The TripCount for a predicated vector loop body will be
ceil(ElementCount/Width). This alters the conversion of an
active.lane.mask to a VCPT intrinsics to match.
Differential Revision: https://reviews.llvm.org/D94608
This strips out a lot of the code that should no longer be needed from
the MVETailPredictionPass, leaving the important part - find active lane
mask instructions and convert them to VCTP operations.
Differential Revision: https://reviews.llvm.org/D91866
This changes the definition of t2DoLoopStart from
t2DoLoopStart rGPR
to
GPRlr = t2DoLoopStart rGPR
This will hopefully mean that low overhead loops are more tied together,
and we can more reliably generate loops without reverting or being at
the whims of the register allocator.
This is a fairly simple change in itself, but leads to a number of other
required alterations.
- The hardware loop pass, if UsePhi is set, now generates loops of the
form:
%start = llvm.start.loop.iterations(%N)
loop:
%p = phi [%start], [%dec]
%dec = llvm.loop.decrement.reg(%p, 1)
%c = icmp ne %dec, 0
br %c, loop, exit
- For this a new llvm.start.loop.iterations intrinsic was added, identical
to llvm.set.loop.iterations but produces a value as seen above, gluing
the loop together more through def-use chains.
- This new instrinsic conceptually produces the same output as input,
which is taught to SCEV so that the checks in MVETailPredication are not
affected.
- Some minor changes are needed to the ARMLowOverheadLoop pass, but it has
been left mostly as before. We should now more reliably be able to tell
that the t2DoLoopStart is correct without having to prove it, but
t2WhileLoopStart and tail-predicated loops will remain the same.
- And all the tests have been updated. There are a lot of them!
This patch on it's own might cause more trouble that it helps, with more
tail-predicated loops being reverted, but some additional patches can
hopefully improve upon that to get to something that is better overall.
Differential Revision: https://reviews.llvm.org/D89881
We have been running tests/benchmarks downstream with tail-predication enabled
for some time now and this behaves as expected: we are not aware of any
correctness issues, and this performs better across the board than with
tail-predication disabled. Time to flip the switch!
Differential Revision: https://reviews.llvm.org/D88093
This is a reimplementation of the overflow checks for the elementcount,
i.e. the 2nd argument of intrinsic get.active.lane.mask. The element
count is lowered in each iteration of the tail-predicated loop, and
we must prove that this expression doesn't overflow.
Many thanks to Eli Friedman and Sam Parker for all their help with
this work.
Differential Revision: https://reviews.llvm.org/D88086
This might be useful for testing. We already have an option -tail-predication
but that controls the MVETailPredication pass. This
-arm-loloops-disable-tail-pred is just for disabling it in the LowoverheadLoops
pass.
Differential Revision: https://reviews.llvm.org/D88212
Additional sanity checks were added to get.active.lane.mask's second argument,
the loop tripcount/elementcount, in rG635b87511ec3. Like the other (overflow)
checks, skip this if tail-predication is forced.
Differential Revision: https://reviews.llvm.org/D87769
This adds additional checks for the original scalar loop tripcount value, i.e.
get.active.lane.mask second argument, and perform several sanity checks to see
if it is of the form that we expect similarly like we already do for the IV
which is the first argument of get.active.lane.
Differential Revision: https://reviews.llvm.org/D86074
These arm_mve_vldr_gather_offset_predicated and
arm_mve_vstr_scatter_offset_predicated have some extra parameters
meaning the predicate is at a later operand. If a loop contains _only_
those masked instructions, we would miss transforming the active lane
mask.
Differential Revision: https://reviews.llvm.org/D86791
This adapts tail-predication to the new semantics of get.active.lane.mask as
defined in D86147. This means that:
- we can remove the BTC + 1 overflow checks because now the loop tripcount is
passed in to the intrinsic,
- we can immediately use that value to setup a counter for the number of
elements processed by the loop and don't need to materialize BTC + 1.
Differential Revision: https://reviews.llvm.org/D86303
Widen the scope of memory operations that are allowed to be tail predicated
to include gathers and scatters, such that loops that are auto-vectorized
with the option -enable-arm-maskedgatscat (and actually end up containing
an MVE gather or scatter) can be tail predicated.
Differential Revision: https://reviews.llvm.org/D85138
This pick ups the work on the overflow checks for get.active.lane.mask,
which ensure that it is safe to insert the VCTP intrinisc that enables
tail-predication. For a 2d auto-correlation kernel and its inner loop j:
M = Size - i;
for (j = 0; j < M; j++)
Sum += Input[j] * Input[j+i];
For this inner loop, the SCEV backedge taken count (BTC) expression is:
(-1 + (sext i16 %Size to i32)),+,-1}<nw><%for.body>
and LoopUtil cannotBeMaxInLoop couldn't calculate a bound on this, thus "BTC
cannot be max" could not be determined. So overflow behaviour had to be assumed
in the loop tripcount expression that uses the BTC. As a result
tail-predication had to be forced (with an option) for this case.
This change solves that by using ScalarEvolution's helper
getConstantMaxBackedgeTakenCount which is able to determine the range of BTC,
thus can determine it is safe, so that we no longer need to force tail-predication
as reflected in the changed test cases.
Differential Revision: https://reviews.llvm.org/D85737
This refactors option -disable-mve-tail-predication to take different arguments
so that we have 1 option to control tail-predication rather than several
different ones.
This is also a prep step for D82953, in which we want to reject reductions
unless that is requested with this option.
Differential Revision: https://reviews.llvm.org/D83133
After the rewrite of this pass (D79175) I missed one thing: the inserted VCTP
intrinsic can be cloned to exit blocks if there are instructions present in it
that perform the same operation, but this wasn't triggering anymore. However,
it turns out that for handling reductions, see D75533, it's actually easier not
not to have the VCTP in exit blocks, so this removes that code.
This was possible because it turned out that some other code that depended on
this, rematerialization of the trip count enabling more dead code removal
later, wasn't doing much anymore due to more aggressive dead code removal that
was added to the low-overhead loops pass.
Differential Revision: https://reviews.llvm.org/D82773
This patch stops the trunc, rint, round, floor and ceil intrinsics from blocking tail predication.
Differential Revision: https://reviews.llvm.org/D82553
To set up a tail-predicated loop, we need to to calculate the number of
elements processed by the loop. We can now use intrinsic
@llvm.get.active.lane.mask() to do this, which is emitted by the vectoriser in
D79100. This intrinsic generates a predicate for the masked loads/stores, and
consumes the Backedge Taken Count (BTC) as its second argument. We can now use
that to reconstruct the loop tripcount, instead of the IR pattern match
approach we were using before.
Many thanks to Eli Friedman and Sam Parker for all their help with this work.
This also adds overflow checks for the different, new expressions that we
create: the loop tripcount, and the sub expression that calculates the
remaining elements to be processed. For the latter, SCEV is not able to
calculate precise enough bounds, so we work around that at the moment, but is
not entirely correct yet, it's conservative. The overflow checks can be
overruled with a force flag, which is thus potentially unsafe (but not really
because the vectoriser is the only place where this intrinsic is emitted at the
moment). It's also good to mention that the tail-predication pass is not yet
enabled by default. We will follow up to see if we can implement these
overflow checks better, either by a change in SCEV or we may want revise the
definition of llvm.get.active.lane.mask.
Differential Revision: https://reviews.llvm.org/D79175
SCEVExpander modifies the underlying function so it is more suitable in
Transforms/Utils, rather than Analysis. This allows using other
transform utils in SCEVExpander.
This patch was originally committed as b8a3c34eee, but broke the
modules build, as LoopAccessAnalysis was using the Expander.
The code-gen part of LAA was moved to lib/Transforms recently, so this
patch can be landed again.
Reviewers: sanjoy.google, efriedma, reames
Reviewed By: sanjoy.google
Differential Revision: https://reviews.llvm.org/D71537
There are some intrinsics like this that currently block tail
predication, but should be fine. This allows fma through, as the one
that I ran into. There may be others that need the same treatment but
I've only done this one here.
Differential Revision: https://reviews.llvm.org/D78385
Finding the loop tripcount is the first crucial step in preparing a loop for
tail-predication, and this adds a debug message if a tripcount cannot be found.
And while I was at it, I added some more comments here and there.
Differential Revision: https://reviews.llvm.org/D78485
Instead, represent the mask as out-of-line data in the instruction. This
should be more efficient in the places that currently use
getShuffleVector(), and paves the way for further changes to add new
shuffles for scalable vectors.
This doesn't change the syntax in textual IR. And I don't currently plan
to change the bitcode encoding in this patch, although we'll probably
need to do something once we extend shufflevector for scalable types.
I expect that once this is finished, we can then replace the raw "mask"
with something more appropriate for scalable vectors. Not sure exactly
what this looks like at the moment, but there are a few different ways
we could handle it. Maybe we could try to describe specific shuffles.
Or maybe we could define it in terms of a function to convert a fixed-length
array into an appropriate scalable vector, using a "step", or something
like that.
Differential Revision: https://reviews.llvm.org/D72467
Summary:
Future patches will make use of TTI to perform cost-model-driven `SCEVExpander::isHighCostExpansionHelper()`
This is a fully NFC patch to make things reviewable.
Reviewers: reames, mkazantsev, wmi, sanjoy
Reviewed By: mkazantsev
Subscribers: hiraditya, zzheng, javed.absar, dmgreen, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D73704
A small IR change in calculating the active lanes resulted in no longer
recognising tail-predication. Now recognise both an 'add' and 'or' in
the expression that calculates the active lanes.
Differential Revision: https://reviews.llvm.org/D74394
We had support for runtime trip count values, but not constants, and this adds
supports for that.
And added a minor optimisation while I was add it: don't invoke Cleanup when
there's nothing to clean up.
Differential Revision: https://reviews.llvm.org/D73198
This patch uses helper function rewriteLoopExitValues that is refactored in
D72602 to rematerialise the iteration count in exit blocks, so that we can
clean-up loop update expressions inside the hardware-loops later in
ARMLowOverheadLoops, which is necessary to get actual performance gains for
tail-predicated loops.
Differential Revision: https://reviews.llvm.org/D72714
I've added a few more debug messages to MVETailPredication because I wanted to
trace better which instructions are added/removed. And while I was at it, I
factored out one function which I thought was clearer, and have added some
comments to describe better the flow between MVETailPredication and
ARMLowOverheadLoops.
Differential Revision: https://reviews.llvm.org/D71549
SCEVExpander modifies the underlying function so it is more suitable in
Transforms/Utils, rather than Analysis. This allows using other
transform utils in SCEVExpander.
Reviewers: sanjoy.google, efriedma, reames
Reviewed By: sanjoy.google
Differential Revision: https://reviews.llvm.org/D71537
This adds ICmp to the list of instructions that we sink a splat to in a
loop, allowing the register forms of instructions to be selected more
often. It does not add FCmp yet as the results look a little odd, trying
to keep the register in an float reg and having to move it back to a GPR.
Differential Revision: https://reviews.llvm.org/D70997
This has two main effects:
- Optimizes debug info size by saving 221.86 MB of obj file size in a
Windows optimized+debug build of 'all'. This is 3.03% of 7,332.7MB of
object file size.
- Incremental step towards decoupling target intrinsics.
The enums are still compact, so adding and removing a single
target-specific intrinsic will trigger a rebuild of all of LLVM.
Assigning distinct target id spaces is potential future work.
Part of PR34259
Reviewers: efriedma, echristo, MaskRay
Reviewed By: echristo, MaskRay
Differential Revision: https://reviews.llvm.org/D71320
Summary:
D65884 added a set of Arm IR intrinsics for the MVE VCTP instruction,
to use in tail predication. But the 64-bit one doesn't work properly:
its predicate type is `<2 x i1>` / `v2i1`, which isn't a legal MVE
type (due to not having a full set of instructions that manipulate it
usefully). The test of `vctp64` in `basic-tail-pred.ll` goes through
`opt` fine, as the test expects, but if you then feed it to `llc` it
causes a type legality failure at isel time.
The usual workaround we've been using in the rest of the MVE
intrinsics family is to bodge `v2i1` into `v4i1`. So I've adjusted the
`vctp64` IR intrinsic to do that, and completely removed the code (and
test) that uses that intrinsic for 64-bit tail predication. That will
allow me to add isel rules (upcoming in D70485) that actually generate
the VCTP64 instruction.
Also renamed all four of these IR intrinsics so that they have `mve`
in the name, since its absence was confusing.
Reviewers: ostannard, MarkMurrayARM, dmgreen
Reviewed By: MarkMurrayARM
Subscribers: samparker, kristof.beyls, hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70592
Remove the restriction, from the mve tail predication pass, that the
all masked vectors instructions need to be 128-bits. This allows us
to supported extending loads and truncating stores.
Differential Revision: https://reviews.llvm.org/D69946
This implements TTI hook 'preferPredicateOverEpilogue' for MVE. This is a
first version and it operates on single block loops only. With this change, the
vectoriser will now determine if tail-folding scalar remainder loops is
possible/desired, which is the first step to generate MVE tail-predicated
vector loops.
This is disabled by default for now. I.e,, this is depends on option
-disable-mve-tail-predication, which is off by default.
I will follow up on this soon with a patch for the vectoriser to respect loop
hint 'vectorize.predicate.enable'. I.e., with this loop hint set to Disabled,
we don't want to tail-fold and we shouldn't query this TTI hook, which is
done in D70125.
Differential Revision: https://reviews.llvm.org/D69845
The VCTP instruction will calculate the predicate masked based upon
the number of elements that need to be processed. I had inserted the
sub before the vctp intrinsic and supplied it as the operand, but
this is incorrect as the phi should directly feed the vctp. The sub
is calculating the value for the next iteration.
Differential Revision: https://reviews.llvm.org/D67921
llvm-svn: 373188
Remove any predicate that we replace with a vctp intrinsic, and try
to remove their operands too. Also look into the exit block to see if
there's any duplicates of the predicates that we've replaced and
clone the vctp to be used there instead.
Differential Revision: https://reviews.llvm.org/D67709
llvm-svn: 372567
The MVE and LOB extensions of Armv8.1m can be combined to enable
'tail predication' which removes the need for a scalar remainder
loop after vectorization. Lane predication is performed implicitly
via a system register. The effects of predication is described in
Section B5.6.3 of the Armv8.1-m Arch Reference Manual, the key points
being:
- For vector operations that perform reduction across the vector and
produce a scalar result, whether the value is accumulated or not.
- For non-load instructions, the predicate flags determine if the
destination register byte is updated with the new value or if the
previous value is preserved.
- For vector store instructions, whether the store occurs or not.
- For vector load instructions, whether the value that is loaded or
whether zeros are written to that element of the destination
register.
This patch implements a pass that takes a hardware loop, containing
masked vector instructions, and converts it something that resembles
an MVE tail predicated loop. Currently, if we had code generation,
we'd generate a loop in which the VCTP would generate the predicate
and VPST would then setup the value of VPR.PO. The loads and stores
would be placed in VPT blocks so this is not tail predication, but
normal VPT predication with the predicate based upon a element
counting induction variable. Further work needs to be done to finally
produce a true tail predicated loop.
Because only the loads and stores are predicated, in both the LLVM IR
and MIR level, we will restrict support to only lane-wise operations
(no horizontal reductions). We will perform a final check on MIR
during loop finalisation too.
Another restriction, specific to MVE, is that all the vector
instructions need operate on the same number of elements. This is
because predication is performed at the byte level and this is set
on entry to the loop, or by the VCTP instead.
Differential Revision: https://reviews.llvm.org/D65884
llvm-svn: 371179
David Green