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PCC: draw the rest of the owl: fully-working PCC on hello-world Wasm on aarch64 #7281

Merged
merged 2 commits into from
Oct 19, 2023
Merged

PCC: draw the rest of the owl: fully-working PCC on hello-world Wasm on aarch64 #7281

merged 2 commits into from
Oct 19, 2023

Commits on Oct 19, 2023

  1. PCC: draw the rest of the owl: fully-working PCC on hello-world Wasm … 

    …on aarch64

This needed a bit more inference / magic than I was hoping for at first,
specifically around constants and adds. Some instructions can now
generate facts on their output registers, even if not stated. This
breaks away from the "breadcrumbs" idea, but seems to be the most
practical solution to a general problem that we have mini-lowering steps
in various places without careful preservation of PCC facts. Two
particular aspects:

- Constants: amodes on aarch64 can decompose into new
  constant-generation instructions, and we need precise ranges on those
  to properly check them. To avoid making the ISLE rules nightmarish,
  it's best to reuse the existing semantics definitions of the Add* ALU
  insts, and add a few rules for MovK/MovZ/MovN.

- Adds: similarly, amodes decompose into de-novo add instructions with
  no facts. To handle this, there's now a notion of "propagating" facts
  that cause an instruction with a propagating fact on the input to
  generate a fact on the output.

Together, these heuristics mean that we'll eagerly generate a fact for
`mem(mt0, 0, 0) + 8 -> mem(mt0, 8, 8)`, but we won't, for example,
generate ranges on every single integer operation.

With these changes and a few other misc fixes, this PR can now check a
nontrivial "hello world" Wasm on aarch64 down to the machine-code level:

```
$ target/release/wasmtime compile -C enable-pcc=y ../wasm-tests/helloworld-rs.wasm
```
    @cfallin
    cfallin committed Oct 19, 2023
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    @cfallin
    cfallin committed Oct 19, 2023
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