librustc: Apply null pointer optimization to slices, closures & trait objects. #15406
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| 1 as *const T | ||
| static PTR_MARKER: u8 = 0; | ||
| if self.ptr.is_null() { | ||
| &PTR_MARKER as *const _ as *const T |
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My stalled-out PR #14436 has a commit that relied on changing this to always return self.ptr. The reason given is that otherwise Vec::from_raw_parts(v.len(), v.cap(), v.as_mut_ptr()) will be incorrect. If at all possible, it would be nice to make that work while you're changing things.
My preference would be to make as_ptr() and as_mut_ptr() return null whenever appropriate, and instead do the null-test when constructing the slice.
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The |
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@thestinger That can't be true, because a brand new |
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As I said, the only required change is to the constructor functions. It can use an arbitrary non-null value because it will never pass it to |
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@thestinger What benefit is there to having
That said, having |
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The non-null enum optimization can and should be extended to library types like |
| // pointer as the first element in the vector, but this will end up | ||
| // being Some(NULL) which is optimized to None. So instead we set ptr | ||
| // to some arbitrary non-null value which is fine since we never call | ||
| // deallocate on the ptr if cap is 0. |
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This comment is misleading. It suggests the non-null pointer is purely because of 0-sized types (not 0-sized vectors!), but one would expect a size_of() test in that case. It should be rewritten to explain that Slice wants to never have a null pointer so it can benefit from null pointer optimization, and the easiest way to do that is to make Vec never have a null pointer.
Extend the null ptr optimization to work with slices, closures, procs, & trait objects by using the internal pointers as the discriminant. This decreases the size of `Option<&[int]>` (and similar) by one word.
ghost
mentioned this pull request
…r=lnicola Don't provide `generate_default_from_new` when impl self ty is missing Also don't provide the assist when the `Default` trait can't be found. Part of rust-lang#15398
Extend the null ptr optimization to work with slices, closures, procs, & trait objects by using the internal pointers as the discriminant.
This decreases the size of
Option<&[int]>(and similar) by one word.