 0 */
> > > +static inline unsigned long insn_get_branch_addr(struct insn *insn)
> > > +{
> > > + switch (insn->opcode.bytes[0]) {
> > > + case 0xe0: /* loopne */
> > > + case 0xe1: /* loope */
> > > + case 0xe2: /* loop */
> >
> > Oh cute, objtool doesn't know about those, let me go add them.

Do they ever appear in the kernel?
They are so slow on Intel cpu that finding one ought to
deemed a bug!

Have you got jcxz (0xe3) in there?
They are fast on both Intel and AMD cpus - so are usable.

David

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