Stream: git-wasmtime

Topic: wasmtime / PR #8133 riscv64: Ensure that we use the same ...

Wasmtime GitHub notifications bot (Mar 14 2024 at 16:56):

afonso360 opened PR #8133 from afonso360:riscv-fix-bitselect-cmp to bytecodealliance:main:

:wave: Hey

We have a special lowering that allows us to fuse a bitselect with a comparison instruction. This saves us a few instructions due to the mismatch that exists between native RISC-V masks and WASM masks.

Native RISC-V masks have a single bit per lane, whereas WASM masks have all bits in a lane set to 1.

The lowering for bitselect+bitcast+{i,f}cmp avoids the need to generate the WASM mask, by directly using the comparison mask with vmerge.

The bug that this fixes was that when we introduce a bitcast in the middle, the comparison and the merge may have different types with different lanes. And if that happens the vmerge will only look at the first n bits of the mask. n being the number of lanes currently configured.

This commit ensures that they are always equal by using the same type for both vmerge and the comparison instruction.

I also manually checked all other uses of gen_{f,i}cmp_mask and they are all using the same type in the subsequent instructions.

With this fix we no longer really care about the type of the bitselect as long as it has the same bitlength as the type of {i,f}cmp, which I think is enforced by the verifier. (i.e. We would have the same bug if bitselect.i8x16+icmp.i8x8 was legal.)

Thanks for reporting this @candymate!

Wasmtime GitHub notifications bot (Mar 14 2024 at 16:56):

afonso360 requested elliottt for a review on PR #8133.

Wasmtime GitHub notifications bot (Mar 14 2024 at 16:56):

afonso360 requested wasmtime-compiler-reviewers for a review on PR #8133.

Wasmtime GitHub notifications bot (Mar 14 2024 at 17:00):

afonso360 edited PR #8133:

:wave: Hey

We have a special lowering that allows us to fuse a bitselect with a comparison instruction. This saves us a few instructions due to the mismatch that exists between native RISC-V masks and WASM masks.

Native RISC-V masks have a single bit per lane, whereas WASM masks have all bits in a lane set to 1.

The lowering for bitselect+bitcast+{i,f}cmp avoids the need to generate the WASM mask, by directly using the native comparison mask with vmerge.

The bug that this fixes was that when we introduce a bitcast in the middle, the comparison and the merge may have different types with different lanes. And if that happens the vmerge will only look at the first n bits of the mask. n being the number of lanes currently configured.

This commit ensures that they are always equal by using the same type for both vmerge and the comparison instruction.

I also manually checked all other uses of gen_{f,i}cmp_mask and they are all using the same type in the subsequent instructions.

With this fix we no longer really care about the type of the bitselect as long as it has the same bitlength as the type of {i,f}cmp, which I think is enforced by the verifier. (i.e. We would have the same bug if bitselect.i8x16+icmp.i8x8 was legal.)

Thanks for reporting this @candymate!

Wasmtime GitHub notifications bot (Mar 14 2024 at 17:08):

afonso360 updated PR #8133.

Wasmtime GitHub notifications bot (Mar 14 2024 at 17:34):

alexcrichton submitted PR review:

Thanks for the quick fix!

Wasmtime GitHub notifications bot (Mar 14 2024 at 18:01):

alexcrichton merged PR #8133.

Last updated: Dec 13 2025 at 21:03 UTC