Add fast-lanes for '++'/2 and '--'/2 on nil arguments #8743
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List++[]
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erts/emulator/beam/erl_bif_lists.c
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| int res; | ||
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| /* First check that lhs is proper. */ | ||
| res = append_is_proper_list(p, &context->iterator); | ||
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| if (res == 1) { | ||
| #ifdef DEBUG | ||
| context->result_cdr = &context->rhs_original; | ||
| #endif | ||
| context->result = context->lhs_original; | ||
| } | ||
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| return res; |
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If we move setting these to the branch returning 1 in append_is_proper_list, we can shorten this a lot. While we're at it we can change its name to append_empty_rhs or the likes.
Otherwise this PR LGTM.
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Currently appending an empty list to another list causes the left-hand side list to be fully copied. We can skip the append operation when the right-hand side is nil since it shouldn't change the left-hand side list. In this fast-lane we scan the list to determine if it is improper so that cases like `[1, 2 | 3] ++ []` behave the same as cases like `[1, 2 | 3] ++ [4]`: return a badarg error. This patch adds similar fast-lanes for '--'/2: we can return the first argument when either the first or second arguments are nil.
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Merged, thanks for the PR! |
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Thanks for your help with this @jhogberg! I wasn't familiar with the internals and trapping so this was a cool learning experience :) |
Currently appending an empty list to another list causes the left-hand side list to be fully copied. We can skip the append operation when the right-hand side is nil since it shouldn't change the left-hand side list.
It looks like there are historical artifacts in the
++/2implementation like its behavior on[] ++ anythingso I'm not sure if I'm missing some context, but I wanted to propose this optimization since it seems needless to allocate forList ++ []. What do you think?