1914005 - Optimize call_indirect for table64

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, enhancement, P3)

Description

[Ryan Hunt [:rhunt]]

All table64 operations in JIT code put index parameters through a clamping operation to turn them UINT32_MAX if they are outside the 32-bit range. This let us keep our bounds checks the same. It works because our maximum table length at runtime is much less than UINT32_MAX. Most table operations are also not that performance sensitive at this time.

However, this extra overhead could be significant for call_indirect. We should measure and possibly optimize.

Comment 1

[Ryan Hunt [:rhunt]]

We should try to do this sometime soon, now that we're shipping memory64.

Comment 2

[Ben Visness [:bvisness]]

Attachment: Bug 1914005: Move bounds checks out of call_indirect. r=rhunt

We currently do bounds checks for call_indirect at the masm level, and
always as 32-bit. This is not ideal now that we have table64, since it
forces us to clamp our address and length values down to 32 bits.

This patch removes the bounds check from masm and the corresponding
optimization logic from lowering. Instead, it moves the optimization
logic into MIR and LIR. Specifically:

• If the index is constant and less than the table's min size, the
  bounds check is eliminated. (To be done in follow-up patch.)
• If the table has a fixed length, a constant is emitted instead of a
  load, and we will compare against an immediate instead of a register
  or memory operand.
• If the bounds check limit is an MWasmLoadInstance (as it is for
  tables), then the load will be emitted at use as a memory operand to
  the bounds check instead of a load to a register.

Comment 3

[Ben Visness [:bvisness]]

Attachment: Bug 1914005: Implement WasmBCE for tables. r=rhunt

This extends our existing WasmBCE pass to work for tables, and also
multiple memories.

Comment 4

[Ben Visness [:bvisness]]

Attachment: WIP: Bug 1914005: Remove phi path from WasmBCE

It seems like the phi path is actually not used. We can remove the code
to avoid future maintenance and bugs.

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/0ec3489ee1b4
https://hg.mozilla.org/integration/autoland/rev/579959211efb
Move bounds checks out of call_indirect. r=rhunt
https://github.com/mozilla-firefox/firefox/commit/173cb54cdeeb
https://hg.mozilla.org/integration/autoland/rev/5c9abbdacf68
Implement WasmBCE for tables. r=rhunt

Comment 6

[pstanciu]

https://hg.mozilla.org/mozilla-central/rev/579959211efb
https://hg.mozilla.org/mozilla-central/rev/5c9abbdacf68