[Mesa-dev] [PATCH 10/32] i965/fs: Remove logic to keep track of MRF metadata in lower_load_payload().

[Francisco Jerez]

Jason Ekstrand <jason at jlekstrand.net> writes:

> On Fri, Feb 20, 2015 at 4:11 AM, Francisco Jerez <currojerez at riseup.net>
> wrote:
>
>> Jason Ekstrand <jason at jlekstrand.net> writes:
>>
>> > I'm still a little pensive.  But
>> >
>> > Reviewed-by: Jason Ekstrand <jason.ekstrand at intel.com>
>> >
>> Thanks.
>>
>> > Now for a little aside.  I have come to the conclusion that I made a
>> grave
>> > mistake when I did the LOAD_PAYLOAD stuff.  In retrospect, I should have
>> > just subclassed fs_inst for load_payload.  The problem is that we need to
>> > snag a bunch of information for the sources when we create the
>> > load_payload.  In particular, we need to know the width of the source so
>> > that we know how much space it consumes in the payload and we need to
>> know
>> > the information required to properly re-create the mov such as
>> > force_sechalf and force_writemask_all.  Really, in order to do things
>> > properly, we need to gather this information *before* we do any
>> > optimizations.  The nasty pile of code that you're editing together with
>> > the "effective_width" parameter is a lame attempt to capture/reconstruct
>> > this information.  Really, we should just subclass, capture the
>> information
>> > up-front, and do it properly.
>> >
>> Yes, absolutely, this lowering pass is a real mess.  There are four more
>> unreviewed patches in the mailing list fixing bugs of the metadata
>> guessing of lower_load_payload() [1][2][3][4], you may want to take a
>> look at them.  There are more bugs I'm aware of but it didn't seem
>> practical to fix them.
>>
>
> Yeah, Matt pointed me at those.  I'll give them a look later today.
>
>
>> That said, I don't think it would be worth subclassing fs_inst.
>>
>> My suggestion would have been to keep it simple and lower LOAD_PAYLOAD
>> into a series of MOVs with force_writemask_all enabled in all cases,
>> simply rely on the optimizer to eliminate those where possible.  Then
>> get rid of the metadata and effective_width guessing.  Instead agree on
>> immediates and uniforms being exec_size-wide by convention
>> (i.e. LOAD_PAYLOAD's exec_size rather than the original instruction's),
>> then prevent constant propagation from propagating an immediate load
>> into a LOAD_PAYLOAD if it would lead to a change in the semantics of the
>> program, and maybe just run constant propagation after lowering so we
>> can be sure those cases are cleaned up properly where register coalesce
>> isn't enough.
>>
>
> First off, simply setting force_writemask_all isn't an option.  Consider
> the following scenario:
>
> a = foo;
> if (bar) {
>    b = 7;
> } else {
>    use(a);
> }
> use(b);
>
> If "use(a)" is the last use of the variable a, then the live ranges of a
> and b don't actually over-lap and we can assign a and b to the same
> register.  However, if force_writemask_all is set on b, it will destroy the
> value in a before its last use.  Right now, we don't actually do this
> because our liveness analysis pass flattens everything so it will think
> that b and a over-lap even though they don't.  However, if we ever decide
> to make the liveness information more accurate, having a pile of
> force_writemask_all instructions will be a major problem.  So, while it is
> *technically* safe for now, it's a really bad idea in the long-term.
>
The thing is we *will* have to deal with that scenario.  Building a
message payload inherently involves crossing channel boundaries (because
of headers, unsupported SIMD modes by some shared functions, etc.).  I'd
say it's a bug in the liveness analysis pass if it wouldn't consider the
liveness interval of a and b to overlap in your example if the
assignment of b had force_writemask_all set.

A reasonable compromise might be to leave it up to the caller whether to
set the force_writemask_all and force_sechalf flags or not.  I.e. just
copy the same flags set on the LOAD_PAYLOAD instruction to the MOV
instructions.  That would still allow reducing the liveness intervals in
cases where we know that the payload respects channel boundaries.

Tracking the flag information per-register in cases where the same
payload has well- and ill-behaved values seems rather premature and
rather useless to me because the optimizer is likely to be able to get
rid of redundant copies with force_writemask_all -- register coalesce
is doing this already AFAIK, maybe by accident.

> Regarding the other suggestion of just requiring width == exec_size for
> immediates and uniforms, that seems pretty reasonable to me.  I'd like to
> know what it will do to optimizations, but it seems ok initially.  I'm
> still a bigger fan of just subclassing and stashing everything we need to
> know up-front.  If we do it right, the only things that will actually need
> to know about the subclass are the function for creating a LOAD_PAYLOAD and
> lower_load_payloads.
>
> --Jason
>
>
>>
>> [1]
>> http://lists.freedesktop.org/archives/mesa-dev/2015-January/074614.html
>> [2]
>> http://lists.freedesktop.org/archives/mesa-dev/2015-February/076094.html
>> [3]
>> http://lists.freedesktop.org/archives/mesa-dev/2015-February/076097.html
>> [4]
>> http://lists.freedesktop.org/archives/mesa-dev/2015-February/076098.html
>>
>>
>> > --Jason
>> >
>> > On Thu, Feb 19, 2015 at 1:53 PM, Jason Ekstrand <jason at jlekstrand.net>
>> > wrote:
>> >
>> >>
>> >>
>> >> On Thu, Feb 19, 2015 at 1:25 PM, Francisco Jerez <currojerez at riseup.net
>> >
>> >> wrote:
>> >>
>> >>> Jason Ekstrand <jason at jlekstrand.net> writes:
>> >>>
>> >>> > On Thu, Feb 19, 2015 at 12:13 PM, Francisco Jerez <
>> >>> currojerez at riseup.net>
>> >>> > wrote:
>> >>> >
>> >>> >> Jason Ekstrand <jason at jlekstrand.net> writes:
>> >>> >>
>> >>> >> > On Fri, Feb 6, 2015 at 4:01 PM, Francisco Jerez <
>> >>> currojerez at riseup.net>
>> >>> >> > wrote:
>> >>> >> >
>> >>> >> >> Hey Matt,
>> >>> >> >>
>> >>> >> >> Matt Turner <mattst88 at gmail.com> writes:
>> >>> >> >>
>> >>> >> >> > On Fri, Feb 6, 2015 at 6:42 AM, Francisco Jerez <
>> >>> >> currojerez at riseup.net>
>> >>> >> >> wrote:
>> >>> >> >> >> MRFs cannot be read from anyway so they cannot possibly be a
>> >>> valid
>> >>> >> >> >> source of LOAD_PAYLOAD.
>> >>> >> >> >> ---
>> >>> >> >> >
>> >>> >> >> > The function only seems to test inst->dst.file == MRF. I don't
>> >>> see any
>> >>> >> >> > code for handling MRF sources. What am I missing?
>> >>> >> >>
>> >>> >> >> That test is for "handling" MRF sources -- More precisely, it's
>> >>> >> >> collecting the writemask and half flags for MRF writes, which can
>> >>> only
>> >>> >> >> possibly be useful if we're going to use them later on to read
>> >>> something
>> >>> >> >> out of an MRF into a payload, which we shouldn't be doing in the
>> >>> first
>> >>> >> >> place.
>> >>> >> >>
>> >>> >> >> Aside from simplifying the function somewhat, that allows us to
>> >>> drop the
>> >>> >> >> 16 register gap reserved for MRFs at register offset zero, what
>> will
>> >>> >> >> allow us to drop the vgrf_to_reg[] offset calculation completely
>> >>> (also
>> >>> >> >> in split_virtual_grfs()) in a future patch (not sent for review
>> >>> yet).
>> >>> >> >>
>> >>> >> >
>> >>> >> > No, we do read from MRF's sort-of...  Send messages have an
>> implicit
>> >>> >> "read"
>> >>> >> > from an MRF.
>> >>> >>
>> >>> >> Heh, and that's pretty much the only way you "read" from it.
>> >>> >>
>> >>> >> > This was written precicely so that we could use LOAD_PAYLOAD
>> >>> >> > to build MRF payloads.  We do on pre-GEN6.
>> >>> >> >
>> >>> >> I'm aware, but you don't need any of this meta-data to LOAD_PAYLOAD
>> >>> >> *into* an MRF, and LOAD_PAYLOAD with an MRF as source should be
>> illegal
>> >>> >> anyway.
>> >>> >>
>> >>> >
>> >>> > And no one is using it that way.  All of the metadata checks you are
>> >>> > deleting are checks on the *destination*.
>> >>> >
>> >>>
>> >>> Didn't you write this code yourself?  The only use for the collected
>> >>> metadata is initializing the instruction flags of the MOVs subsequent
>> >>> LOAD_PAYLOAD instructions are lowered to, based on the metadata already
>> >>> collected for its source registers, which can never be MRFs, so the
>> >>> metadata you collect from MRF writes is never actually used.
>> >>>
>> >>
>> >> Right... I misred something initially.  Yes, we should never be tracking
>> >> MRF's as a source of a LOAD_PAYLOAD.  I'll give it a better look a bit
>> >> later, but it looks better.
>> >>
>>
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