[Mesa-dev] [PATCH v4 34/40] intel/compiler: validate region restrictions for half-float conversions

Tue Mar 12 22:42:27 UTC 2019

Iago Toral <itoral at igalia.com> writes:

> On Mon, 2019-03-04 at 15:36 -0800, Francisco Jerez wrote:
>> Iago Toral <itoral at igalia.com> writes:
>> 
>> > On Fri, 2019-03-01 at 19:04 -0800, Francisco Jerez wrote:
>> > > Iago Toral <itoral at igalia.com> writes:
>> > > 
>> > > > On Thu, 2019-02-28 at 09:54 -0800, Francisco Jerez wrote:
>> > > > > Iago Toral <itoral at igalia.com> writes:
>> > > > > 
>> > > > > > On Wed, 2019-02-27 at 13:47 -0800, Francisco Jerez wrote:
>> > > > > > > Iago Toral <itoral at igalia.com> writes:
>> > > > > > > 
>> > > > > > > > On Tue, 2019-02-26 at 14:54 -0800, Francisco Jerez
>> > > > > > > > wrote:
>> > > > > > > > > Iago Toral Quiroga <itoral at igalia.com> writes:
>> > > > > > > > > 
>> > > > > > > > > > ---
>> > > > > > > > > >  src/intel/compiler/brw_eu_validate.c    |  64
>> > > > > > > > > > ++++++++++++-
>> > > > > > > > > >  src/intel/compiler/test_eu_validate.cpp | 122
>> > > > > > > > > > ++++++++++++++++++++++++
>> > > > > > > > > >  2 files changed, 185 insertions(+), 1 deletion(-)
>> > > > > > > > > > 
>> > > > > > > > > > diff --git a/src/intel/compiler/brw_eu_validate.c
>> > > > > > > > > > b/src/intel/compiler/brw_eu_validate.c
>> > > > > > > > > > index 000a05cb6ac..203641fecb9 100644
>> > > > > > > > > > --- a/src/intel/compiler/brw_eu_validate.c
>> > > > > > > > > > +++ b/src/intel/compiler/brw_eu_validate.c
>> > > > > > > > > > @@ -531,7 +531,69 @@
>> > > > > > > > > > general_restrictions_based_on_operand_types(const
>> > > > > > > > > > struct
>> > > > > > > > > > gen_device_info *devinf
>> > > > > > > > > >         exec_type_size == 8 && dst_type_size == 4)
>> > > > > > > > > >        dst_type_size = 8;
>> > > > > > > > > >  
>> > > > > > > > > > -   if (exec_type_size > dst_type_size) {
>> > > > > > > > > > +   /* From the BDW+ PRM:
>> > > > > > > > > > +    *
>> > > > > > > > > > +    *    "There is no direct conversion from HF to
>> > > > > > > > > > DF
>> > > > > > > > > > or
>> > > > > > > > > > DF to
>> > > > > > > > > > HF.
>> > > > > > > > > > +    *     There is no direct conversion from HF to
>> > > > > > > > > > Q/UQ or
>> > > > > > > > > > Q/UQ to
>> > > > > > > > > > HF."
>> > > > > > > > > > +    */
>> > > > > > > > > > +   enum brw_reg_type src0_type =
>> > > > > > > > > > brw_inst_src0_type(devinfo,
>> > > > > > > > > > inst);
>> > > > > > > > > > +   ERROR_IF(brw_inst_opcode(devinfo, inst) ==
>> > > > > > > > > > BRW_OPCODE_MOV
>> > > > > > > > > > &&
>> > > > > > > > > 
>> > > > > > > > > Why is only the MOV instruction handled here and
>> > > > > > > > > below?  Aren't
>> > > > > > > > > other
>> > > > > > > > > instructions able to do implicit
>> > > > > > > > > conversions?  Probably
>> > > > > > > > > means
>> > > > > > > > > you
>> > > > > > > > > need
>> > > > > > > > > to deal with two sources rather than one.
>> > > > > > > > 
>> > > > > > > > This comes from the programming notes of the MOV
>> > > > > > > > instruction
>> > > > > > > > (Volume
>> > > > > > > > 2a, Command Reference - Instructions - MOV), so it is
>> > > > > > > > described
>> > > > > > > > specifically for the MOV instruction. I should probably
>> > > > > > > > have
>> > > > > > > > made
>> > > > > > > > this
>> > > > > > > > clear in the comment.
>> > > > > > > > 
>> > > > > > > 
>> > > > > > > Maybe the one above is specified in the MOV page only,
>> > > > > > > probably
>> > > > > > > due
>> > > > > > > to
>> > > > > > > an oversight (If these restrictions were really specific
>> > > > > > > to
>> > > > > > > the
>> > > > > > > MOV
>> > > > > > > instruction, what would prevent you from implementing
>> > > > > > > such
>> > > > > > > conversions
>> > > > > > > through a different instruction?  E.g. "ADD dst:df,
>> > > > > > > src:hf,
>> > > > > > > 0"
>> > > > > > > which
>> > > > > > > would be substantially more efficient than what you're
>> > > > > > > doing
>> > > > > > > in
>> > > > > > > PATCH
>> > > > > > > 02)
>> > > > > > 
>> > > > > > Instructions that take HF can only be strictly HF or mix F
>> > > > > > and
>> > > > > > HF
>> > > > > > (mixed mode float), with MOV being the only exception. That
>> > > > > > means
>> > > > > > that
>> > > > > > any instruction like the one you use above are invalid.
>> > > > > > Maybe
>> > > > > > we
>> > > > > > should
>> > > > > > validate explicitly that instructions that use HF are
>> > > > > > strictly
>> > > > > > HF
>> > > > > > or
>> > > > > > mixed-float mode only?
>> > > > > > 
>> > > > > 
>> > > > > So you're acknowledging that the conversions checked above
>> > > > > are
>> > > > > illegal
>> > > > > whether the instruction is a MOV or something else.  Where
>> > > > > are we
>> > > > > preventing instructions other than MOV with such conversions
>> > > > > from
>> > > > > being
>> > > > > accepted by the validator?
>> > > > 
>> > > > We aren't, because the validator is not checking, in general,
>> > > > for
>> > > > accepted type combinations for specific instructions anywhere
>> > > > as
>> > > > far as
>> > > > I can see.
>> > > 
>> > > Luckily these type conversion restrictions aren't really specific
>> > > to
>> > > any
>> > > instruction AFAICT, even though they only seem to be mentioned
>> > > explicitly for the MOV instruction...
>> > > 
>> > > > If we want to start doing this with HF conversions
>> > > > specifically, I
>> > > > guess it is fine, but in case it is not clear, I think it won't
>> > > > bring
>> > > > any immediate benefits with the VK_KHR_shader_float16_int8
>> > > > implementation since this series only ever emits conversions
>> > > > involving
>> > > > HF operands via MOV instructions,
>> > > 
>> > > Yes, I can see that's the intention of this series, but how can
>> > > you
>> > > make
>> > > sure that nothing in the optimizer is breaking your assumption if
>> > > you
>> > > don't add some validator code to verify the claim of your last
>> > > paragraph?
>> > > 
>> > > > which is why I thought that validating that no direct MOV
>> > > > conversions
>> > > > from DF/Q types ever happen was useful, since we have code in
>> > > > the
>> > > > driver to handle this scenario.
>> > > > 
>> > > > [...]
>> > > > > 
>> > > > > I still don't understand why you want to implement the same
>> > > > > restriction
>> > > > > twice, once for MOV and once for all other mixed-mode
>> > > > > instructions.  How
>> > > > > is that more convenient?
>> > > > 
>> > > > The restrictions based on operand types that are checked in the
>> > > > validator are specific to Byte or cases where the execution
>> > > > type is
>> > > > larger than the destination stride, for which mixed float has a
>> > > > different set of restrictions.
>> > > > 
>> > > > For example, for mixed float we have this rule:
>> > > > 
>> > > > "In Align1, destination stride can be smaller than execution
>> > > > type"
>> > > > 
>> > > > Which overrides this other from "General restrictions based on
>> > > > operand
>> > > > types":
>> > > > 
>> > > > "Destination stride must be equal to the ratio of the sizes of
>> > > > the
>> > > > execution data type to the destination type"
>> > > > 
>> > > > So I thought that it would make things easier to keep all
>> > > > restrictions
>> > > > for mixed float separate and make sure that we skipped mixed
>> > > > float
>> > > > instructions in general_restrictions_based_on_operand_types()
>> > > > so we
>> > > > avoid having to add code to skip individual general
>> > > > restrictions
>> > > > that that are overriden for mixed float mode anyway.
>> > > > 
>> > > 
>> > > I'm fine with that, but it doesn't seem like what this patch is
>> > > doing...
>> > > Isn't it adding code to validate mixed-mode float MOV
>> > > instructions in
>> > > general_restrictions_based_on_operand_types()?
>> > 
>> > I guess this could be arguable, but I was not considering
>> > conversion
>> > MOVs to be mixed-float instructions. There are two reasons for
>> > this:
>> > 
>> > A conversion MOV involving F/HF doesn't seem to be fundamentally
>> > different from any other conversion instruction involving other
>> > types,
>> > other than the requirement of aligning the destination to a Dword,
>> > which is not a resriction explictly made for mixed-float mode.
>> > 
>> > Then, for mixed-float mode, there is this other rule:
>> > 
>> > "In Align1, destination stride can be smaller than execution type.
>> > When
>> > destination is stride of 1, 16 bit packed data
>> > is updated on the destination. However, output packed f16 data must
>> > be
>> > oword aligned, no oword crossing in
>> > packed f16."
>> > 
>> > Which contradicts the other rule that conversions from F to HF need
>> > to
>> > be DWord aligned on the destination.
>> > 
>> > So it seems to me that conversion MOVs are not following the same
>> > principles of mixed-float instructions and we should skip
>> > validating
>> > mixed-float restrictions for them. What do you think?
>> > 
>> 
>> That all seems fairly ambiguous...  And the restriction on DWORD
>> alignment for conversions includes a mixed-mode ADD instruction among
>> the examples, so I'm skeptical that MOV could be truly fundamentally
>> different.
>
> Ok... in that case what do we do about the mixed-float restriction I
> quoted above? Since it is incompatible with the mixed-float MOV
> conversion I guess we only have two options: ether we don't validate it
> at all or we only validate it for mixed-float instructions that are not
> MOV. I guess we can do the latter?
>

Are you referring to this restriction you quoted above?

|  "In Align1, destination stride can be smaller than execution type. When
|  destination is stride of 1, 16 bit packed data
|  is updated on the destination. However, output packed f16 data must be
|  oword aligned, no oword crossing in
|  packed f16."

Why do you say it's incompatible with mixed-float MOV conversions?  It
seems less strict than the general rule actually...

>> > > > Anyway, I'll try to rework the patches to do more generic
>> > > > validation of
>> > > > HF conversions like you ask and see what kind of code we end up
>> > > > with.
>> > > > 
>> > > 
>> > > Thanks.
>> > > 
>> > > > [...]
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 227 bytes
Desc: not available
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20190312/aa1d294a/attachment-0001.sig>