This was added as an incremental step to improve AArch64 code quality in
PR #2278. At the time, we did not have a way to pattern-match the load +
splat opcode sequence that the relevant Wasm opcodes lowered to.
However, now with PR #2366, we can merge effectful instructions such as
loads into other ops, and so we can do this pattern matching directly.
The pattern-matching update will come in a subsequent commit.
This patch implements, for aarch64, the following wasm SIMD extensions.
v128.load32_zero and v128.load64_zero instructions
https://github.com/WebAssembly/simd/pull/237
The changes are straightforward:
* no new CLIF instructions. They are translated into an existing CLIF scalar
load followed by a CLIF `scalar_to_vector`.
* the comment/specification for CLIF `scalar_to_vector` has been changed to
match the actual intended semantics, per consulation with Andrew Brown.
* translation from `scalar_to_vector` to aarch64 `fmov` instruction. This
has been generalised slightly so as to allow both 32- and 64-bit transfers.
* special-case zero in `lower_constant_f128` in order to avoid a
potentially slow call to `Inst::load_fp_constant128`.
* Once "Allow loads to merge into other operations during instruction
selection in MachInst backends"
(https://github.com/bytecodealliance/wasmtime/issues/2340) lands,
we can use that functionality to pattern match the two-CLIF pair and
emit a single AArch64 instruction.
* A simple filetest has been added.
There is no comprehensive testcase in this commit, because that is a separate
repo. The implementation has been tested, nevertheless.
This patch implements, for aarch64, the following wasm SIMD extensions
i32x4.dot_i16x8_s instruction
https://github.com/WebAssembly/simd/pull/127
It also updates dependencies as follows, in order that the new instruction can
be parsed, decoded, etc:
wat to 1.0.27
wast to 26.0.1
wasmparser to 0.65.0
wasmprinter to 0.2.12
The changes are straightforward:
* new CLIF instruction `widening_pairwise_dot_product_s`
* translation from wasm into `widening_pairwise_dot_product_s`
* new AArch64 instructions `smull`, `smull2` (part of the `VecRRR` group)
* translation from `widening_pairwise_dot_product_s` to `smull ; smull2 ; addv`
There is no testcase in this commit, because that is a separate repo. The
implementation has been tested, nevertheless.
This patch implements, for aarch64, the following wasm SIMD extensions
Floating-point rounding instructions
https://github.com/WebAssembly/simd/pull/232
Pseudo-Minimum and Pseudo-Maximum instructions
https://github.com/WebAssembly/simd/pull/122
The changes are straightforward:
* `build.rs`: the relevant tests have been enabled
* `cranelift/codegen/meta/src/shared/instructions.rs`: new CLIF instructions
`fmin_pseudo` and `fmax_pseudo`. The wasm rounding instructions do not need
any new CLIF instructions.
* `cranelift/wasm/src/code_translator.rs`: translation into CLIF; this is
pretty much the same as any other unary or binary vector instruction (for
the rounding and the pmin/max respectively)
* `cranelift/codegen/src/isa/aarch64/lower_inst.rs`:
- `fmin_pseudo` and `fmax_pseudo` are converted into a two instruction
sequence, `fcmpgt` followed by `bsl`
- the CLIF rounding instructions are converted to a suitable vector
`frint{n,z,p,m}` instruction.
* `cranelift/codegen/src/isa/aarch64/inst/mod.rs`: minor extension of `pub
enum VecMisc2` to handle the rounding operations. And corresponding `emit`
cases.
The `bitmask.{8x16,16x8,32x4}` instructions do not map neatly to any single
AArch64 SIMD instruction, and instead need a sequence of around ten
instructions. Because of this, this patch is somewhat longer and more complex
than it would be for (eg) x64.
Main changes are:
* the relevant testsuite test (`simd_boolean.wast`) has been enabled on aarch64.
* at the CLIF level, add a new instruction `vhigh_bits`, into which these wasm
instructions are to be translated.
* in the wasm->CLIF translation (code_translator.rs), translate into
`vhigh_bits`. This is straightforward.
* in the CLIF->AArch64 translation (lower_inst.rs), translate `vhigh_bits`
into equivalent sequences of AArch64 instructions. There is a different
sequence for each of the `{8x16, 16x8, 32x4}` variants.
All other changes are AArch64-specific, and add instruction definitions needed
by the previous step:
* Add two new families of AArch64 instructions: `VecShiftImm` (vector shift by
immediate) and `VecExtract` (effectively a double-length vector shift)
* To the existing AArch64 family `VecRRR`, add a `zip1` variant. To the
`VecLanesOp` family add an `addv` variant.
* Add supporting code for the above changes to AArch64 instructions:
- getting the register uses (`aarch64_get_regs`)
- mapping the registers (`aarch64_map_regs`)
- printing instructions
- emitting instructions (`impl MachInstEmit for Inst`). The handling of
`VecShiftImm` is a bit complex.
- emission tests for new instructions and variants.
It corresponds to WebAssembly's `load*_splat` operations, which
were previously represented as a combination of `Load` and `Splat`
instructions. However, there are architectures such as Armv8-A
that have a single machine instruction equivalent to the Wasm
operations. In order to generate it, it is necessary to merge the
`Load` and the `Splat` in the backend, which is not possible
because the load may have side effects. The new IR instruction
works around this limitation.
The AArch64 backend leverages the new instruction to improve code
generation.
Copyright (c) 2020, Arm Limited.
The implementation is pretty straightforward. Wasm atomic instructions fall
into 5 groups
* atomic read-modify-write
* atomic compare-and-swap
* atomic loads
* atomic stores
* fences
and the implementation mirrors that structure, at both the CLIF and AArch64
levels.
At the CLIF level, there are five new instructions, one for each group. Some
comments about these:
* for those that take addresses (all except fences), the address is contained
entirely in a single `Value`; there is no offset field as there is with
normal loads and stores. Wasm atomics require alignment checks, and
removing the offset makes implementation of those checks a bit simpler.
* atomic loads and stores get their own instructions, rather than reusing the
existing load and store instructions, for two reasons:
- per above comment, makes alignment checking simpler
- reuse of existing loads and stores would require extension of `MemFlags`
to indicate atomicity, which sounds semantically unclean. For example,
then *any* instruction carrying `MemFlags` could be marked as atomic, even
in cases where it is meaningless or ambiguous.
* I tried to specify, in comments, the behaviour of these instructions as
tightly as I could. Unfortunately there is no way (per my limited CLIF
knowledge) to enforce the constraint that they may only be used on I8, I16,
I32 and I64 types, and in particular not on floating point or vector types.
The translation from Wasm to CLIF, in `code_translator.rs` is unremarkable.
At the AArch64 level, there are also five new instructions, one for each
group. All of them except `::Fence` contain multiple real machine
instructions. Atomic r-m-w and atomic c-a-s are emitted as the usual
load-linked store-conditional loops, guarded at both ends by memory fences.
Atomic loads and stores are emitted as a load preceded by a fence, and a store
followed by a fence, respectively. The amount of fencing may be overkill, but
it reflects exactly what the SM Wasm baseline compiler for AArch64 does.
One reason to implement r-m-w and c-a-s as a single insn which is expanded
only at emission time is that we must be very careful what instructions we
allow in between the load-linked and store-conditional. In particular, we
cannot allow *any* extra memory transactions in there, since -- particularly
on low-end hardware -- that might cause the transaction to fail, hence
deadlocking the generated code. That implies that we can't present the LL/SC
loop to the register allocator as its constituent instructions, since it might
insert spills anywhere. Hence we must present it as a single indivisible
unit, as we do here. It also has the benefit of reducing the total amount of
work the RA has to do.
The only other notable feature of the r-m-w and c-a-s translations into
AArch64 code, is that they both need a scratch register internally. Rather
than faking one up by claiming, in `get_regs` that it modifies an extra
scratch register, and having to have a dummy initialisation of it, these new
instructions (`::LLSC` and `::CAS`) simply use fixed registers in the range
x24-x28. We rely on the RA's ability to coalesce V<-->R copies to make the
cost of the resulting extra copies zero or almost zero. x24-x28 are chosen so
as to be call-clobbered, hence their use is less likely to interfere with long
live ranges that span calls.
One subtlety regarding the use of completely fixed input and output registers
is that we must be careful how the surrounding copy from/to of the arg/result
registers is done. In particular, it is not safe to simply emit copies in
some arbitrary order if one of the arg registers is a real reg. For that
reason, the arguments are first moved into virtual regs if they are not
already there, using a new method `<LowerCtx for Lower>::ensure_in_vreg`.
Again, we rely on coalescing to turn them into no-ops in the common case.
There is also a ridealong fix for the AArch64 lowering case for
`Opcode::Trapif | Opcode::Trapff`, which removes a bug in which two trap insns
in a row were generated.
In the patch as submitted there are 6 "FIXME JRS" comments, which mark things
which I believe to be correct, but for which I would appreciate a second
opinion. Unless otherwise directed, I will remove them for the final commit
but leave the associated code/comments unchanged.
This PR adds a conditional move following a heap bounds check through
which the address to be accessed flows. This conditional move ensures
that even if the branch is mispredicted (access is actually out of
bounds, but speculation goes down in-bounds path), the acually accessed
address is zero (a NULL pointer) rather than the out-of-bounds address.
The mitigation is controlled by a flag that is off by default, but can
be set by the embedding. Note that in order to turn it on by default,
we would need to add conditional-move support to the current x86
backend; this does not appear to be present. Once the deprecated
backend is removed in favor of the new backend, IMHO we should turn
this flag on by default.
Note that the mitigation is unneccessary when we use the "huge heap"
technique on 64-bit systems, in which we allocate a range of virtual
address space such that no 32-bit offset can reach other data. Hence,
this only affects small-heap configurations.
These instructions have fast, inline JIT paths for the common cases, and only
call out to host VM functions for the slow paths. This required some changes to
`cranelift-wasm`'s `FuncEnvironment`: instead of taking a `FuncCursor` to insert
an instruction sequence within the current basic block,
`FuncEnvironment::translate_table_{get,set}` now take a `&mut FunctionBuilder`
so that they can create whole new basic blocks. This is necessary for
implementing GC read/write barriers that involve branching (e.g. checking for
null, or whether a store buffer is at capacity).
Furthermore, it required that the `load`, `load_complex`, and `store`
instructions handle loading and storing through an `r{32,64}` rather than just
`i{32,64}` addresses. This involved making `r{32,64}` types acceptable
instantiations of the `iAddr` type variable, plus a few new instruction
encodings.
Part of #929
This is useful to have to allow resumable_trap to happen in loop
headers, for instance. This is the correct way to implement interrupt
checks in Spidermonkey, which are effectively resumable traps. Previous
implementation was using traps, which is wrong, since traps semantically
can't be resumed after.
The InsertLane format has an ordering (`value().imm().value()`) and immediate name (`"lane"`) that make it awkward to use for other instructions. This changes the ordering (`value().value().imm()`) and uses the default name (`"imm"`) throughout the codebase.
* Manually rename BasicBlock to BlockPredecessor
BasicBlock is a pair of (Ebb, Inst) that is used to represent the
basic block subcomponent of an Ebb that is a predecessor to an Ebb.
Eventually we will be able to remove this struct, but for now it
makes sense to give it a non-conflicting name so that we can start
to transition Ebb to represent a basic block.
I have not updated any comments that refer to BasicBlock, as
eventually we will remove BlockPredecessor and replace with Block,
which is a basic block, so the comments will become correct.
* Manually rename SSABuilder block types to avoid conflict
SSABuilder has its own Block and BlockData types. These along with
associated identifier will cause conflicts in a later commit, so
they are renamed to be more verbose here.
* Automatically rename 'Ebb' to 'Block' in *.rs
* Automatically rename 'EBB' to 'block' in *.rs
* Automatically rename 'ebb' to 'block' in *.rs
* Automatically rename 'extended basic block' to 'basic block' in *.rs
* Automatically rename 'an basic block' to 'a basic block' in *.rs
* Manually update comment for `Block`
`Block`'s wikipedia article required an update.
* Automatically rename 'an `Block`' to 'a `Block`' in *.rs
* Automatically rename 'extended_basic_block' to 'basic_block' in *.rs
* Automatically rename 'ebb' to 'block' in *.clif
* Manually rename clif constant that contains 'ebb' as substring to avoid conflict
* Automatically rename filecheck uses of 'EBB' to 'BB'
'regex: EBB' -> 'regex: BB'
'$EBB' -> '$BB'
* Automatically rename 'EBB' 'Ebb' to 'block' in *.clif
* Automatically rename 'an block' to 'a block' in *.clif
* Fix broken testcase when function name length increases
Test function names are limited to 16 characters. This causes
the new longer name to be truncated and fail a filecheck test. An
outdated comment was also fixed.
Accessing Wasm reference globals that are reference types will
want to use the plain load/store instructions. This commit adds
encodings for these instructions to match loading a i32/i64.
Producers of IR are required to insert the appropriate barriers
around the loads/stores.
Spidermonkey returns a sentinel ref value of '-1' from some VM functions
to indicate failure. This commit adds an instruction analagous to ref.is_null
that checks for this value.
In order to implement SIMD's all_true (https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#all-lanes-true), we must legalize some instruction (I chose `vall_true`) to a comparison against 0 and a similar reduction as vany_true using `PTEST` and `SETNZ`. Since `icmp` only allows integers but `vall_true` could allow more vector types, `raw_bitcast` is used to convert the lane types into integers, e.g. b32x4 to i32x4. To do so without runtime type-checking, the `raw_bitcast` instruction (which emits no instruction) can now bitcast from any vector type to the same type, e.g. i32x4 to i32x4.
This does a lot at once, since there was no clear way to split the three
commits:
- Instruction need to be passed an explicit InstructionFormat,
- InstructionFormat deduplication is checked once all entities have been
defined;
Only i16x8 and i32x4 are encoded in this commit mainly because i8x16 and i64x2 do not have simple encodings in x86. i64x2 is not required by the SIMD spec and there is discussion (https://github.com/WebAssembly/simd/pull/98#issuecomment-530092217) about removing i8x16.
