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Rollup merge of #125038 - ivan-shrimp:checked_sub, r=joboet
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Invert comparison in `uN::checked_sub`

After #124114, LLVM no longer combines the comparison and subtraction in `uN::checked_sub` when either operand is a constant (demo: https://rust.godbolt.org/z/MaeoYbsP1). The difference is more pronounced when the expression is slightly more complex (https://rust.godbolt.org/z/4rPavsYdc).

This is due to the use of `>=` here:

https://github.com/rust-lang/rust/blob/ee97564e3a9f9ac8c65103abb37c6aa48d95bfa2/library/core/src/num/uint_macros.rs#L581-L593

For constant `C`, LLVM eagerly converts `a >= C` into `a > C - 1`, but the backend can only combine `a < C` with `a - C`, not `C - 1 < a` and `a - C`: https://github.com/llvm/llvm-project/blob/e586556e375fc5c4f7e76b5c299cb981f2016108/llvm/lib/CodeGen/CodeGenPrepare.cpp#L1697-L1742

This PR[^1] simply inverts the `>=` into `<` to restore the LLVM magic, and somewhat align this with the implementation of `uN::overflowing_sub` from #103299.

When the result is stored as an `Option` (rather than being branched/cmoved on), the discriminant is `self >= rhs`. This PR doesn't affect the codegen (and relevant tests) of that since LLVM will negate `self < rhs` to `self >= rhs` when necessary.

[^1]: Note to `self`: My very first contribution to publicly-used code. Hopefully like what I should learn to always be, tiny and humble.
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fmease authored May 15, 2024
2 parents 2804d42 + 7fde730 commit 4f7d9d4
Showing 1 changed file with 3 additions and 3 deletions.
6 changes: 3 additions & 3 deletions library/core/src/num/uint_macros.rs
Original file line number Diff line number Diff line change
Expand Up @@ -584,11 +584,11 @@ macro_rules! uint_impl {
// Thus, rather than using `overflowing_sub` that produces a wrapping
// subtraction, check it ourself so we can use an unchecked one.

if self >= rhs {
if self < rhs {
None
} else {
// SAFETY: just checked this can't overflow
Some(unsafe { intrinsics::unchecked_sub(self, rhs) })
} else {
None
}
}

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