[llvm-dev] [RFC] Supporting ARM's SVE in LLVM
Graham Hunter via llvm-dev
llvm-dev at lists.llvm.org
Fri Nov 4 07:13:56 PDT 2016
Hi,
We've been working for the last two years on support for ARM's Scalable Vector Extension in LLVM, and we'd like to upstream our work. We've had to make several design decisions without community input, and would like to discuss the major changes we've made. To help with the discussions, I've attached a technical document (also in plain text below) to describe the semantics of our changes. We've made the entire patch available as a github repository at https://github.com/ARM-software/LLVM-SVE and our presentation slides and video from the DevMeeting should be available on the llvm website soon.
A few caveats:
* This is not intended for code review, just to serve as context for discussion when we post patches for individual pieces (the first of which should be posted within the next two weeks)
* It's based on a version of upstream llvm which is a few months old at this point
* This is a warts-and-all release of our development tree, with plenty of TODOs and unfinished experiments
* We haven't posted our clang changes yet
-Graham
Support for Scalable Vector Architectures in LLVM IR
# Background
*ARMv8-A Scalable Vector Extensions* is a vector ISA extension for AArch64,
featuring lane predication, scatter-gather, and speculative loads. It is
intended to scale with hardware such that the same binary running on a processor
with wider vector registers can take advantage of the increased compute power
without recompilation. This document describes changes made to LLVM that allow
its vectorizer to better target SVE.
As the vector length is no longer a statically known value, the way in which the
LLVM vectorizer generates code required modifications such that certain values
are now runtime evaluated expressions. The following section documents the
additional IR instructions needed to achieve this. During the design of these
extensions, we endeavoured to strike a balance between elegant and succinct
design and the pragmatic realities of generating efficient code for the SVE
architecture. We believe that the current design has a relatively small impact
on the IR and type system given the unusual characteristics of the target.
# Types
## Overview:
## IR Class Changes:
To represent a vector of unknown length a scaling property is added to the
`VectorType` class whose element count becomes an unknown multiple of a known
minimum element count. To be specific `unsigned NumElements` is replaced by
`class ElementCount` with its two members:
* `unsigned Min`: the minimum number of elements.
* `bool Scalable`: is the element count an unknown multiple of `Min`?
For non-scalable vectors (`Scalable=false`) the scale is assumed to be one and
thus `Min` becomes identical to the original `NumElements` property.
This mixture of static and runtime quantities allow us to reason about the
relationship between different scalable vector types without knowing their
exact length.
## IR Interface:
```cpp
static VectorType *get(Type *ELType, ElementCount EC);
// Legacy interface whilst code is migrated.
static VectorType *get(Type *ElType, unsigned NumEls, bool Scalable=false);
ElementCount getElementCount();
bool isScalable();
```
## IR Textual Form:
The textual IR for vectors becomes:
`<[n x] <m> x <type>>`
where `type` is the scalar type of each element, `m` corresponds to `Min` and
the optional string literal `n x ` signifies `Scalable` is *true* when present,
false otherwise. The textual IR for non-scalable vectors is thus unchanged.
Scalable vectors with the same `Min` value have the same number of elements,
and the same number of bytes if `Min * sizeof(type)` is the same:
`<n x 4 x i32>` and `<n x 4 x i8>` have the same number of elements.
`<n x 4 x i32>` and `<n x 8 x i16>` have the same number of bytes.
## SelectionDAG
New scalable vector MVTs are added, one for each existing vector type. Scalable
vector MVTs are modelled in the same way as the IR. Hence, `<n x 4 x i32>`
becomes `nxv4i32`.
## MVT Interface:
```cpp
static MVT getVectorVT(MVT VT, ElementCount EC);
bool isScalableVector() const;
static mvt_range integer_scalable_valuetypes();
static mvt_range fp_scalable_valuetypes();
```
To minimise the effort required for common code to preserve the scalable flag
we extend the helper functions within MVT/EVT classes to cover more cases. For
example:
```cpp
/// Return a VT for a vector type whose attributes match ourselves
/// but with an element type chosen by the caller.
EVT changeVectorElementType(EVT EltVT)`
```
# Instructions
The majority of instructions work seamlessly when applied to scalable vector
types. However, on occasion assumptions are made that allow vectorization logic
to be reduced directly to constants completely bypassing the IR (e.g. when the
element count is known). These assumption are generally unsafe for scalable
vectors which forces a requirement to express more logic in IR. To help with
this the following instruction are added to the IR.
## *elementcount*
### Syntax:
`<result> = elementcount <n x m x ty> <v1> as <ty2>`
### Overview:
This instruction returns the actual number of elements in the input vector of
type `<n x m x ty>` as the scalar type `<ty2>`. This is primarily used to
increment induction variables and replaces many uses of `VF` within the loop
vectorizer.
### IRBuilder Interface:
```cpp
Value *CreateElementCount(Type *Ty, Value *V, const Twine &Name = "");
```
### Fixed-Width Behaviour:
A constant with the value of `Min` is created.
### SelectionDAG:
See [*ISD::ELEMENT_COUNT*](#isdelementcount).
## *seriesvector*
### Syntax:
`<result> = seriesvector <ty> <v1>, <v2> as <n x m x ty>`
### Overview:
This instruction returns a vector of type `<n x m x ty>` with elements forming
the arithmetic series:
`elt[z] = v1 + z * v2`
where `z` is the element index and `<ty>` a scalar type. This is primarily used
to represent a vector of induction values leading to:
* Predicate creation using vector compares for fully predicated loops (see also:
[*propff*](#propff), [*test*](#test)).
* Creating offset vectors for gather/scatter via `getelementptr`.
* Creating masks for `shufflevector`.
For the following loop, a seriesvector instruction based on `i` is used to
create the data vector to store:
```cpp
unsigned a[LIMIT];
for (unsigned i = 0; i < LIMIT; i++) {
a[i] = i;
}
```
*seriesvector* instructions can optionally have *NUW* and *NSW* flags attached
to them. The semantics of these flags are intended to match those of the usual
scalar instruction wrap flags, but apply element-wise to the result vector. If
any element addition of the current value and step results in a signed or
unsigned overflow with the corresponding flag set, then the result is a poison
value for the entire vector. However, this feature is not currently used.
### IRBuilder Interface:
```cpp
Value *CreateSeriesVector(VectorType::ElementCount EC, Value *Start,
Value* Step, const Twine &Name = "",
bool HasNUW = false, bool HasNSW = false);
```
### Fixed-Width Behaviour:
A constant vector is created with the same arithmetic series.
### SelectionDAG:
See [*ISD::SERIES_VECTOR*](#isdseriesvector).
## *test*
### Syntax:
`<result> = test <cond> <ty> <v1>`
### Overview:
This instruction returns a scalar boolean value based on the comparison of a
boolean or vector boolean operand. It allows us to reason about a value as a
whole rather than the per scalar comparison obtained from say *icmp*. The most
common use is testing the result of *icmp*/*fcmp*.
We use this for the controlling predicate of fully predicated loops. Loops with
a termination condition as follows:
```cpp
for (i = 0; i < LIMIT; ++i)
```
are handled by testing their control predicate for `first true` to signify
another iteration is required.
Support for scalar booleans is simply to provide symmetry so that all variants
of *icmp*/*fcmp* can be passed as input to *test*.
#### Supported Conditions:
* all false
* all true
* any false
* any true
* first false
* first true
* last false
* last true
### IRBuilder Interface:
```cpp
Value *CreateTest(TestInst::Predicate P, Value *V, const Twine &Name = "");
```
### Fixed-Width Behaviour
Same as scalable.
### SelectionDAG:
See [*ISD::TEST_VECTOR*](#isdtestvector).
## *propff*
### Syntax:
`<result> = propff <ty> <v1>, <v2>`
### Overview:
This instruction creates a partitioned boolean vector based on the position of
the first boolean false value in the concatenation of input boolean vectors `v1`
and `v2`. Given vectors of length `k`, an element `x[i]` in the result vector is
true if, and only if, each element `y[0]...y[i+k]` in the concatenated input
vector `y=v1:v2` is true.
The following examples show the results for a full `v1` and a `v2` which has
reached a termination condition, and a `v1` which previously reached a
termination condition with any `v2`.
\ 
A core use of this instruction is to protect against integer overflow that can
occur when maintaining the induction vector of a fully predicated loop. For SVE
the issue is further compounded with its support of non-power-of-2 vector
lengths.
We believe *propff* to be the least complex partitioning instruction to provide
sufficient abstraction yet achieve the expected SVE code quality. Although
other partitioning schemes can be modelled using *propff* we feel a more
[capable partitioning instruction](#partition) is worth highlighting because
it can simplify the vectorization of more loops.
### IRBuilder Interface:
```cpp
Value *CreatePropFF(Value* P1, Value *P2, const Twine &Name = "");
```
### Fixed-Width Behaviour:
Same as scalable.
### SelectionDAG:
See [*ISD::PROPAGATE\_FIRST\_ZERO*](#isdpropagatefirstzero).
## *shufflevector*
### Syntax:
`<result> = shufflevector <ty1> <v1>, <ty1> <v2>, <ty2> <mask>`
### Overview:
Not a new instruction but *shufflevector* is extended to accept non-constant
masks. For code that expects the original restriction
`ShuffleVectorInst::getMaskValue` has changed to guard against unsafe uses.
```cpp
// [old] Returns the mask value.
static int getMaskValue(Constant *Mask, unsigned i);
// [new] Returns true when Result contains the mask value, false otherwise.
static bool getMaskValue(Value *Mask, unsigned i, int &Result);
```
Similar changes are made to related interfaces and their users.
### IRBuilder Interface:
```cpp
Value *CreateShuffleVector(Value *V1, Value *V2, Value *Mask,
const Twine &Name = "");
```
### Fixed-Width Behaviour:
No change.
### SelectionDAG:
See [*ISD::VECTOR\_SHUFFLE\_VAR*](#isdvectorshufflevar).
# Constants
Scalable vectors of known constants cannot be represented within LLVM due to
their unknown element count. Optimizations performed on scalable vectors become
much more reliant on `ConstantExpr`s. Most of the instructions talked about in
this document also have a matching `ConstantExpr` with the most common constant
folds duplicated to work with them.
This is most prevalent when considering vector splats. These are simulated via
a sequence of `insertelement`and `shufflevector` that are usually resolved to a
`Constant`. For scalable vectors this cannot be done and the original sequence
is maintained. To mitigate this, `PatternMatch` is extended to better support
`ConstantExpr`s along with new helpers like:
```cpp
#define m_SplatVector(X) \
m_ShuffleVector( \
m_InsertElement(m_Undef(), X, m_Zero()), \
m_Value(), \
m_Zero()) \
```
Zero is the exception with `zeroinitializer` applying equally well to scalable
and non-scalable vectors.
# Predicated floating point arithmetic {#predfparith}
When a loop is fully predicated it becomes necessary to have masked versions of
faulting instructions. Today we use the existing masked memory intrinsics to
ensure we do not access memory that would not have been accessed by the
original scalar loop.
The same principle applies to floating point instructions whereby we should not
trigger an exception that would not have been produced by the original scalar
loop.
We don't handle this today but are seeking advice as to the preferred method.
Options include:
1. Add masked versions of the affected instructions.
2. Extend existing instructions to take a predicate.
3. Construct IR that contains "safe" values for undefined locations.
Given the choice, Option 1 is our preferred route as it's in line with the
introduction of the masked memory intrinsics. Although, given the greater
potential for optimization, instruction forms may be preferable to intrinsics.
See also [*Predicated floating point intrinsics*](#predfpint)
# Intrinsics
## *llvm.masked.spec.load*
### Syntax:
`<result> = call {<data>, <pred>} @llvm.masked.spec.load.<type>(<ptr>, <align>, <mask>, <merge>)`
### Overview:
This intrinsic represents a load whereby only the first active lane is expected
to succeed with remaining lanes loaded speculatively. Along with the data this
intrinsic returns a predicate indicating which lanes of data are valid.
### Interface:
```cpp
CallInst *CreateMaskedSpecLoad(Value *Ptr, unsigned Align, Value *Mask,
Value *PassThru = 0, const Twine &Name = "");
```
### Fixed-Width Behaviour:
Same as scalable.
## *llvm.ctvpop*
### Syntax:
### Overview:
This intrinsic returns a count of the set bits across a complete vector. It's
most useful when operating on predicates as it allows a portable way to create
predicate vectors that are partitioned differently (e.g. including the lane
with the first change, rather than *propff*'s exclusive behaviour).
### Interface:
```cpp
CallInst *CreateCntVPop(Value *Vec, const Twine &Name);
```
### Fixed-Width Behaviour
Same as scalable. However, it can also be modelled using `*llvm.ctpop*`
followed by a scalarized horizontal reduction. This is not possible with
scalable vectors because scalarization is not an option.
## *horizontal reductions*
Loops containing reductions are first vectorized as if no such reduction
exists, thus building a vector of accumulated results. When exiting the vector
loop a final in-vector reduction is done by effectively scalarizing the
operation across each of the result vector's elements.
For scalable vectors, whose element count is unknown, such scalarization is not
possible. For this reason we introduce target specific intrinsics to support
common reductions (e.g. horizontal add), along with TargetTransformInfo
extensions to query their availability. If a scalable vector loop requires a
reduction that's not provided by the target its vectorization is prevented.
When fully predicated vectorization is required, additional work is done within
the vector loop to ensure inactive lanes don't affect the accumulated result.
See also [Predicated floating point arithmetic](#predfparith).
## *Predicated floating point intrinsics* {#predfpint}
Libcalls not directly supported by the code generator must be serialized (See
[here](#pass_libcall)). Also, for fully predicated loops the scalar calls must
only occur for active lanes so we introduce predicated versions of the most
common routines (e.g. `*llvm.pow`). This is mostly transparent to the loop
vectorizer beyond the requirement to pass an extra parameter.
### TTI Interface:
```cpp
bool canReduceInVector(const RecurrenceDescriptor &Desc, bool NoNaN) const;
Value* getReductionIntrinsic(IRBuilder<> &Builder,
const RecurrenceDescriptor& Desc, bool NoNaN,
Value* Src) const;
```
# SelectionDAG Nodes
## *ISD::BUILD_VECTOR* {#isdbuildvector}
`BUILD_VECTOR(ELT0, ELT1, ELT2, ELT3,...)`
This node takes as input a set of scalars to concatenate into a single vector.
By its very nature this node is incompatible with scalable vectors because
we don't know how many scalars it will takes to fill one. All common code
using this node has either been rewritten or bypassed.
## *ISD::ELEMENT_COUNT* {#isdelementcount}
`ELEMENT_COUNT(TYPE)`
See [*elementcount*](#elementcount).
## *ISD::EXTRACT_SUBVECTOR* {#isdextractsubvector}
`EXTRACT_SUBVECTOR(VECTOR, IDX)`
Usage of this node is typically linked to legalization where it's used to split
vectors. The index parameter is often absolute and proportional to the input
vector's element count.
For scalable vectors an absolute index makes little sense. We have changed this
parameter's meaning to become implicitly multiplied by `n` to match its main
usage.
The change has no effect when applied to non-scalable vectors, because `n ==
1`. No target specific code is affected and in many cases common code becomes
compatible with scalable vectors. For example:
`nxv2i64 extract_subvector(nxv4i64, 2)`
The real first lane becomes `n * 2`, resulting in the extraction of the top
half of the input vector. This maintains the intension of the original code for
both scalable and non-scalable vectors.
For common code that truly requires an absolute index we recommend a new
distinct ISD node to better differentiate such patterns.
## *ISD::INSERT_SUBVECTOR* {#isdinsertsubvector}
`INSERT_SUBVECTOR(VECTOR1, VECTOR2, IDX)`
We have made the equivalent change to this node's index parameter to match the
behaviour of [ISD::EXTRACT_SUBVECTOR](#ISD::EXTRACT_SUBVECTOR).
## *ISD::PROPAGATE\_FIRST\_ZERO* {#isdpropagatefirstzero}
`PROPAGATE_FIRST_ZERO(VEC1, VEC2)`
See [*propff*](#propff).
## *ISD::SERIES_VECTOR* {#isdseriesvector}
`SERIES_VECTOR(INITIAL, STEP)`
See [*seriesvector*](#seriesvector).
## *ISD::SPLAT_VECTOR* {#isdsplatvector}
`SPLAT_VECTOR(VAL)`
Within the code generator a splat is represented by ISD::BUILD_VECTOR and is
thus incompatible with scalable vectors.
We initially made use of [ISD::SERIES_VECTOR](#seriesvector) with a zero stride
but that brings with it a requirement for [ISD::SERIES_VECTOR](#seriesvector)
to support floating-point types. For this reason we created an explicit node
that also allowed less complex looking legalization/selection code.
## *ISD::TEST_VECTOR* {#isdtestvector}
`TEST_VECTOR(VEC, PRED)`
`VEC` is the boolean vector being tested, and `PRED` is a `TestCode` enum under
the `llvm::ISD` namespace which contains all the supported conditions.
See [*test*](#test).
## *ISD::VECTOR\_SHUFFLE\_VAR* {#isdvectorshufflevar}
`VECTOR_SHUFFLE_VAR(VEC1, VEC2, VEC3)`
See [*shufflevector*](#shufflevector).
# AArch64 Specific Changes
## Instruction Selection
In order to allow proper instruction selection there must be a direct mapping
from MVTs to SVE registers. SVE data registers have a length in multiples of
128bits (with 128bits being the minimum) and predicate registers have a bit for
every byte of a data register.
Given the 128bit minimum we map scalable vector MVTs whose static component is
also 128bits (e.g. MVT::nxv4i32) directly to SVE data registers. Scalable
vector MVTs with an `i1` element type and a static element count of 16
(128/8 = 16) or fewer (e.g. MVT::nxv4i1) are mapped to SVE predicate registers.
All other integer MVTs are considered illegal and are either split or promoted
accordingly. A similar rule applies to vector floating point MVTs but those
types whose static component is less that 128bits (MVT::nx2f32) are also mapped
directly to SVE data registers but in a form whereby elements are effectively
interleaved with enough undefined elements to fulfil the 128bit requirement.
We do this so that predicate bits correctly align to their data counterpart.
For example, for all vector MVTs with two elements, a predicate of nxv2i1 is
used, regardless of the data vector's element type.
## Stack Regions for SVE Objects
As SVE registers have an unknown size their associated fill/spill instructions
require an offset that will be implicitly scaled by the vector length instead
of bytes or element size. To accommodate this we introduce the concept of
`Stack Regions` that are areas on the stack associated with specific data types
or register classes.
Each `Stack Region` controls its own allocation, deallocation and internal
offset calculations. For SVE we create separate `Stack Region`s for its data
and predicate registers. Objects not belonging to a `Stack Region` use a
default so that existing targets are not affected.
## SVE-Specific Optimizations
The following SVE specific IR transformation passes are added to better guide
code generation. In some instances this has affected how loops are vectorized
because it's assumed they will occur. They also point to work other target's
may wish to consider.
### SVE Post-Vectorization Optimization Pass
This pass is responsible for converting generically vectorized loops into a
form that more closely resembles SVE's style of vectorization. For example,
rewriting a vectorized loop's control flow to utilize SVE's `while` instruction.
NOTE: This pass is very sensitive to the wrap flags (i.e. NSW/NUW). Much effort
has gone into ensuring they are preserved during vectorization to the extent of
introducing `llvm.assume` calls when beneficial.
### SVE Expand Libcall Pass{#pass_libcall}
In order to keep the loop vectorizer generic we maintain the ability to
vectorize libcalls we know the code generator cannot handle because scalable
vectors cannot be scalarized. This pass serializes such calls by introducing
loops using SVE's predicate iteration instructions.
Note that although such serialization can be achieved generically using
`extractelement`/`insertelement`, our experiments showed no route to efficient
code generation.
### SVE Addressing Modes Pass
This pass alleviates the negative effects of `Loop Strength Reduction` on
address computations for SVE targeted loops, leading to better code quality.
# Full Example
The following section shows how a C loop with a sum reduction is represented in
scalable IR (assuming -Ofast) and the final generated code.
```cpp
int SimpleReduction(int *a, int count) {
int res = 0;
for (int i = 0; i < count; ++i)
res += a[i];
return res;
}
```
The IR representation shows each of the new instructions being used to control
the loop, along with one of the horizontal reduction intrinsics. The main
sequence (noted by the *;; Control* comments below) for controlling loop
iterations is:
1. Using *elementcount* to increment the induction variable
2. Using *seriesvector* starting from the current induction variable value to
compare against a splat of the loop's trip count
3. Using *propff* to ensure the resulting predicate strictly partitions the
predicate and does not wrap
4. Using *test* to determine whether the first lane is active (and thus at
least one more iteration is required)
```llvm
define i32 @SimpleReduction(i32* nocapture readonly %a, i32 %count) #0 {
entry:
%cmp6 = icmp sgt i32 %count, 0
br i1 %cmp6, label %min.iters.checked, label %for.cond.cleanup
min.iters.checked:
%0 = add i32 %count, -1
%1 = zext i32 %0 to i64
%2 = add nuw nsw i64 %1, 1
%wide.end.idx.splatinsert = insertelement <n x 4 x i64> undef, i64 %2, i32 0
%wide.end.idx.splat = shufflevector <n x 4 x i64> %wide.end.idx.splatinsert,
<n x 4 x i64> undef, <n x 4 x i32> zeroinitializer
%3 = icmp ugt <n x 4 x i64> %wide.end.idx.splat, seriesvector (i64 0, i64 1)
%predicate.entry = propff <n x 4 x i1> shufflevector (<n x 4 x i1> insertelement
(<n x 4 x i1> undef, i1 true, i32 0), <n x 4 x i1> undef,
<n x 4 x i32> zeroinitializer), %3
br label %vector.body
vector.body:
%min.iters.checked
%index = phi i64 [ 0, %min.iters.checked ], [ %index.next, %vector.body ]
%predicate = phi <n x 4 x i1> [ %predicate.entry, %min.iters.checked ],
[ %predicate.next, %vector.body ]
%vec.phi = phi <n x 4 x i32> [ zeroinitializer, %min.iters.checked ],
[ %8, %vector.body ]
%4 = icmp ult i64 %index, 4294967296
call void @llvm.assume(i1 %4)
%5 = getelementptr inbounds i32, i32* %a, i64 %index
%6 = bitcast i32* %5 to <n x 4 x i32>*
%wide.masked.load = call <n x 4 x i32> @llvm.masked.load.nxv4i32(<n x 4 x i32>* %6,
i32 4, <n x 4 x i1> %predicate, <n x 4 x i32> undef), !tbaa !1
%7 = select <n x 4 x i1> %predicate, <n x 4 x i32> %wide.masked.load,
<n x 4 x i32> zeroinitializer
%8 = add nsw <n x 4 x i32> %vec.phi, %7
%index.next = add nuw nsw i64 %index, elementcount (<n x 4 x i64> undef) ;; Control 1
%9 = add nuw nsw i64 %index, elementcount (<n x 4 x i64> undef)
%10 = seriesvector i64 %9, i64 1 as <n x 4 x i64> ;; Control 2
%11 = icmp ult <n x 4 x i64> %10, %wide.end.idx.splat
%predicate.next = propff <n x 4 x i1> %predicate, %11 ;; Control 3
%12 = test first true <n x 4 x i1> %predicate.next ;; Control 4
br i1 %12, label %vector.body, label %middle.block, !llvm.loop !5
middle.block:
%.lcssa = phi <n x 4 x i32> [ %8, %vector.body ]
%13 = call i64 @llvm.aarch64.sve.uaddv.nxv4i32(<n x 4 x i1> shufflevector
(<n x 4 x i1> insertelement (<n x 4 x i1> undef, i1 true, i32 0),
<n x 4 x i1> undef, <n x 4 x i32> zeroinitializer), <n x 4 x i32> %.lcssa)
%14 = trunc i64 %13 to i32
br label %for.cond.cleanup
for.cond.cleanup:
%res.0.lcssa = phi i32 [ 0, %entry ], [ %14, %middle.block ]
ret i32 %res.0.lcssa
}
```
The main vector body of the resulting code is one instruction longer than it
would be for NEON, but no scalar tail is required and performance will scale
with register length. The *seriesvector*, *shufflevector*(splat), *icmp*,
*propff*, *test* sequence has been recognized and transformed into the `whilelo`
instruction.
\newpage
```nasm
SimpleReduction:
// BB#0:
subs w9, w1, #1
b.lt .LBB0_4
// BB#1:
add x9, x9, #1
mov x8, xzr
whilelo p0.s, xzr, x9
mov z0.s, #0
.LBB0_2:
ld1w {z1.s}, p0/z, [x0, x8, lsl #2]
incw x8
add z0.s, p0/m, z0.s, z1.s
whilelo p0.s, x8, x9
b.mi .LBB0_2
// BB#3:
ptrue p0.s
uaddv d0, p0, z0.s
fmov w0, s0
ret
.LBB0_4:
mov w0, wzr
ret
```
<!--
Commented out until after DevMeeting talk
# Clang Changes
## ACLE Types
(TODO)
-->
# Appendix
The following is an alternative to the [*propff*](#propff) instruction described
above. We haven't implemented this (or worked out all the specific details)
since we can synthesize the initial partitions required with extra
operations, although matching those and transforming to appropriate AArch64 SVE
instructions is fragile given other optimization passes. This instruction is
more complex, but would allow us to explicitly specify the exact partition
desired.
## *partition*
### Syntax:
`<result> = partition first <part_on> [inclusive] <ty> <p> [, propagating <pprop>]`
`<result> = partition next <part_on> [inclusive] <ty> <p> from <pcont>`
### Overview:
This instruction creates a partitioned boolean vector based on an input vector
`p`. There are two main modes to consider. The *first* mode is an extended
version of [*propff*](#propff) where `part_on` is a boolean which determines
whether the partition is made based on the first false value or the first true
value. Propagating from a previous vector is optional. The `inclusive` option
produces a partition which includes the first true or false value; the default
is an exclusive partition which only covers the elements before the first true
or false value.
In cases where there are multiple subsets of data in a vector which must be
processed independently, we need a way to take the original boolean vector and
an existing partition to create the next partition to work on. This is provided
by the *next* form of *partition*. The `pcont` argument is the existing
partitioned vector, generated either by a *partition first* or another
*partition next*. If there are no more partitions after the current one, a
boolean vector of all false will be returned.
### Fixed-Width Behaviour:
Same as scalable.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ScalableIR.pdf
Type: application/pdf
Size: 385844 bytes
Desc: ScalableIR.pdf
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161104/6e48b071/attachment-0001.pdf>
More information about the llvm-dev
mailing list