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[docs] Document the lost variables statistics option

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Emil Pedersen
2024-04-30 13:18:38 -07:00
parent 59d0bd4f64
commit 7e81fd60f1
3 changed files with 38 additions and 6 deletions
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docs/HowToUpdateDebugInfo.md
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@@ -255,7 +255,8 @@ debug_value %1 : $Int, var, name "pair", type $Pair, expr op_fragment:#Pair.a //
## Rules of thumb
- Optimization passes may never drop a variable entirely. If a variable is
entirely optimized away, an `undef` debug value should still be kept.
entirely optimized away, an `undef` debug value should still be kept. The only
exception is an unreachable function or scope, which is entirely removed.
- A `debug_value` must always describe a correct value for that source variable
at that source location. If a value is only correct on some paths through that
instruction, it must be replaced with `undef`. Debug info never speculates.
@@ -263,3 +264,9 @@ debug_value %1 : $Int, var, name "pair", type $Pair, expr op_fragment:#Pair.a //
capture the effect of the deleted instruction in a debug expression, so the
location can be preserved. You can also use an `InstructionDeleter` which will
automatically call `salvageDebugInfo`.
> [!Tip]
> To detect when a pass drops a variable, you can use the
> `-Xllvm -sil-stats-lost-variables` to print when a variable is lost by a pass.
> More information about this option is available in
> [Optimizer Counter Analysis](OptimizerCountersAnalysis.md)