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Indicate that multiplication in Layout::array cannot overflow

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This allows LLVM to optimize comparisons to zero before & after the
multiplication into one, saving on code size and eliminating an (always
true) branch from most Vec allocations.
This commit is contained in:
Mark Rousskov 2023-11-23 21:47:08 -05:00
parent a4a5c976fe
commit b81e788d16
1 changed files with 5 additions and 1 deletions
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6
library/core/src/alloc/layout.rs
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@ -450,7 +450,11 @@ impl Layout {
return Err(LayoutError); return Err(LayoutError);
} }
let array_size = element_size * n; // SAFETY: We just checked that we won't overflow `usize` when we multiply.
// This is a useless hint inside this function, but after inlining this helps
// deduplicate checks for whether the overall capacity is zero (e.g., in RawVec's
// allocation path) before/after this multiplication.
let array_size = unsafe { element_size.unchecked_mul(n) };
// SAFETY: We just checked above that the `array_size` will not // SAFETY: We just checked above that the `array_size` will not
// exceed `isize::MAX` even when rounded up to the alignment. // exceed `isize::MAX` even when rounded up to the alignment.