Generate better code for "large" vec![] macro invocations #17857
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Even when optimized, something like vec![1i,2,3,4,5,6,7,8,9,10] will allocate three times, because LLVM can only drop the call based on the observed size of the previous allocation. Using a simple macro to count the number of elements, we can initialize the vector with the correct size and avoid reallocations. Since LLVM already does constant folding for us, the new count!() macro results in a single integer constant even if building without optimizations, so we don't end up with an awful lot of additions in the generated code.
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This has come up before, see #16204. There was pushback on the basis that counting should be a proper macro feature, not something that spits out 1+1+1... |
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@dotdash: Can you think of a way to hide this as an implementation detail (no new macro export)? As long as it's an internal implementation detail then it's fine. |
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…p, r=Veykril feat: Allow declaring cfg groups in rust-project.json, to help sharing common cfgs Closes rust-lang#17815.
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Even when optimized, something like vec![1i,2,3,4,5,6,7,8,9,10] will
allocate three times, because LLVM can only drop the call based on the
observed size of the previous allocation. Using a simple macro to count
the number of elements, we can initialize the vector with the correct
size and avoid reallocations.
Since LLVM already does constant folding for us, the new count!() macro
results in a single integer constant even if building without
optimizations, so we don't end up with an awful lot of additions in the
generated code.