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Auto merge of #105446 - erikdesjardins:vt-size, r=nikic

Add 0..=isize::MAX range metadata to size loads from vtables

This is the (much belated) size counterpart to #91569.

Inspired by https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm/topic/Range.20metadata.20for.20.60size_of_val.60.20and.20other.20isize.3A.3AMAX.20limits. This could help optimize layout computations based on the size of a dyn trait. Though, admittedly, adding this to vtables wouldn't be as beneficial as adding it to slice len, which is used much more often.

Miri detects this UB already: b7cc99142a/compiler/rustc_const_eval/src/interpret/traits.rs (L119-L121)
(In fact Miri goes further, [assuming a 48-bit address space on 64-bit platforms](9db224fc90/compiler/rustc_abi/src/lib.rs (L312-L331)), but I don't think we can assume that in an optimization.)
This commit is contained in:
bors 2022-12-18 22:01:39 +00:00
commit 2b094b1ede
5 changed files with 76 additions and 4 deletions

View file

@ -774,6 +774,18 @@ impl Integer {
}
}
/// Returns the largest signed value that can be represented by this Integer.
#[inline]
pub fn signed_max(self) -> i128 {
match self {
I8 => i8::MAX as i128,
I16 => i16::MAX as i128,
I32 => i32::MAX as i128,
I64 => i64::MAX as i128,
I128 => i128::MAX,
}
}
/// Finds the smallest Integer type which can represent the signed value.
#[inline]
pub fn fit_signed(x: i128) -> Integer {