Fix scratch local optimizations when emitting string slice #6649
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The binary writing of `stringview_wtf16.slice` requires scratch locals to store the `start` and `end` operands while the string operand is converted to a stringview. To avoid unbounded binary bloat when round-tripping, we detect the case that `start` and `end` are already `local.get`s and avoid using scratch locals by deferring the binary writing of the `local.get` operands until after the stringview conversoins is emitted. We previously optimized the scratch locals for `start` and `end` independently, but this could produce incorrect code in the case where the `local.get` for `start` is deferred but its value is changed by a `local.set` in the code for `end`. Fix the problem by only optimizing to avoid scratch locals in the case where both `start` and `end` are already `local.get`s, so they will still be emitted in the original relative order and they cannot interfere with each other anyway.
kripken
approved these changes
Jun 11, 2024
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The binary writing of
stringview_wtf16.slicerequires scratch locals to store thestartandendoperands while the string operand is converted to a stringview. To avoid unbounded binary bloat when round-tripping, we detect the case thatstartandendare alreadylocal.gets and avoid using scratch locals by deferring the binary writing of thelocal.getoperands until after the stringview conversoins is emitted.We previously optimized the scratch locals for
startandendindependently, but this could produce incorrect code in the case where thelocal.getforstartis deferred but its value is changed by alocal.setin the code forend. Fix the problem by only optimizing to avoid scratch locals in the case where bothstartandendare alreadylocal.gets, so they will still be emitted in the original relative order and they cannot interfere with each other anyway.