Stream: git-wasmtime

Topic: wasmtime / PR #4068 x64 backend: fix a load-op merging bu...

Wasmtime GitHub notifications bot (Apr 25 2022 at 16:46):

cfallin opened PR #4068 from fix-cmp-and-choose-load-op-merging to main:

The recent work in #4061 introduced a notion of "unique uses" for CLIF
values that both simplified the load-op merging rules and allowed
loads to merge in some more places.

Unfortunately there's one factor that PR didn't account for: a unique
use at the CLIF level could become a multiple-use at the VCode level,
when a lowering uses a value multiple times!

Making this less error-prone in general is hard, because we don't know
the lowering in VCode until it's emitted, so we can't ahead-of-time
know that a value will be used multiple times and prevent its
merging. But we can know in the lowerings themselves when we're
doing this. At least we get a panic from regalloc when we get this
wrong; no bad code (uninitialized register being read) should ever
come from a backend bug like this.

This is still a bit less than ideal, but for now the fix is: in
cmp_and_choose in the x64 backend (which compares values, then
picks one or the other with a cmove), explicitly put values in
registers.

Fixes #4067 (thanks @Mrmaxmeier for the report!).

<!--

Please ensure that the following steps are all taken care of before submitting
the PR.

• [ ] This has been discussed in issue #..., or if not, please tell us why
  here.

• [ ] A short description of what this does, why it is needed; if the
  description becomes long, the matter should probably be discussed in an issue
  first.

• [ ] This PR contains test cases, if meaningful.

• [ ] A reviewer from the core maintainer team has been assigned for this PR.
  If you don't know who could review this, please indicate so. The list of
  suggested reviewers on the right can help you.

Please ensure all communication adheres to the code of conduct.
-->

Wasmtime GitHub notifications bot (Apr 25 2022 at 16:46):

cfallin requested fitzgen for a review on PR #4068.

Wasmtime GitHub notifications bot (Apr 25 2022 at 16:46):

cfallin requested abrown for a review on PR #4068.

Wasmtime GitHub notifications bot (Apr 25 2022 at 17:22):

abrown submitted PR review.

Wasmtime GitHub notifications bot (Apr 25 2022 at 17:22):

abrown submitted PR review.

Wasmtime GitHub notifications bot (Apr 25 2022 at 17:22):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 25 2022 at 17:32):

cfallin merged PR #4068.

Last updated: Nov 22 2024 at 16:03 UTC