Re: Patch ping

[Jakub Jelinek <jakub at redhat dot com>]

On Wed, Jul 26, 2017 at 04:13:30PM +0200, Richard Biener wrote:
> > >  You don't seem to use 'size' anywhere.
> > 
> > size I thought about but then decided not to do anything with it.
> > There are two cases, one is where there is no ADDR_EXPR and it actually
> > a memory reference.  
> > In that case in theory the size could be used, but it would need
> > to be used only for the positive offsets, so like:
> > if (off > 0) {
> >   if (ptr + off + size < ptr)
> >     runtime_fail;
> > } else if (ptr + off > ptr)
> >   runtime_fail;
> > but when it is actually a memory reference, I suppose it will fail
> > at runtime anyway when performing such an access, so I think it is
> > unnecessary.  And for the ADDR_EXPR case, the size is irrelevant, we
> > are just taking address of the start of the object.
> > 
> > > You fail to allow other handled components -- for no good reason?
> > 
> > I was trying to have a quick bail out.  What other handled components might
> > be relevant?  I guess IMAGPART_EXPR.  For say BIT_FIELD_REF I don't think
> > I can
> >   tree ptr = build1 (ADDR_EXPR, build_pointer_type (TREE_TYPE (t)), t);
> 
> REALPART/IMAGPART_EXPR, yes.  You can't address BIT_FIELD_REF
> apart those on byte boundary (&vector[4] is eventually folded to
> a BIT_FIELD_REF).  Similar for VIEW_CONVERT_EXPR, but you are
> only building the address on the base?
> 
> > >  You fail to handle
> > > &MEM[ptr + CST] a canonical gimple invariant way of ptr +p CST,
> > > the early out bitpos == 0 will cause non-instrumentation here.
> > 
> > Guess I could use:
> >   if ((offset == NULL_TREE
> >        && bitpos == 0
> >        && (TREE_CODE (inner) != MEM_REF
> > 	   || integer_zerop (TREE_OPERAND (inner, 1))))
> > The rest of the code will handle it.
> 
> Yeah.
> 
> > 
> > > (I'd just round down in the case of bitpos % BITS_PER_UNIT != 0)
> > 
> > But then the
> >   tree ptr = build1 (ADDR_EXPR, build_pointer_type (TREE_TYPE (t)), t);
> > won't work again.
> 
> Hmm.  So instead of building the address on the original tree you
> could build the difference based on what get_inner_reference returns
> in bitpos/offset?

I'm building both addresses and subtracting them to get the offset.
I guess the other option is to compute just the address of the base
(i.e. base_addr), and add offset (if non-NULL) plus bitpos / BITS_PER_UNIT
plus offset from the MEM_REF (if any).  In that case it would probably
handle any handled_component_p and bitfields too.

	Jakub