Stream: git-wasmtime

Topic: wasmtime / PR #12585 Cranelift/Wasmtime/Pulley/Debugging:...

Wasmtime GitHub notifications bot (Feb 12 2026 at 21:47):

cfallin opened PR #12585 from cfallin:pulley-little-endian-vecs-on-big-endian-when-debugging to bytecodealliance:main:

When running Pulley on an s390x (or other big-endian) host, and enabling guest-debugging instrumentationa very strange confluence of events occurs:

• Pulley uses "native endian" of the host by default for loads and stores.
• Patchable calls to debug hooks use the preserve_all ABI, which spills all registers in the trampoline adapter (callee in this ABI), including vector registers.
• Saving vector-typed locals/operand stack values to the debugger state slot also uses vector stores.
• All of these stores were thus big-endian on big-endian hosts.
• Pulley's bytecode only supports little-endian vector loads/stores.

We were thus hitting an assert in Pulley codegen (the Cranelift backend) when encountering a VStore VCode instruction with a big-endian mode.

This PR makes two changes that avoid this issue:

• The ABI code for Pulley is careful to specify little-endian mode explicitly for any vector load/store.
• The debug instrumentation code is refactored to use little-endian explicitly for vector types only.
  • (Why not for all types? Because we GC-root GC ref values, and these need to be provided to the collector as mutable storage cells, so need to be in native endianness.)

Test will come as part of #12575 incorporating a
Pulley-with-guest-debugging test and running on s390x amongst our platforms.

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Wasmtime GitHub notifications bot (Feb 12 2026 at 21:47):

cfallin requested fitzgen for a review on PR #12585.

Wasmtime GitHub notifications bot (Feb 12 2026 at 21:47):

cfallin requested wasmtime-compiler-reviewers for a review on PR #12585.

Wasmtime GitHub notifications bot (Feb 12 2026 at 21:47):

cfallin requested wasmtime-core-reviewers for a review on PR #12585.

Wasmtime GitHub notifications bot (Feb 12 2026 at 21:48):

cfallin edited PR #12585:

When running Pulley on an s390x (or other big-endian) host, and enabling guest-debugging instrumentation, a very strange confluence of events occurs:

• Pulley uses "native endian" of the host by default for loads and stores.
• Patchable calls to debug hooks use the preserve_all ABI, which spills all registers in the trampoline adapter (callee in this ABI), including vector registers.
• Saving vector-typed locals/operand stack values to the debugger state slot also uses vector stores.
• All of these stores were thus big-endian on big-endian hosts.
• Pulley's bytecode only supports little-endian vector loads/stores.

We were thus hitting an assert in Pulley codegen (the Cranelift backend) when encountering a VStore VCode instruction with a big-endian mode.

This PR makes two changes that avoid this issue:

• The ABI code for Pulley is careful to specify little-endian mode explicitly for any vector load/store.
• The debug instrumentation code is refactored to use little-endian explicitly for vector types only.
  • (Why not for all types? Because we GC-root GC ref values, and these need to be provided to the collector as mutable storage cells, so need to be in native endianness.)

Test will come as part of #12575 incorporating a
Pulley-with-guest-debugging test and running on s390x amongst our platforms.

<!--
Please make sure you include the following information:

• If this work has been discussed elsewhere, please include a link to that
  conversation. If it was discussed in an issue, just mention "issue #...".

• Explain why this change is needed. If the details are in an issue already,
  this can be brief.

Our development process is documented in the Wasmtime book:
https://docs.wasmtime.dev/contributing-development-process.html

Please ensure all communication follows the code of conduct:
https://github.com/bytecodealliance/wasmtime/blob/main/CODE_OF_CONDUCT.md
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Wasmtime GitHub notifications bot (Feb 12 2026 at 21:48):

cfallin updated PR #12585.

Wasmtime GitHub notifications bot (Feb 12 2026 at 22:01):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Feb 12 2026 at 22:01):

fitzgen created PR review comment:

    }

    fn memflags_for_debug_slot_value_clif_ty(&self, ty: ir::Type) -> MemFlags {

Wasmtime GitHub notifications bot (Feb 12 2026 at 22:27):

cfallin updated PR #12585.

Wasmtime GitHub notifications bot (Feb 12 2026 at 22:27):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Feb 12 2026 at 22:27):

cfallin created PR review comment:

Fixed, thanks!

Wasmtime GitHub notifications bot (Feb 12 2026 at 22:27):

cfallin has enabled auto merge for PR #12585.

Wasmtime GitHub notifications bot (Feb 12 2026 at 22:35):

cfallin updated PR #12585.

Wasmtime GitHub notifications bot (Feb 12 2026 at 23:13):

cfallin added PR #12585 Cranelift/Wasmtime/Pulley/Debugging: use little-endian mode to spill/reload vectors in guest-debugging slot and ABI clobbers. to the merge queue

Wasmtime GitHub notifications bot (Feb 12 2026 at 23:35):

cfallin merged PR #12585.

Wasmtime GitHub notifications bot (Feb 12 2026 at 23:35):

cfallin removed PR #12585 Cranelift/Wasmtime/Pulley/Debugging: use little-endian mode to spill/reload vectors in guest-debugging slot and ABI clobbers. from the merge queue

Last updated: Feb 24 2026 at 04:36 UTC