Re: [Design notes, RFC] Address-lowering prototype design (PR46556)

[Richard Guenther <richard dot guenther at gmail dot com>]

On Wed, Jun 29, 2011 at 8:18 PM, William J. Schmidt
<wschmidt@linux.vnet.ibm.com> wrote:
> On Tue, 2011-06-14 at 15:39 +0200, Richard Guenther wrote:
>> On Fri, Jun 10, 2011 at 5:11 PM, William J. Schmidt
>> <wschmidt@linux.vnet.ibm.com> wrote:
>> > On Tue, 2011-06-07 at 16:49 +0200, Richard Guenther wrote:
>> >> On Tue, Jun 7, 2011 at 4:14 PM, William J. Schmidt
>> >> <wschmidt@linux.vnet.ibm.com> wrote:
>> >
>> > <snip>
>> >
>> >> >> > Loss of aliasing information
>> >> >> > ============================
>> >> >> > The most serious problem I've run into is degraded performance due to poorer
>> >> >> > instruction scheduling choices. I tracked this down to
>> >> >> > alias.c:nonoverlapping_component_refs_p.
>> >> >> >
>> >> >> > This code proves that two memory accesses don't overlap by attempting to prove
>> >> >> > that they access different fields of the same structure. This is done using
>> >> >> > the MEM_EXPRs of the two rtx's, which record the expression trees that were
>> >> >> > translated into the rtx's during expand. When address lowering is not
>> >> >> > present, a simple COMPONENT_REF will appear in the MEM_EXPR: x.a, for
>> >> >> > example. However, address lowering changes the simple COMPONENT_REF into a
>> >> >> > [TARGET_]MEM_REF that is no longer necessarily identifiable as a field
>> >> >> > reference. Thus the aliasing machinery can no longer prove that two such
>> >> >> > field references are disjoint.
>> >> >> >
>> >> >> > This has severe consequences for performance, and has to be dealt with if
>> >> >> > address lowering is to be successful.
>> >> >> >
>> >> >> > I've worked around this with an admittedly fragile solution; I'll discuss the
>> >> >> > drawbacks below. The idea is to construct a mapping from replacement mem_refs
>> >> >> > to the original expressions that they replaced. When a MEM_EXPR is being set
>> >> >> > during expand, we first look up the mem_ref in the mapping. If present, the
>> >> >> > MEM_EXPR is set to the original expression, rather than to the mem_ref. This
>> >> >> > essentially duplicates the behavior in the absence of address lowering.
>> >> >>
>> >> >> Ick. We had this in the past via TMR_ORIGINAL which caused all sorts
>> >> >> of problems. Removing it didn't cause much degradation because we now
>> >> >> preserve points-to information.
>> >> >>
>> >> >> Originally I played with lowering all memory accesses to MEM_REFs
>> >> >> (see the old mem-ref branch), and the loss of type-based alias
>> >> >> disambiguation was indeed an issue.
>> >> >>
>> >> >> But - I definitely do not like the idea of preserving something similar
>> >> >> to TMR_ORIGINAL. Instead we can try preserving some information
>> >> >> we derive from it. We keep the original access type that we can use
>> >> >> for TBAA but do not retain knowledge on whether the type of the
>> >> >> MEM_REF is valid for TBAA or if it is view-converted.
>> >> >
>> >> > Yes, I really don't like what I have at the moment, either. I put it in
>> >> > place as a stopgap to let me proceed to look for other performance
>> >> > problems.
>> >> >
>> >> > The question is how we can infer useful information for TBAA from the
>> >> > MEM_REFs and TMRs. I poked at trying to identify types and offsets from
>> >> > the MEM_EXPRs, but this ended up being useless; I had to constrain too
>> >> > many cases to maintain correctness, and couldn't prove the type
>> >> > information for the important cases in SPEC I was trying to address.
>> >> >
>> >> > Unfortunately, the whole design goes down the drain if we can't find a
>> >> > way to solve the TBAA issue. The performance degradations are too
>> >> > costly.
>> >>
>> >> If you look at what basic TBAA the alias oracle performs then it boils
>> >> down to the fact that get_alias_set for a.b.c might end up using the
>> >> alias-set of the type of C but for MEM[&a + 4] it will use the alias set
>> >> of the type of a. The tree alias-oracle extracts both alias sets, that
>> >> of the outermost valid type and that of the innermost as both are
>> >> equally useful. But the MEM_REF (or TARGET_MEM_REF) tree
>> >> only have storage for one such alias-set. Thus my idea at some point
>> >> was to store the other one as well in some form. It will not be
>> >> the full information (after all, the complete access path does provide
>> >> some extra information - see aliasing_component_refs_p).
>> >
>> > This is what concerns me. TBAA information for the outer and inner
>> > components doesn't seem sufficient to provide what
>> > nonoverlapping_component_refs_p is currently able to prove. The latter
>> > searches for a common RECORD_TYPE somewhere along the two access paths,
>> > and then disambiguates if the two associated referenced fields differ.
>> > For a simple case like "struct x { int a; int b; };", a and b have the
>> > same type and alias-set, so the alias-set information doesn't add
>> > anything. It isn't sufficient alone for the disambiguation of x1.a =
>> > MEM_REF[&x1, 0] and x2.b = MEM_REF[&x2, 4].
>> >
>> > Obviously the offset is sufficient to disambiguate for this simple case
>> > with a common base type, but when the shared record types aren't at the
>> > outermost level, we can't detect whether it is.
>> >
>> > At the moment I don't see how we can avoid degradation unless we keep
>> > the full access path around somewhere, for [TARGET_]MEM_REFs built from
>> > COMPONENT_REFs. I hope I'm wrong.
>>
>> You are not wrong. ?But the question is, does it make a difference?
>>
> After some rework and measurements, I'm going to change my answer to
> this question. ?The loss of same-type field disambiguation does make a
> difference under limited circumstances. ?But at this time, I don't think
> those differences are significant enough to warrant the complexities
> required to avoid them.
>
> The changes you made a couple weeks ago to tree-ssa-address.c, and the
> associated aliasing changes, caused me to remove some heuristics and add
> others. ?My measurements changed noticeably as a result. ?At this point,
> the address-lowering patch I've been working on has detrimental effects
> (on powerpc64) only on 188.ammp. ?As discussed before, this is caused by
> the lack of field disambiguation during scheduling of a large hot loop
> with high register pressure. ?However, the degradation I am seeing is
> now roughly 2%.
>
> So with only 2% degradation on a single benchmark, my thought is to add
> some commentary to the code about this concern, but not to attempt to
> address it. ?I would include the salient points from this discussion in
> the commentary.
>
> Sound reasonable?

Yes.

Thanks,
Richard.

> Bill
>
>> Richard.
>
>
>