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Auto merge of #126852 - scottmcm:more-checked-math-tweaks, r=Amanieu

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Also get `add nuw` from `uN::checked_add`

When I was doing this for `checked_{sub,shl,shr}`, it was mentioned https://github.com/rust-lang/rust/pull/124114#issuecomment-2066173305 that it'd be worth trying for `checked_add` too.

It makes a particularly-big difference for `x.checked_add(C)`, as doing this means that LLVM removes the intrinsic and does it as a normal `x <= MAX - C` instead.

cc `@DianQK` who had commented about `checked_add` related to https://github.com/rust-lang/hashbrown/issues/509 before

cc https://github.com/llvm/llvm-project/issues/80637 for how LLVM is unlikely to do this itself
This commit is contained in:
bors 2024-06-25 02:50:37 +00:00
commit fc555cd832
4 changed files with 47 additions and 32 deletions
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15
library/core/src/num/uint_macros.rs
View File

@ -454,8 +454,19 @@ pub const fn to_le(self) -> Self {
without modifying the original"]
#[inline]
pub const fn checked_add(self, rhs: Self) -> Option<Self> {
let (a, b) = self.overflowing_add(rhs);
if unlikely!(b) { None } else { Some(a) }
// This used to use `overflowing_add`, but that means it ends up being
// a `wrapping_add`, losing some optimization opportunities. Notably,
// phrasing it this way helps `.checked_add(1)` optimize to a check
// against `MAX` and a `add nuw`.
// Per <https://github.com/rust-lang/rust/pull/124114#issuecomment-2066173305>,
// LLVM is happy to re-form the intrinsic later if useful.
if unlikely!(intrinsics::add_with_overflow(self, rhs).1) {
None
} else {
// SAFETY: Just checked it doesn't overflow
Some(unsafe { intrinsics::unchecked_add(self, rhs) })
}
}
/// Strict integer addition. Computes `self + rhs`, panicking

14
tests/codegen/checked_math.rs
View File

@ -84,3 +84,17 @@ pub fn checked_shr_signed(a: i32, b: u32) -> Option<i32> {
// CHECK: ret { i32, i32 } %[[R1]]
a.checked_shr(b)
}
// CHECK-LABEL: @checked_add_one_unwrap_unsigned
// CHECK-SAME: (i32 noundef %x)
#[no_mangle]
pub fn checked_add_one_unwrap_unsigned(x: u32) -> u32 {
// CHECK: %[[IS_MAX:.+]] = icmp eq i32 %x, -1
// CHECK: br i1 %[[IS_MAX]], label %[[NONE_BB:.+]], label %[[SOME_BB:.+]]
// CHECK: [[SOME_BB]]:
// CHECK: %[[R:.+]] = add nuw i32 %x, 1
// CHECK: ret i32 %[[R]]
// CHECK: [[NONE_BB]]:
// CHECK: call {{.+}}unwrap_failed
x.checked_add(1).unwrap()
}

25
tests/mir-opt/pre-codegen/checked_ops.step_forward.PreCodegen.after.panic-abort.mir
View File

@ -11,15 +11,9 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
scope 3 (inlined <u16 as Step>::forward_checked) {
scope 4 {
scope 6 (inlined core::num::<impl u16>::checked_add) {
let mut _5: (u16, bool);
let mut _6: bool;
let mut _7: bool;
scope 7 {
}
scope 8 (inlined core::num::<impl u16>::overflowing_add) {
let mut _5: (u16, bool);
let _6: bool;
scope 9 {
}
}
}
}
scope 5 (inlined convert::num::ptr_try_from_impls::<impl TryFrom<usize> for u16>::try_from) {
@ -27,11 +21,11 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
let mut _4: u16;
}
}
scope 10 (inlined Option::<u16>::is_none) {
scope 11 (inlined Option::<u16>::is_some) {
scope 7 (inlined Option::<u16>::is_none) {
scope 8 (inlined Option::<u16>::is_some) {
}
}
scope 12 (inlined core::num::<impl u16>::wrapping_add) {
scope 9 (inlined core::num::<impl u16>::wrapping_add) {
}
}
@ -45,12 +39,11 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
bb1: {
_4 = _2 as u16 (IntToInt);
StorageDead(_3);
StorageLive(_7);
StorageLive(_6);
StorageLive(_5);
_5 = AddWithOverflow(_1, _4);
_6 = (_5.1: bool);
StorageDead(_5);
StorageLive(_7);
_7 = unlikely(move _6) -> [return: bb2, unwind unreachable];
}
@ -59,14 +52,16 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
}
bb3: {
StorageDead(_7);
StorageDead(_5);
StorageDead(_6);
StorageDead(_7);
goto -> bb7;
}
bb4: {
StorageDead(_7);
StorageDead(_5);
StorageDead(_6);
StorageDead(_7);
goto -> bb6;
}

25
tests/mir-opt/pre-codegen/checked_ops.step_forward.PreCodegen.after.panic-unwind.mir
View File

@ -11,15 +11,9 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
scope 3 (inlined <u16 as Step>::forward_checked) {
scope 4 {
scope 6 (inlined core::num::<impl u16>::checked_add) {
let mut _5: (u16, bool);
let mut _6: bool;
let mut _7: bool;
scope 7 {
}
scope 8 (inlined core::num::<impl u16>::overflowing_add) {
let mut _5: (u16, bool);
let _6: bool;
scope 9 {
}
}
}
}
scope 5 (inlined convert::num::ptr_try_from_impls::<impl TryFrom<usize> for u16>::try_from) {
@ -27,11 +21,11 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
let mut _4: u16;
}
}
scope 10 (inlined Option::<u16>::is_none) {
scope 11 (inlined Option::<u16>::is_some) {
scope 7 (inlined Option::<u16>::is_none) {
scope 8 (inlined Option::<u16>::is_some) {
}
}
scope 12 (inlined core::num::<impl u16>::wrapping_add) {
scope 9 (inlined core::num::<impl u16>::wrapping_add) {
}
}
@ -45,12 +39,11 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
bb1: {
_4 = _2 as u16 (IntToInt);
StorageDead(_3);
StorageLive(_7);
StorageLive(_6);
StorageLive(_5);
_5 = AddWithOverflow(_1, _4);
_6 = (_5.1: bool);
StorageDead(_5);
StorageLive(_7);
_7 = unlikely(move _6) -> [return: bb2, unwind unreachable];
}
@ -59,14 +52,16 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
}
bb3: {
StorageDead(_7);
StorageDead(_5);
StorageDead(_6);
StorageDead(_7);
goto -> bb7;
}
bb4: {
StorageDead(_7);
StorageDead(_5);
StorageDead(_6);
StorageDead(_7);
goto -> bb6;
}