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Division by NonZero* checks for zero #76960

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bugadani opened this issue Sep 20, 2020 · 4 comments
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Division by NonZero* checks for zero #76960

bugadani opened this issue Sep 20, 2020 · 4 comments

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@bugadani
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bugadani commented Sep 20, 2020 •
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https://rust.godbolt.org/z/eTPzE6

Technically, NonZero* types are not meant to be used for optimizing arithmetic, but people (like myself) may or may not know that or may assume otherwise.

It is possible to create a NonZero* object with 0 inner value, but since that is a misuse of the type and deliberate violation of the safety guarantees, I think it would make sense to teach the compiler that get() can't return 0.

Edit: unfortunately, since NonZero* methods are const, it's not as simple as adding an assume.
@oli-obk
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oli-obk commented Sep 20, 2020

since NonZero* methods are const, it's not as simple as adding an assume.

we can make assume const, it's a no-op in const eval anyway (though we use it to assert that the assumption holds in miri)

@tesuji
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tesuji commented Sep 20, 2020

I'm preparing a PR for that.

@bugadani
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I can add the assume after that's done.

@bugadani
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Duplicate of #49572

@tesuji tesuji mentioned this issue Sep 20, 2020
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RalfJung added a commit to RalfJung/rust that referenced this issue Sep 25, 2020
@RalfJung
Rollup merge of rust-lang#76973 - lzutao:unstably-const-assume, r=oli…
287fdff 
…-obk

Unstably allow assume intrinsic in const contexts

Not sure much about this usage because there are concerns
about [blocking  optimization][1] and [slowing down LLVM][2] when using `assme` intrinsic
in inline functions.
But since Oli suggested in rust-lang#76960 (comment),
here we are.

[1]: rust-lang#54995 (comment)
[2]: rust-lang#49572 (comment)
RalfJung added a commit to RalfJung/rust that referenced this issue Sep 25, 2020
@RalfJung
Rollup merge of rust-lang#76973 - lzutao:unstably-const-assume, r=oli…
c822e91 
…-obk

Unstably allow assume intrinsic in const contexts

Not sure much about this usage because there are concerns
about [blocking  optimization][1] and [slowing down LLVM][2] when using `assme` intrinsic
in inline functions.
But since Oli suggested in rust-lang#76960 (comment),
here we are.

[1]: rust-lang#54995 (comment)
[2]: rust-lang#49572 (comment)
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@oli-obk @bugadani @tesuji