Select optimization: Avoid backtracking on mutually exclusive branches #158
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Andersama
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When we match against characters that belong to no other select branch we can avoid backtracking by a significant amount. Note: HeadOptions could be a fairly large sequence, here we use calculate_first() to evaluate the initial condition for a mutually exclusive branch*
This was referenced Jul 18, 2021
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I actually did this once, and found out the resulting assembly is worse, the first character lookahead is useful mostly only for cycles. And only for optimizations. |
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Was it more like this pr or the other one? I just rewrote this because with that bug patched something kinda clicked, and this one only handles when the branch is mutually exclusive from everything else. The newer approach could be useful not necessarily for a one character lookahead it could be just to reorder the select expression around for the branch predictor. |
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When we match against characters that belong to no other select branch we can avoid backtracking by a significant amount.
Note: HeadOptions could be a fairly large sequence*. I updated to use calculate_first() to evaluate the initial condition and backtrack by at worst one character*
See comparison: https://compiler-explorer.com/z/8ea8Pr
*Updated: https://compiler-explorer.com/z/vPEq1T