[Mesa-dev] [PATCH 04/15] i965/fs: Get rid of reladdr

Francisco Jerez currojerez at riseup.net
Mon Dec 14 06:23:44 PST 2015 

Jason Ekstrand <jason at jlekstrand.net> writes:

> On Dec 11, 2015 5:44 AM, "Francisco Jerez" <currojerez at riseup.net> wrote:
>>
>> Jason Ekstrand <jason at jlekstrand.net> writes:
>>
>> > On Dec 10, 2015 6:58 AM, "Francisco Jerez" <currojerez at riseup.net>
> wrote:
>> >>
>> >> Jason Ekstrand <jason at jlekstrand.net> writes:
>> >>
>> >> > We aren't using it anymore.
>> >>
>> >> It seems useful to me to be able to represent indirect access as part
> of
>> >> any instruction source or destination register.
>> >>
>> >> The following:
>> >>
>> >> | mov_indirect g0, g1, a0
>> >> | foo g2, g0
>> >>
>> >> and the converse case with indirect destination offset (which you don't
>> >> seem to represent currently) can be implemented by the hardware more
>> >> efficiently using a single instruction in certain cases.  The current
> IR
>> >> is able to represent what the hardware can do, but supporting the
>> >> MOV_INDIRECT instruction only would force us to keep the indirection
>> >> separate from the instruction that uses it, so it seems like a less
>> >> expressive representation to me than the current approach, unless
> you're
>> >> willing to add _INDIRECT variants of most hardware opcodes.
>> >
>> > Yes and, mostly, no.  Yes, you can put an indirect on almost anything
> but
>> > it has substantial restrictions:
>> >
>> Yes, I'm aware of the restrictions of indirect addressing on Gen
>> hardware.  The fact that reladdr can represent addressing modes which
>> aren't directly implemented in the hardware doesn't invalidate the
>> abstraction, nor implies that optimization and translation passes must
>> be made aware of such restrictions.  It just means that our abstraction
>> is a superset of the hardware indirect addressing scheme, which is fine
>> given a lowering pass to convert indirect addressing into a legal form.
>>
>> Your proposal instead goes the opposite direction and replaces the
>> preexisting abstraction with a new abstraction which can only represent
>> a tiny subset of the functionality of hardware instructions, which I
>> don't think is acceptable for a low-level IR.
>>
>> > 1) Destination indirects must be uniform (I'm only 95% sure this is the
>> > case)
>> >
>> > 2) We only have 8 address subregisters on HSW and prior and 16 on BDW
> which
>> > means:
>> >
>>
>> That's the kind of thing that SIMD lowering would be able to take care
>> of easily.
>
> Yes, and we use it for MOV_INDIRECT.
>
Right, and there's no reason why it couldn't be used for reladdr in the
same way to handle the above restrictions.

>> >     a) All indirects must be uniform OR
>> >
>> >     b) We must be on Broadwell, have only two indirects, and split the
>> > instruction OR
>> >
>> >     c) We can only have one indirect (and still split the instruction on
>> > HSW)
>> >
>> > So, yes, "everthing can be uniform", but not in real life.
>>
>> > The reladdr abstraction, while maybe ok for a high-level IR, doesn't
>> > accurately represent the hardware at all because it can't represent
>> > any of the restrictions without piles of helper functions and checks.
>>
>> The reladdr abstraction can represent the full flexibility of the
>> hardware, while MOV_INDIRECT cannot, that makes reladdr a more accurate
>> low-level representation of the program.  For that reason I'd argue the
>> exact opposite: MOV_INDIRECT would be a great abstraction and a welcome
>> simplification for a high-level IR (e.g. NIR), in which nothing is lost
>> by sacrificing the expressiveness of individual instructions in favour
>> of instruction composition, because the back-end can always combine
>> multiple high-level instructions into a single low-level instruction
>> where the hardware ISA allows, as long as the low-level IR is able to
>> represent the full functionality of hardware instructions (IOW the
>> mapping between low-level IR instructions and hardware instructions is
>> surjective), which is further from being the case after this series.
>
> Flexibility and expressiveness isn't always a good thing.

Expressiveness is a good thing as long as it's required to represent the
capabilities of the hardware.

> Yes, reladdr is more "expressive", but it's not getting used because
> that expressiveness makes it a pain.

AFAIK nobody has bothered to implement it completely down to the
back-end generator so it's not a wonder that it's not extensively used.
I'd like to see some evidence that it would actually be a pain
(e.g. some proof of concept implementation) before we give up and commit
to an inherently more limited model.  If you already have such a proof
of concept, please share it.  If you don't and are fully convinced that
it will be a pain I suggest you hold up this series for a while and let
me write the proof of concept -- Doing something with the expectation
that it's going to be a pain is the best way to make sure it actually
becomes a pain.  Worst-case scenario you're right and it does become a
pain, at which point I come back and review this series.  Best-case
scenario you're wrong and we don't give up useful functionality of the
IR based on an expectation.

> You could say it is getting used, but it's current use is just an
> intermediate to something less expressive than MOV_INDIRECT.  In that
> sense, we are expressing *more* with MOV_INDIRECT than we were before
> since we are also expressing in directing on registers and not just
> pull constants.
>
Yes, but MOV_INDIRECT is a dead end in the long-term, and the same
functionality you implement here could have been implemented based on
the preexisting model.

> Also, while reladdr is "expressive", it doesn't actually represent the
> hardware.  If we wanted that, we would expose the address register file
> (which I did consider doing).  However, that comes with it's own set of
> issues.
>
It represents the hardware more accurately than MOV_INDIRECT in the
following sense: It allows us to represent a wide subset of the indirect
addressing capabilities of the hardware down to the generator, on any
instruction, source or destination we can possibly address indirectly.
MOV_INDIRECT on the other hand allows us to represent a MOV instruction
with indirectly-addressed source *only*.

>> > The MOV_INDIRECT instruction, on the other hand, is something we can
>> > always do because it falls in the "one indirect" case above.  The
>> > reladdr abstraction also suffers from the recursive reladdr problem
>> > which MOV_INDIRECT neatly side-steps by design.
>>
>> I don't think the reladdr "problem" requires any fundamentally more
>> difficult logic in optimization passes than what you need in order to
>> handle MOV_INDIRECT instructions reasonably.
>
> No, it just requires rewriting them because iterating over sources just
> became recursive.
>
That's quite an overstatement, most optimization passes only need to
care about the top-level register of a source or destination, because
wherever a reladdr is present it's not statically determined so you're
unlikely to be able to optimize through it anyway regardless of whether
you recurse or not.  Cases where you actually want to iterate over each
register used by an instruction are a minority and don't require
fundamentally changing the optimization pass, the iteration can be
factored out as in the straightforward for_each_use() helper I
originally implemented to solve a different problem but which happens to
handle arbitrarily-nested reladdr as one would expect.

>> > We can't even use reladdr past initial NIR -> FS stage because none of
>> > the optimization passes know what to do with it (and no, I don't want
>> > to add reladdr support to any of them).
>> >
>> > Yes, removing reladdr makes the IR "less expressive", but we're not
> really
>> > using that expressiveness nor is reladdr the abstraction we want.  If we
>> > want to start taking more advantage of relative addressing in the
> future,
>> > we can come up with a better abstraction then.
>> >
>> > --Jason
>> >
>> >> > ---
>> >> >  src/mesa/drivers/dri/i965/brw_fs.cpp  | 7 +------
>> >> >  src/mesa/drivers/dri/i965/brw_ir_fs.h | 5 +----
>> >> >  2 files changed, 2 insertions(+), 10 deletions(-)
>> >> >
>> >> > diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp
>> > b/src/mesa/drivers/dri/i965/brw_fs.cpp
>> >> > index 7cc03c5..786c5fb 100644
>> >> > --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
>> >> > +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
>> >> > @@ -433,7 +433,6 @@ fs_reg::fs_reg(struct ::brw_reg reg) :
>> >> >  {
>> >> >     this->reg_offset = 0;
>> >> >     this->subreg_offset = 0;
>> >> > -   this->reladdr = NULL;
>> >> >     this->stride = 1;
>> >> >     if (this->file == IMM &&
>> >> >         (this->type != BRW_REGISTER_TYPE_V &&
>> >> > @@ -448,7 +447,6 @@ fs_reg::equals(const fs_reg &r) const
>> >> >  {
>> >> >     return (this->backend_reg::equals(r) &&
>> >> >             subreg_offset == r.subreg_offset &&
>> >> > -           !reladdr && !r.reladdr &&
>> >> >             stride == r.stride);
>> >> >  }
>> >> >
>> >> > @@ -4716,9 +4714,7 @@
> fs_visitor::dump_instruction(backend_instruction
>> > *be_inst, FILE *file)
>> >> >           break;
>> >> >        case UNIFORM:
>> >> >           fprintf(file, "u%d", inst->src[i].nr +
>> > inst->src[i].reg_offset);
>> >> > -         if (inst->src[i].reladdr) {
>> >> > -            fprintf(file, "+reladdr");
>> >> > -         } else if (inst->src[i].subreg_offset) {
>> >> > +         if (inst->src[i].subreg_offset) {
>> >> >              fprintf(file, "+%d.%d", inst->src[i].reg_offset,
>> >> >                      inst->src[i].subreg_offset);
>> >> >           }
>> >> > @@ -4829,7 +4825,6 @@
>> > fs_visitor::get_instruction_generating_reg(fs_inst *start,
>> >> >  {
>> >> >     if (end == start ||
>> >> >         end->is_partial_write() ||
>> >> > -       reg.reladdr ||
>> >> >         !reg.equals(end->dst)) {
>> >> >        return NULL;
>> >> >     } else {
>> >> > diff --git a/src/mesa/drivers/dri/i965/brw_ir_fs.h
>> > b/src/mesa/drivers/dri/i965/brw_ir_fs.h
>> >> > index c3eec2e..e4f20f4 100644
>> >> > --- a/src/mesa/drivers/dri/i965/brw_ir_fs.h
>> >> > +++ b/src/mesa/drivers/dri/i965/brw_ir_fs.h
>> >> > @@ -58,8 +58,6 @@ public:
>> >> >      */
>> >> >     int subreg_offset;
>> >> >
>> >> > -   fs_reg *reladdr;
>> >> > -
>> >> >     /** Register region horizontal stride */
>> >> >     uint8_t stride;
>> >> >  };
>> >> > @@ -136,8 +134,7 @@ component(fs_reg reg, unsigned idx)
>> >> >  static inline bool
>> >> >  is_uniform(const fs_reg &reg)
>> >> >  {
>> >> > -   return (reg.stride == 0 || reg.is_null()) &&
>> >> > -          (!reg.reladdr || is_uniform(*reg.reladdr));
>> >> > +   return (reg.stride == 0 || reg.is_null());
>> >> >  }
>> >> >
>> >> >  /**
>> >> > --
>> >> > 2.5.0.400.gff86faf
>> >> >
>> >> > _______________________________________________
>> >> > mesa-dev mailing list
>> >> > mesa-dev at lists.freedesktop.org
>> >> > http://lists.freedesktop.org/mailman/listinfo/mesa-dev
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