Open
Description
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.
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.
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.
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.