[llvm-dev] globalisel: cross-bank constant propagation?

[Nicolai Hähnle via llvm-dev]

Hi Jay,

On Sat, Mar 27, 2021 at 9:56 AM Jay Foad <jay.foad at gmail.com> wrote:

> For simplicity our regbankselect says that all operands of VALU
> instructions have to go in vgprs. Moving some of them into sgprs is
> left as an optimisation for a later pass. As you know there are limits
> on //how many// operands of a VALU instruction can be sgprs or
> constants, which are not simple to express in terms of alternative
> operand mappings.
>

Okay, that makes sense: the initial register bank selection is biased to
generate legal code.

It sounds likely that similar issues of obstructive COPYs will appear in
other patterns as well, right? For example, if you have original code of
the form (x * (-y)), where x is divergent and y is uniform, it seems likely
you'd get something like (typed completely off the top of my head):

  t:sgpr = G_FNEG y:sgpr
  t':vgpr = COPY t:sgpr
  r:vgpr = G_FMUL x:vgpt, t':vgpr

... and then an instruction selection pattern that pulls the negation into
a source modifier would fail due to the COPY? The point is that this is not
just an issue for constants, so a constant-specific pass seems the wrong
way to go.

The remaining options I see right now are:

(1) Allow pattern matching to look through COPYs.
(2) Eliminate those COPYs before instruction selection.

Not having thought about this very long, the relative merits seem to be:

- (2) results in smaller GMIR and a simpler pattern matcher, which is
better for compile time
- (2) requires legalization after(?) instruction selection (constant bus
limit etc.), which is going to be a pain in the existing CodeGen framework

Neither option makes me happy :)  I guess I would go with (2) if this was a
greenfield development, but (1) fits better in the existing LLVM code base.

Cheers,
Nicolai


Thanks,
> Jay.
>
> On Sat, 27 Mar 2021 at 07:58, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> >
> > Hi Jay,
> >
> > Where does the copy of a constant from SGPR to VGPR come from? Naively,
> I'd think that better GMIR would be
> >
> >     %3:sgpr(s32) = G_CONSTANT i32 5
> >     ...
> >     %8:vgpr(s64) = G_LSHR %7, %3(s32)
> >
> > Cheers,
> > Nicolai
> >
> > On Fri, Mar 26, 2021 at 3:43 PM Jay Foad via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> Hi,
> >>
> >> While experimenting with alternative ways of lowering funnel shifts on
> >> AMDGPU I have run into a case where this GMIR:
> >>
> >>     %0:vgpr(s32) = COPY $vgpr0
> >>     %1:vgpr(s32) = COPY $vgpr1
> >>     %2:sgpr_64 = COPY $sgpr30_sgpr31
> >>     %3:sgpr(s32) = G_CONSTANT i32 5
> >>     %7:vgpr(s64) = G_MERGE_VALUES %1(s32), %0(s32)
> >>     %9:vgpr(s32) = COPY %3(s32)
> >>     %8:vgpr(s64) = G_LSHR %7, %9(s32)
> >>     %4:vgpr(s32) = G_TRUNC %8(s64)
> >>     $vgpr0 = COPY %4(s32)
> >>     %5:ccr_sgpr_64 = COPY %2
> >>     S_SETPC_B64_return %5, implicit $vgpr0
> >>
> >> does not get matched by this selection pattern:
> >>
> >> def : GCNPat<(i32 (trunc (srl i64:$src0, (i32 ShiftAmt32Imm:$src1)))),
> >>           (V_ALIGNBIT_B32_e64 (i32 (EXTRACT_SUBREG VReg_64:$src0,
> sub1)),
> >>                           (i32 (EXTRACT_SUBREG VReg_64:$src0, sub0)),
> >> VSrc_b32:$src1)>;
> >>
> >> because the COPY of G_CONSTANT 5 from sgpr to vgpr gets in the way of
> >> matching the (i32 ShiftAmt32Imm:$src1).
> >>
> >> What's a good way of fixing this? I know there has been some
> >> discussion before about whether the generated matcher should look
> >> through copies, either automatically or only when the tablegen pattern
> >> is augmented with some kind of "please look through copies here" node.
> >>
> >> As an alternative, would it make sense to run some cross-bank constant
> >> propagation after regbankselect? It seems pretty obvious to me that %9
> >> above should be simplified to:
> >>
> >> %9:vgpr(s32) = G_CONSTANT i32 5
> >>
> >> so long as it's legal, which we can easily check, can't we?
> >>
> >> Thanks,
> >> Jay.
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
> >
> > --
> > Lerne, wie die Welt wirklich ist,
> > aber vergiss niemals, wie sie sein sollte.
>


-- 
Lerne, wie die Welt wirklich ist,
aber vergiss niemals, wie sie sein sollte.
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