[slow_vector_initialization]: catch Vec::new() followed by .resize(len, 0)
#11198
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r? @Manishearth (rustbot has picked a reviewer for you, use r? to override) |
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As requested by Manishearth on zulip, r? @Centri3 |
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Jul 24, 2023
| .size_expr | ||
| .expect("length expression must be set by this point"), | ||
| "len", | ||
| ); | ||
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| span_lint_and_then(cx, SLOW_VECTOR_INITIALIZATION, slow_fill.span, msg, |diag| { |
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Not part of your PR, but if this could also suggest to remove the following call that'd be nice. Not sure if that'd have adverse side effects though
(should probably also be a separate PR ^^)
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Thanks! @bors r+ |
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☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test |
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…,djc [`slow_vector_initialization`]: clarify why `Vec::new()` + resize is worse #11198 extended this lint to also warn on `Vec::new()` + `resize(0, len)`, but did not update the lint documentation, so it left some confused (#10938 (comment)). This PR should make it a bit more clear. (cc `@djc` `@vi` what do you think about this?) <details> <summary>More details</summary> Godbolt for `Vec::new()` + `.resize(x, 0)`: https://godbolt.org/z/e7q9xc9rG The resize call first does a normal allocation (`__rust_alloc`): ```asm alloc::raw_vec::finish_grow: ... cmp qword ptr [rcx + 8], 0 je .LBB1_7 ; if capacity == 0 -> LBB1_7 .LBB1_7: ... call qword ptr [rip + __rust_alloc@GOTPCREL] ``` *Then* a memset for zero initialization: ```asm example::f: ... xor esi, esi ; 0 call qword ptr [rip + memset@GOTPCREL] ``` ------------ Godbolt for `vec![0; len]`: https://godbolt.org/z/M3vr53vWY Important bit: ```asm example::f: ... call qword ptr [rip + __rust_alloc_zeroed@GOTPCREL] ``` </details> changelog: [`slow_vector_initialization`]: clarify why `Vec::new()` + resize is worse than `vec![0; len]`
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Closes #10938
changelog: [
slow_vector_initialization]: catchVec::new()followed by.resize(len, 0)