Re: [PATCH v3] powerpc/64: Fix memcmp reading past the end of src/dest

[Segher Boessenkool <segher@kernel.crashing.org>]

On Mon, Mar 25, 2019 at 11:33:56PM +1100, Michael Ellerman wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> > On Fri, Mar 22, 2019 at 11:37:24PM +1100, Michael Ellerman wrote:
> >> +	clrldi	r6,r4,(64-12)	// r6 = r4 & 0xfff
> >
> > You can just write
> >   rlwinm r6,r4,0,0x0fff
> 
> > if that is clearer?  Or do you still want a comment with that :-)
> 
> I don't think it's clearer doing a rotate of zero bits :)
> 
> And yeah I'd probably still leave the comment, so I'm inclined to stick
> with the clrldi?

I always have to think what the clrldi etc. do exactly, while with rlwinm
it is obvious.  But yeah this may be different for other people who are
used to different idiom.

> Would be nice if the assembler could support:
> 
> 	andi	r6, r4, 0x0fff
> 
> And turn it into the rlwinm, or rldicl :)

The extended mnemonics are *simple*, *one-to-one* mappings.  Having
"andi. 6,4,0x0f0f" a valid insn, but an extended mnemonic "andi 6,4,0x0f0f"
that is not (and the other way around for say 0xff0000ff) would violate that.

You could do some assembler macro, that can also expand to multiple insns
where that is useful.  Also one for loading constants, etc.  The downside
to that is you often do care how many insns are generated.

Instead you could do a macro for only those cases that can be done with *one*
insn.  But that then is pretty restricted in use, and people have to learn
what values are valid.

I don't see a perfect solution.


Segher