impl more traits for ptr::Alignment, add mask method #115249

clarfonthey · 2023-08-26T16:11:22Z

Changes:

Adds rustc_const_unstable attributes where missing
Makes log2 method const
Adds mask method
Implements Default, which is equivalent to Alignment::MIN

No longer included in PR:

Removes indirection of AlignmentEnum type alias (this was intentional)
Implements Display, Binary, Octal, LowerHex, and UpperHex (should go through libs-api instead)
Controversially implements LowerExp and UpperExp using p instead of e to indicate a power of 2 (also should go through libs-api)

Tracking issue for ptr::Alignment: #102070

rustbot · 2023-08-26T16:11:29Z

r? @cuviper

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clarfonthey · 2023-08-26T16:14:37Z

library/core/src/ptr/alignment.rs

+}
+
+impl Alignment {
+    // longest string: 1p-128


even though no targets have 128-bit usize, I figured I might as well future-proof it

scottmcm · 2023-08-27T08:25:54Z

library/core/src/ptr/alignment.rs

+    #[inline]
+    pub const fn mask(self) -> usize {
+        // SAFETY: The alignment is always nonzero, and therefore decrementing won't overflow.
+        !(unsafe { self.as_usize().unchecked_sub(1) })


Sadly, unchecked_sub(1) is useless on unsigned types llvm/llvm-project#53377

I guess it's fine to leave it, though, since it's clearly correct. And trying to do it in isize would be UB.

Oh, that's unfortunate. I even was considering at some point adding a decrement() method to NonZeroU* that could use unchecked_sub since it would never overflow, but I guess that that also wouldn't work.

scottmcm · 2023-08-27T08:27:55Z

library/core/src/ptr/alignment.rs

+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl fmt::Display for Alignment {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&self.as_nonzero(), f)


Pondering: I have no idea when we should be including Display on types. I guess it's not a problem, but it feels like alignment isn't something I'd usually be showing to people. But showing it in binary or hex seems logical, so maybe display is ok too.

Yeah, that was my thought too. It's less that it's necessarily a common operation, but more that it's well-defined and not a whole lot of extra code to do. I'm fine removing it if it seems too weird.

library/core/src/ptr/alignment.rs

scottmcm · 2023-10-13T15:59:46Z

r? @scottmcm

Would you be willing to split this change? I feel like it's got a bunch of things that are easy to take, but also some unprecedented things (as you note in the op), and I'd be happy to just merge the former to try to make progress here for you, since this has been open a while.

If you make this PR be these bits:

Adds rustc_const_unstable attributes where missing
Makes log2 method const
Adds mask method
Implements Default, which is equivalent to Alignment::MIN
- Please write a doc comment on it saying that it's the same as MIN, though

Then I'd be happy to just r+ that, and we can leave the question of Display and Exp and such to libs-api for their opinions in another PR.

@rustbot author

bors · 2023-10-14T00:03:47Z

⌛ Testing commit 5dda0c9 with merge 2c4b73c...

impl more traits for ptr::Alignment, add mask method Changes: * Adds `rustc_const_unstable` attributes where missing * Makes `log2` method const * Adds `mask` method * Implements `Default`, which is equivalent to `Alignment::MIN` No longer included in PR: * Removes indirection of `AlignmentEnum` type alias (this was intentional) * Implements `Display`, `Binary`, `Octal`, `LowerHex`, and `UpperHex` (should go through libs-api instead) * Controversially implements `LowerExp` and `UpperExp` using `p` instead of `e` to indicate a power of 2 (also should go through libs-api) Tracking issue for `ptr::Alignment`: rust-lang#102070

bors · 2023-10-14T00:56:01Z

💔 Test failed - checks-actions

scottmcm · 2023-10-14T01:01:50Z

library/core/src/ptr/alignment.rs

+    /// assert_eq!(four.mask(Alignment::of::<u8>().mask()), four);
+    /// assert_eq!(four.mask(Alignment::of::<u16>().mask()), four);
+    /// assert_eq!(four.mask(Alignment::of::<u32>().mask()), four);
+    /// assert_ne!(four.mask(Alignment::of::<u64>().mask()), four);


Looks like you need to not make assumptions about the alignment of primitives like this in these tests. There's no guarantee for alignof(u64), and on i686 it looks like it's 4. Maybe these demos should use Alignment::new or have specific repr(align(…)) types, rather than get them from primitives?

@bors r-

clarfonthey · 2023-10-15T18:34:04Z

I don't like it, but I updated the example to have explicitly-aligned structs.

Right now, it's operating way more as a test than an example of behaviour, but that can be resolved during stabilisation.

clarfonthey · 2023-10-28T05:16:42Z

Ping @scottmcm when you have time to take a look at this

clarfonthey · 2023-11-18T05:04:18Z

Ping @scottmcm again when you're free.

scottmcm · 2023-11-18T05:14:38Z

Oops, thanks for the ping!

@bors r+ rollup=iffy (had a per-target failure last time)

bors · 2023-11-18T05:14:40Z

📌 Commit 114873d has been approved by scottmcm

It is now in the queue for this repository.

bors · 2023-11-18T06:51:18Z

⌛ Testing commit 114873d with merge 61d3b26...

bors · 2023-11-18T08:47:54Z

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing 61d3b26 to master...

rust-timer · 2023-11-18T10:22:46Z

Finished benchmarking commit (61d3b26): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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Regressions ❌ (secondary)	2.4%	[1.8%, 2.9%]	10
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Bootstrap: 675.975s -> 678.192s (0.33%)
Artifact size: 313.82 MiB -> 313.84 MiB (0.01%)

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Tracking Issue for ptr::Alignment type #102070

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impl more traits for ptr::Alignment, add mask method

114873d

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impl more traits for ptr::Alignment, add mask method #115249

impl more traits for ptr::Alignment, add mask method #115249

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impl more traits for ptr::Alignment, add mask method #115249

impl more traits for ptr::Alignment, add mask method #115249

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