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

[David Laight]

From: Peter Zijlstra
> Sent: 07 September 2022 10:40
> 
> On Wed, Sep 07, 2022 at 09:06:12AM +0000, David Laight wrote:
> > 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?
> 
> No; that is, not on any of the random vmlinux.o images I checked this
> morning.
> 
> Still, best to properly decode them anyway.

It is annoying that cpu with adox/adcx have slow loop.
You really want to be able to do:
	1:	adox ...
		adcx ...
		loop	1b
That would never run with one iteration/clock.
But unrolling once would probably be enough.

What you can do (and gives the fastest IPcsum loop) is:
	1:	jcxz	2f
		....
		lea	%rcx,...
		jmp	1b
	2:
The extra instructions mean that needs unrolling 4 times.
I've got over 12 bytes/clock that way.

	David

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