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Re: [PATCH][RFC] Remove quadratic loop with component_uses_parent_alias_set
- From: Richard Biener <rguenther at suse dot de>
- To: Eric Botcazou <ebotcazou at adacore dot com>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Thu, 26 Sep 2013 11:46:07 +0200 (CEST)
- Subject: Re: [PATCH][RFC] Remove quadratic loop with component_uses_parent_alias_set
- Authentication-results: sourceware.org; auth=none
- References: <alpine dot LNX dot 2 dot 00 dot 1309241338320 dot 29411 at zhemvz dot fhfr dot qr> <1812454 dot 1D1WXCO7Y6 at polaris>
On Thu, 26 Sep 2013, Eric Botcazou wrote:
> > Eric, the current way component_uses_parent_alias_set is called from
> > get_alias_set causes the reference tree chain to be walked O(n^2)
> > in case there is any DECL_NONADDRESSABLE_P or TYPE_NONALIASED_COMPONENT
> > reference in the tree chain. Also the comment above
> > component_uses_parent_alias_set doesn't seem to match behavior
> > in get_alias_set. get_alias_set ends up using exactly the type of
> > the parent
> > of the DECL_NONADDRESSABLE_P / TYPE_NONALIASED_COMPONENT reference
> > instead of "the alias set provided by the object at the heart of T"
>
> That's what "the object at the heart of T" means I think: given the loop in
> get_alias_set (or reference_alias_ptr_type_1 now), we end up retrieving the
> parent of the outermost non-addressable component in the component chain
> (outermost in the type layout sense), which is what is wanted: when you go
> down the component chain from the base object to find the alias set, you need
> to stop at the first non-addressable component. That's what is achieved in
> the RTL expander by means of MEM_KEEP_ALIAS_SET_P: you expand first the base
> object, then set MEM_KEEP_ALIAS_SET_P for the first non-addressable component,
> so that the alias set isn't touched any more after that.
>
> But I agree that the head comment and the interface of c_u_p_a_s are awkward,
> to say the least, and the quadratic aspect isn't very nice either.
Ok.
> > I also fail to see why component_uses_parent_alias_set invokes
> > get_alias_set (is that to make 'a.c' in struct { char c; } a;
> > use the alias set of 'a' instead of the alias set of 'char'?
>
> Yes, I think it's intended to stop the walk as soon as you have a type with
> alias set 0: if 'a' has alias set 0, then 'a.c' will have it.
That wasn't the question - I was asking if we have a struct type
with non-zero alias set, like struct { char c; int i } a; then
if we have an access like a.c does the code want to use the
alias set of 'a' (non-zero) for the access for optimization purposes?
It seems not, given your comment below...
[...]
> However, I think the handling of the "parent has alias set zero" is wrong in
> your patch, you should test
>
> if (get_alias_set (TREE_TYPE (TREE_OPERAND (t, 0))) == 0)
but I fail to see how this can happen in practice? It can happen
for ref-all bases but that case is handled separately.
But ok, I'll leave the code as-is functionality wise, we should change
semantics as followup if at all.
> > The only other use besides from get_alias_set is to set
> > MEM_KEEP_ALIAS_SET_P -
>
> It means that the alias set already on the MEM may not be changed afterwards,
> even if you look into its sub-components later.
>
> > whatever that is exactly for, I can
> > see a single non-obvious use in store_constructor_field
> > (didn't bother to lookup how callers specify the alias_set argument).
> > In
> >
> > if (MEM_P (to_rtx) && !MEM_KEEP_ALIAS_SET_P (to_rtx)
> > && DECL_NONADDRESSABLE_P (field))
> > {
> > to_rtx = copy_rtx (to_rtx);
> > MEM_KEEP_ALIAS_SET_P (to_rtx) = 1;
> > }
> >
> > we can just drop the MEM_KEEP_ALIAS_SET_P check (well, in case
> > MEM_KEEP_ALIAS_SET_P is dropped).
>
> Do you mean dropped in set_mem_attributes_minus_bitpos? No, I don't think we
> can do that.
Yeah, I thought of dropping it completely - we know the effective
alias-set of the load/store stmt we are expanding via get_alias_set
of the expression. I don't see why we need to invent new alias sets
in any place down the road when creating sub-accesses (either from
storing constructor components or from storing bitfield pieces).
Thanks for the comments, I'll prepare a patch to only remove the
quadraticness without changing anything else.
Richard.