* cranelift-wasm: translate Wasm loads into lower-level CLIF operations
Rather than using `heap_{load,store,addr}`.
* cranelift: Remove the `heap_{addr,load,store}` instructions
These are now legalized in the `cranelift-wasm` frontend.
* cranelift: Remove the `ir::Heap` entity from CLIF
* Port basic memory operation tests to .wat filetests
* Remove test for verifying CLIF heaps
* Remove `heap_addr` from replace_branching_instructions_and_cfg_predecessors.clif test
* Remove `heap_addr` from readonly.clif test
* Remove `heap_addr` from `table_addr.clif` test
* Remove `heap_addr` from the simd-fvpromote_low.clif test
* Remove `heap_addr` from simd-fvdemote.clif test
* Remove `heap_addr` from the load-op-store.clif test
* Remove the CLIF heap runtest
* Remove `heap_addr` from the global_value.clif test
* Remove `heap_addr` from fpromote.clif runtests
* Remove `heap_addr` from fdemote.clif runtests
* Remove `heap_addr` from memory.clif parser test
* Remove `heap_addr` from reject_load_readonly.clif test
* Remove `heap_addr` from reject_load_notrap.clif test
* Remove `heap_addr` from load_readonly_notrap.clif test
* Remove `static-heap-without-guard-pages.clif` test
Will be subsumed when we port `make-heap-load-store-tests.sh` to generating
`.wat` tests.
* Remove `static-heap-with-guard-pages.clif` test
Will be subsumed when we port `make-heap-load-store-tests.sh` over to `.wat`
tests.
* Remove more heap tests
These will be subsumed by porting `make-heap-load-store-tests.sh` over to `.wat`
tests.
* Remove `heap_addr` from `simple-alias.clif` test
* Remove `heap_addr` from partial-redundancy.clif test
* Remove `heap_addr` from multiple-blocks.clif test
* Remove `heap_addr` from fence.clif test
* Remove `heap_addr` from extends.clif test
* Remove runtests that rely on heaps
Heaps are not a thing in CLIF or the interpreter anymore
* Add generated load/store `.wat` tests
* Enable memory-related wasm features in `.wat` tests
* Remove CLIF heap from fcmp-mem-bug.clif test
* Add a mode for compiling `.wat` all the way to assembly in filetests
* Also generate WAT to assembly tests in `make-load-store-tests.sh`
* cargo fmt
* Reinstate `f{de,pro}mote.clif` tests without the heap bits
* Remove undefined doc link
* Remove outdated SVG and dot file from docs
* Add docs about `None` returns for base address computation helpers
* Factor out `env.heap_access_spectre_mitigation()` to a local
* Expand docs for `FuncEnvironment::heaps` trait method
* Restore f{de,pro}mote+load clif runtests with stack memory
* Cranelift: Make `heap_addr` return calculated `base + index + offset`
Rather than return just the `base + index`.
(Note: I've chosen to use the nomenclature "index" for the dynamic operand and
"offset" for the static immediate.)
This move the addition of the `offset` into `heap_addr`, instead of leaving it
for the subsequent memory operation, so that we can Spectre-guard the full
address, and not allow speculative execution to read the first 4GiB of memory.
Before this commit, we were effectively doing
load(spectre_guard(base + index) + offset)
Now we are effectively doing
load(spectre_guard(base + index + offset))
Finally, this also corrects `heap_addr`'s documented semantics to say that it
returns an address that will trap on access if `index + offset + access_size` is
out of bounds for the given heap, rather than saying that the `heap_addr` itself
will trap. This matches the implemented behavior for static memories, and after
https://github.com/bytecodealliance/wasmtime/pull/5190 lands (which is blocked
on this commit) will also match the implemented behavior for dynamic memories.
* Update heap_addr docs
* Factor out `offset + size` to a helper
This PR adds a basic *alias analysis*, and optimizations that use it.
This is a "mid-end optimization": it operates on CLIF, the
machine-independent IR, before lowering occurs.
The alias analysis (or maybe more properly, a sort of memory-value
analysis) determines when it can prove a particular memory
location is equal to a given SSA value, and when it can, it replaces any
loads of that location.
This subsumes two common optimizations:
* Redundant load elimination: when the same memory address is loaded two
times, and it can be proven that no intervening operations will write
to that memory, then the second load is *redundant* and its result
must be the same as the first. We can use the first load's result and
remove the second load.
* Store-to-load forwarding: when a load can be proven to access exactly
the memory written by a preceding store, we can replace the load's
result with the store's data operand, and remove the load.
Both of these optimizations rely on a "last store" analysis that is a
sort of coloring mechanism, split across disjoint categories of abstract
state. The basic idea is that every memory-accessing operation is put
into one of N disjoint categories; it is disallowed for memory to ever
be accessed by an op in one category and later accessed by an op in
another category. (The frontend must ensure this.)
Then, given this, we scan the code and determine, for each
memory-accessing op, when a single prior instruction is a store to the
same category. This "colors" the instruction: it is, in a sense, a
static name for that version of memory.
This analysis provides an important invariant: if two operations access
memory with the same last-store, then *no other store can alias* in the
time between that last store and these operations. This must-not-alias
property, together with a check that the accessed address is *exactly
the same* (same SSA value and offset), and other attributes of the
access (type, extension mode) are the same, let us prove that the
results are the same.
Given last-store info, we scan the instructions and build a table from
"memory location" key (last store, address, offset, type, extension) to
known SSA value stored in that location. A store inserts a new mapping.
A load may also insert a new mapping, if we didn't already have one.
Then when a load occurs and an entry already exists for its "location",
we can reuse the value. This will be either RLE or St-to-Ld depending on
where the value came from.
Note that this *does* work across basic blocks: the last-store analysis
is a full iterative dataflow pass, and we are careful to check dominance
of a previously-defined value before aliasing to it at a potentially
redundant load. So we will do the right thing if we only have a
"partially redundant" load (loaded already but only in one predecessor
block), but we will also correctly reuse a value if there is a store or
load above a loop and a redundant load of that value within the loop, as
long as no potentially-aliasing stores happen within the loop.
