Stream: git-wasmtime

Topic: wasmtime / PR #6804 WIP: two-tier processing for MachBuff...

Wasmtime GitHub notifications bot (Aug 04 2023 at 16:51):

cfallin opened PR #6804 from cfallin:quadratic-islands to bytecodealliance:main:

Work-in-progress solution to #6798

Wasmtime GitHub notifications bot (Aug 04 2023 at 17:08):

cfallin updated PR #6804.

Wasmtime GitHub notifications bot (Aug 04 2023 at 22:44):

cfallin updated PR #6804.

Wasmtime GitHub notifications bot (Aug 04 2023 at 22:54):

cfallin edited PR #6804:

Currently, MachBuffer is used to resolve forward and backward references to labels (basic-block entry points and other branch targets) within function bodies as well as calls to functions within a whole module. It implements a single-pass algorithm that tracks "pending fixups" -- references to some other place -- and fills in the offsets when known, also handling upgrading ranges by emitting veneers on architectures that need that behavior.

In #6798 it was reported that on aarch64, module emission is slow for a certain large module. This module ends up forcing many islands of veneers but has branches whose ranges cross those islands and need not be extended with a veneer yet. This case exposes quadratic behavior: the island-emission logic puts the fixup back in a list and replaces the fixup list with that leftover-fixup list after processing all fixups.

This PR instead does the following:

• When an island is emitted, all unresolved forward references get a veneer, even if their current label kind would still be in range. This prevents fixups from hanging around arbitrarily long when they have longer range kinds (e.g., aarch64's Branch26) while lots of islands for shorter-range references (e.g. Branch19) are emitted.
• When a label fixup reaches its "longest-range" kind (PCRel32 for branches, for example), we put it in a separate list that we don't process again until the very last "last-chance island" after all emission.

These two rules together will bound the amount of processing to O(|fixups| * |label kinds|) rather than O(|fixups|^2).

The first new rule theoretically causes more veneers to be emitted, but in practice is not too likely to make a difference, I think. Unfortunately my aarch64 laptop is unavailable at the moment; if someone on that platform could benchmark the impact that would be quite appreciated!

Fixes #6798.

Last updated: Dec 23 2024 at 13:07 UTC