Stream: git-wasmtime

Topic: wasmtime / Issue #2633 Cranelift: add ghost_use instruction

Wasmtime GitHub notifications bot (Feb 03 2021 at 22:08):

bjorn3 edited Issue #2633:

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Feature

Add a new ghost_use instruction that accepts a value and ensures that the value can be found somewhere at runtime. It will codegen to no instructions.
<!-- What is the feature or code improvement you would like to do in
Cranelift/Wasmtime? -->

Benefit

This allows extending the range for which variables are available in debuginfo beyond the last actual use of the variable. In addition it would for exanple allow a jit runtime to inspect the value for switching to and from speculatively optimized code.
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Implementation

The instruction will be marked as has_other_sideeffects. It will be lowered by backends to an instruction that doesn't result in any bytes being emitted. It will tell regalloc that the value needs to be available on either in a register or on the stack.
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Alternatives

• Force all variables to the stack, which regressed runtime performance
• Accept that variables are only available until the last use, resulting in a sub-optimal debugging experience.
  <!-- Have you considered alternative implementations? If so, how are they
  better or worse than your proposal? -->
  

Last updated: Nov 22 2024 at 16:03 UTC