1. Restricts max nop size to 15 instead of 16.
2. Fixes an edge case where gen_nop() would return a zero sized intruction on multiples of 16.
3. Clarifies the documentation of the gen_nop interface to state that returning zero is allowed when preferred_size is zero.
This PR propagates "value labels" all the way from CLIF to DWARF
metadata on the emitted machine code. The key idea is as follows:
- Translate value-label metadata on the input into "value_label"
pseudo-instructions when lowering into VCode. These
pseudo-instructions take a register as input, denote a value label,
and semantically are like a "move into value label" -- i.e., they
update the current value (as seen by debugging tools) of the given
local. These pseudo-instructions emit no machine code.
- Perform a dataflow analysis *at the machine-code level*, tracking
value-labels that propagate into registers and into [SP+constant]
stack storage. This is a forward dataflow fixpoint analysis where each
storage location can contain a *set* of value labels, and each value
label can reside in a *set* of storage locations. (Meet function is
pairwise intersection by storage location.)
This analysis traces value labels symbolically through loads and
stores and reg-to-reg moves, so it will naturally handle spills and
reloads without knowing anything special about them.
- When this analysis converges, we have, at each machine-code offset, a
mapping from value labels to some number of storage locations; for
each offset for each label, we choose the best location (prefer
registers). Note that we can choose any location, as the symbolic
dataflow analysis is sound and guarantees that the value at the
value_label instruction propagates to all of the named locations.
- Then we can convert this mapping into a format that the DWARF
generation code (wasmtime's debug crate) can use.
This PR also adds the new-backend variant to the gdb tests on CI.
This follows the implementation in the legacy x86 backend, including
hardcoded sequence that is compatible with what the linker expects. We
could potentially do better here, but it is likely not necessary.
Thanks to @bjorn3 for a bugfix to an earlier version of this.
This implements all of the ops on I128 that are implemented by the
legacy x86 backend, and includes all that are required by at least one
major use-case (cg_clif rustc backend).
The sequences are open-coded where necessary; for e.g. the bit
operations, this can be somewhat complex, but these sequences have been
tested carefully. This PR also includes a drive-by fix of clz/ctz for 8-
and 16-bit cases where they were incorrect previously.
Also includes ridealong fixes developed while bringing up cg_clif
support, because they are difficult to completely separate due to
other refactors that occurred in this PR:
- fix REX prefix logic for some 8-bit instructions.
When using an 8-bit register in 64-bit mode on x86-64, the REX prefix
semantics are somewhat subtle: without the REX prefix, register numbers
4--7 correspond to the second-to-lowest byte of the first four registers
(AH, CH, BH, DH), whereas with the REX prefix, these register numbers
correspond to the usual encoding (SPL, BPL, SIL, DIL). We could always
emit a REX byte for instructions with 8-bit cases (this is harmless even
if unneeded), but this would unnecessarily inflate code size; instead,
the usual approach is to emit it only for these registers.
This logic was present in some cases but missing for some other
instructions: divide, not, negate, shifts.
Fixes#2508.
- avoid unaligned SSE loads on some f64 ops.
The implementations of several FP ops, such as fabs/fneg, used SSE
instructions. This is not a problem per-se, except that load-op merging
did not take *alignment* into account. Specifically, if an op on an f64
loaded from memory happened to merge that load, and the instruction into
which it was merged was an SSE instruction, then the SSE instruction
imposes stricter (128-bit) alignment requirements than the load.f64 did.
This PR simply forces any instruction lowerings that could use SSE
instructions to implement non-SIMD operations to take inputs in
registers only, and avoid load-op merging.
Fixes#2507.
- two bugfixes exposed by cg_clif: urem/srem.i8, select.b1.
- urem/srem.i8: the 8-bit form of the DIV instruction on x86-64 places
the remainder in AH, not RDX, different from all the other width-forms
of this instruction.
- select.b1: we were not recognizing selects of boolean values as
integer-typed operations, so we were generating XMM moves instead (!).
This will allow for support for `I128` values everywhere, and `I64`
values on 32-bit targets (e.g., ARM32 and x86-32). It does not alter the
machine backends to build such support; it just adds the framework for
the MachInst backends to *reason* about a `Value` residing in more than
one register.
Previously, `select` and `brz`/`brnz` instructions, when given a `b1`
boolean argument, would test whether that boolean argument was nonzero,
rather than whether its LSB was nonzero. Since our invariant for mapping
CLIF state to machine state is that bits beyond the width of a value are
undefined, the proper lowering is to test only the LSB.
(aarch64 does not have the same issue because its `Extend` pseudoinst
already properly handles masking of b1 values when a zero-extend is
requested, as it is for select/brz/brnz.)
Found by Nathan Ringo on Zulip [1] (thanks!).
[1]
https://bytecodealliance.zulipchat.com/#narrow/stream/217117-cranelift/topic/bnot.20on.20b1s
In existing MachInst backends, many instructions -- any that can trap or
result in a relocation -- carry `SourceLoc` values in order to propagate
the location-in-original-source to use to describe resulting traps or
relocation errors.
This is quite tedious, and also error-prone: it is likely that the
necessary plumbing will be missed in some cases, and in any case, it's
unnecessarily verbose.
This PR factors out the `SourceLoc` handling so that it is tracked
during emission as part of the `EmitState`, and plumbed through
automatically by the machine-independent framework. Instruction emission
code that directly emits trap or relocation records can query the
current location as necessary. Then we only need to ensure that memory
references and trap instructions, at their (one) emission point rather
than their (many) lowering/generation points, are wired up correctly.
This does have the side-effect that some loads and stores that do not
correspond directly to user code's heap accesses will have unnecessary
but harmless trap metadata. For example, the load that fetches a code
offset from a jump table will have a 'heap out of bounds' trap record
attached to it; but because it is bounds-checked, and will never
actually trap if the lowering is correct, this should be harmless. The
simplicity improvement here seemed more worthwhile to me than plumbing
through a "corresponds to user-level load/store" bit, because the latter
is a bit complex when we allow for op merging.
Closes#2290: though it does not implement a full "metadata" scheme as
described in that issue, this seems simpler overall.
This approach suffers from memory-size bloat during compile time due to the desire to de-duplicate the constants emitted and reduce runtime memory-size. As a first step, though, this provides an end-to-end mechanism for constants to be emitted in the MachBuffer islands.
In order to register traps for `load_splat`, several instruction formats need knowledge of `SourceLoc`s; however, since the x64 backend does not correctly and completely register traps for `RegMem::Mem` variants I opened https://github.com/bytecodealliance/wasmtime/issues/2290 to discuss and resolve this issue. In the meantime, the current behavior (i.e. remaining largely unaware of `SourceLoc`s) is retained.
A new associated type Info is added to MachInstEmit, which is the
immutable counterpart to State. It can't easily be constructed from an
ABICallee, since it would require adding an associated type to the
latter, and making so leaks the associated type in a lot of places in
the code base and makes the code harder to read. Instead, the EmitInfo
state can simply be passed to the `Vcode::emit` function directly.
As found by @julian-seward1, movss/movsd aren't included in the
zero-latency move instructions section of the Intel optimization manual.
Use MOVAPS instead for those moves.
This PR updates the AArch64 ABI implementation so that it (i) properly
respects that v8-v15 inclusive have callee-save lower halves, and
caller-save upper halves, by conservatively approximating (to full
registers) in the appropriate directions when generating prologue
caller-saves and when informing the regalloc of clobbered regs across
callsites.
In order to prevent saving all of these vector registers in the prologue
of every non-leaf function due to the above approximation, this also
makes use of a new regalloc.rs feature to exclude call instructions'
writes from the clobber set returned by register allocation. This is
safe whenever the caller and callee have the same ABI (because anything
the callee could clobber, the caller is allowed to clobber as well
without saving it in the prologue).
Fixes#2254.
This approach is not the best but avoids an extra instruction; perhaps at some point, as mentioned in https://github.com/bytecodealliance/wasmtime/pull/2248, we will add the extra instruction or refactor things in such a way that this `Inst` variant is unnecessary.
In particular:
- try to optimize the integer emission into a 32-bit emission, when the
high bits are all zero, and stop relying on the caller of `imm_r` to
ensure this.
- rename `Inst::imm_r`/`Inst::Imm_R` to `Inst::imm`/`Inst::Imm`.
- generate a sign-extending mov 32-bit immediate to 64-bits, whenever
possible.
- fix a few places where the previous commit did introduce the
generation of zero-constants with xor, when calling `put_input_to_reg`,
thus clobbering the flags before they were read.
Previously, in #2128, we factored out a common "vanilla 64-bit ABI"
implementation from the AArch64 ABI code, with the idea that this should
be largely compatible with x64. This PR alters the new x64 backend to
make use of the shared infrastructure, removing the duplication that
existed previously. The generated code is nearly (not exactly) the same;
the only difference relates to how the clobber-save region is padded in
the prologue.
This also changes some register allocations in the aarch64 code because
call support in the shared ABI infra now passes a temp vreg in, rather
than requiring use of a fixed, non-allocable temp; tests have been
updated, and the runtime behavior is unchanged.
It does this by providing an implementation of the CLIF instructions `AtomicRmw`, `AtomicCas`,
`AtomicLoad`, `AtomicStore` and `Fence`.
The translation is straightforward. `AtomicCas` is translated into x64 `cmpxchg`, `AtomicLoad`
becomes a normal load because x64-TSO provides adequate sequencing, `AtomicStore` becomes a
normal store followed by `mfence`, and `Fence` becomes `mfence`. `AtomicRmw` is the only
complex case: it becomes a normal load, followed by a loop which computes an updated value,
tries to `cmpxchg` it back to memory, and repeats if necessary.
This is a minimum-effort initial implementation. `AtomicRmw` could be implemented more
efficiently using LOCK-prefixed integer read-modify-write instructions in the case where the old
value in memory is not required. Subsequent work could add that, if required.
The x64 emitter has been updated to emit the new instructions, obviously. The `LegacyPrefix`
mechanism has been revised to handle multiple prefix bytes, not just one, since it is now
sometimes necessary to emit both 0x66 (Operand Size Override) and F0 (Lock).
In the aarch64 implementation of atomics, there has been some minor renaming for the sake of
clarity, and for consistency with this x64 implementation.
This change primarily adds the ability to lower packed `[move|load|store]` instructions (the vector types were previously unimplemented), but with the addition of the utility `Inst::[move|load|store]` functions it became possible to remove duplicated code (e.g. `stack_load` and `stack_store`) and use these utility functions elsewhere (though not exhaustively).
