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Make `checked` ops emit *unchecked* LLVM operations where feasible 

For things with easily pre-checked overflow conditions -- shifts and unsigned subtraction -- write then checked methods in such a way that we stop emitting wrapping versions of them.

For example, today <https://rust.godbolt.org/z/qM9YK8Txb> neither
```rust
a.checked_sub(b).unwrap()
```
nor
```rust
a.checked_sub(b).unwrap_unchecked()
```
actually optimizes to `sub nuw`.  After this PR they do.
This commit is contained in:
Scott McMurray 2024-04-17 23:55:35 -07:00
parent e3181b091e
commit 986d9f104b
6 changed files with 160 additions and 91 deletions
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22
library/core/src/num/int_macros.rs
View File

@ -1163,12 +1163,19 @@ macro_rules! int_impl {
/// ```
#[stable(feature = "wrapping", since = "1.7.0")]
#[rustc_const_stable(feature = "const_checked_int_methods", since = "1.47.0")]
// We could always go back to wrapping
#[rustc_allow_const_fn_unstable(unchecked_shifts)]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn checked_shl(self, rhs: u32) -> Option<Self> {
let (a, b) = self.overflowing_shl(rhs);
if unlikely!(b) { None } else { Some(a) }
// Not using overflowing_shl as that's a wrapping shift
if rhs < Self::BITS {
// SAFETY: just checked the RHS is in-range
Some(unsafe { self.unchecked_shl(rhs) })
} else {
None
}
}
/// Strict shift left. Computes `self << rhs`, panicking if `rhs` is larger
@ -1254,12 +1261,19 @@ macro_rules! int_impl {
/// ```
#[stable(feature = "wrapping", since = "1.7.0")]
#[rustc_const_stable(feature = "const_checked_int_methods", since = "1.47.0")]
// We could always go back to wrapping
#[rustc_allow_const_fn_unstable(unchecked_shifts)]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn checked_shr(self, rhs: u32) -> Option<Self> {
let (a, b) = self.overflowing_shr(rhs);
if unlikely!(b) { None } else { Some(a) }
// Not using overflowing_shr as that's a wrapping shift
if rhs < Self::BITS {
// SAFETY: just checked the RHS is in-range
Some(unsafe { self.unchecked_shr(rhs) })
} else {
None
}
}
/// Strict shift right. Computes `self >> rhs`, panicking `rhs` is

35
library/core/src/num/uint_macros.rs
View File

@ -579,8 +579,17 @@ macro_rules! uint_impl {
without modifying the original"]
#[inline]
pub const fn checked_sub(self, rhs: Self) -> Option<Self> {
let (a, b) = self.overflowing_sub(rhs);
if unlikely!(b) { None } else { Some(a) }
// Per PR#103299, there's no advantage to the `overflowing` intrinsic
// for *unsigned* subtraction and we just emit the manual check anyway.
// Thus, rather than using `overflowing_sub` that produces a wrapping
// subtraction, check it ourself so we can use an unchecked one.
if self >= rhs {
// SAFETY: just checked this can't overflow
Some(unsafe { intrinsics::unchecked_sub(self, rhs) })
} else {
None
}
}
/// Strict integer subtraction. Computes `self - rhs`, panicking if
@ -1222,12 +1231,19 @@ macro_rules! uint_impl {
/// ```
#[stable(feature = "wrapping", since = "1.7.0")]
#[rustc_const_stable(feature = "const_checked_int_methods", since = "1.47.0")]
// We could always go back to wrapping
#[rustc_allow_const_fn_unstable(unchecked_shifts)]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn checked_shl(self, rhs: u32) -> Option<Self> {
let (a, b) = self.overflowing_shl(rhs);
if unlikely!(b) { None } else { Some(a) }
// Not using overflowing_shl as that's a wrapping shift
if rhs < Self::BITS {
// SAFETY: just checked the RHS is in-range
Some(unsafe { self.unchecked_shl(rhs) })
} else {
None
}
}
/// Strict shift left. Computes `self << rhs`, panicking if `rhs` is larger
@ -1313,12 +1329,19 @@ macro_rules! uint_impl {
/// ```
#[stable(feature = "wrapping", since = "1.7.0")]
#[rustc_const_stable(feature = "const_checked_int_methods", since = "1.47.0")]
// We could always go back to wrapping
#[rustc_allow_const_fn_unstable(unchecked_shifts)]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn checked_shr(self, rhs: u32) -> Option<Self> {
let (a, b) = self.overflowing_shr(rhs);
if unlikely!(b) { None } else { Some(a) }
// Not using overflowing_shr as that's a wrapping shift
if rhs < Self::BITS {
// SAFETY: just checked the RHS is in-range
Some(unsafe { self.unchecked_shr(rhs) })
} else {
None
}
}
/// Strict shift right. Computes `self >> rhs`, panicking `rhs` is

86
tests/codegen/checked_math.rs Normal file
View File

@ -0,0 +1,86 @@
//@ compile-flags: -O -Z merge-functions=disabled
#![crate_type = "lib"]
#![feature(unchecked_shifts)]
// Because the result of something like `u32::checked_sub` can only be used if it
// didn't overflow, make sure that LLVM actually knows that in optimized builds.
// Thanks to poison semantics, this doesn't even need branches.
// CHECK-LABEL: @checked_sub_unsigned
// CHECK-SAME: (i16 noundef %a, i16 noundef %b)
#[no_mangle]
pub fn checked_sub_unsigned(a: u16, b: u16) -> Option<u16> {
// CHECK-DAG: %[[IS_SOME:.+]] = icmp uge i16 %a, %b
// CHECK-DAG: %[[DIFF_P:.+]] = sub nuw i16 %a, %b
// CHECK-DAG: %[[DISCR:.+]] = zext i1 %[[IS_SOME]] to i16
// CHECK-DAG: %[[DIFF_U:.+]] = select i1 %[[IS_SOME]], i16 %[[DIFF_P]], i16 undef
// CHECK: %[[R0:.+]] = insertvalue { i16, i16 } poison, i16 %[[DISCR]], 0
// CHECK: %[[R1:.+]] = insertvalue { i16, i16 } %[[R0]], i16 %[[DIFF_U]], 1
// CHECK: ret { i16, i16 } %[[R1]]
a.checked_sub(b)
}
// Note that `shl` and `shr` in LLVM are already unchecked. So rather than
// looking for no-wrap flags, we just need there to not be any masking.
// CHECK-LABEL: @checked_shl_unsigned
// CHECK-SAME: (i32 noundef %a, i32 noundef %b)
#[no_mangle]
pub fn checked_shl_unsigned(a: u32, b: u32) -> Option<u32> {
// CHECK-DAG: %[[IS_SOME:.+]] = icmp ult i32 %b, 32
// CHECK-DAG: %[[SHIFTED_P:.+]] = shl i32 %a, %b
// CHECK-DAG: %[[DISCR:.+]] = zext i1 %[[IS_SOME]] to i32
// CHECK-DAG: %[[SHIFTED_U:.+]] = select i1 %[[IS_SOME]], i32 %[[SHIFTED_P]], i32 undef
// CHECK: %[[R0:.+]] = insertvalue { i32, i32 } poison, i32 %[[DISCR]], 0
// CHECK: %[[R1:.+]] = insertvalue { i32, i32 } %[[R0]], i32 %[[SHIFTED_U]], 1
// CHECK: ret { i32, i32 } %[[R1]]
a.checked_shl(b)
}
// CHECK-LABEL: @checked_shr_unsigned
// CHECK-SAME: (i32 noundef %a, i32 noundef %b)
#[no_mangle]
pub fn checked_shr_unsigned(a: u32, b: u32) -> Option<u32> {
// CHECK-DAG: %[[IS_SOME:.+]] = icmp ult i32 %b, 32
// CHECK-DAG: %[[SHIFTED_P:.+]] = lshr i32 %a, %b
// CHECK-DAG: %[[DISCR:.+]] = zext i1 %[[IS_SOME]] to i32
// CHECK-DAG: %[[SHIFTED_U:.+]] = select i1 %[[IS_SOME]], i32 %[[SHIFTED_P]], i32 undef
// CHECK: %[[R0:.+]] = insertvalue { i32, i32 } poison, i32 %[[DISCR]], 0
// CHECK: %[[R1:.+]] = insertvalue { i32, i32 } %[[R0]], i32 %[[SHIFTED_U]], 1
// CHECK: ret { i32, i32 } %[[R1]]
a.checked_shr(b)
}
// CHECK-LABEL: @checked_shl_signed
// CHECK-SAME: (i32 noundef %a, i32 noundef %b)
#[no_mangle]
pub fn checked_shl_signed(a: i32, b: u32) -> Option<i32> {
// CHECK-DAG: %[[IS_SOME:.+]] = icmp ult i32 %b, 32
// CHECK-DAG: %[[SHIFTED_P:.+]] = shl i32 %a, %b
// CHECK-DAG: %[[DISCR:.+]] = zext i1 %[[IS_SOME]] to i32
// CHECK-DAG: %[[SHIFTED_U:.+]] = select i1 %[[IS_SOME]], i32 %[[SHIFTED_P]], i32 undef
// CHECK: %[[R0:.+]] = insertvalue { i32, i32 } poison, i32 %[[DISCR]], 0
// CHECK: %[[R1:.+]] = insertvalue { i32, i32 } %[[R0]], i32 %[[SHIFTED_U]], 1
// CHECK: ret { i32, i32 } %[[R1]]
a.checked_shl(b)
}
// CHECK-LABEL: @checked_shr_signed
// CHECK-SAME: (i32 noundef %a, i32 noundef %b)
#[no_mangle]
pub fn checked_shr_signed(a: i32, b: u32) -> Option<i32> {
// CHECK-DAG: %[[IS_SOME:.+]] = icmp ult i32 %b, 32
// CHECK-DAG: %[[SHIFTED_P:.+]] = ashr i32 %a, %b
// CHECK-DAG: %[[DISCR:.+]] = zext i1 %[[IS_SOME]] to i32
// CHECK-DAG: %[[SHIFTED_U:.+]] = select i1 %[[IS_SOME]], i32 %[[SHIFTED_P]], i32 undef
// CHECK: %[[R0:.+]] = insertvalue { i32, i32 } poison, i32 %[[DISCR]], 0
// CHECK: %[[R1:.+]] = insertvalue { i32, i32 } %[[R0]], i32 %[[SHIFTED_U]], 1
// CHECK: ret { i32, i32 } %[[R1]]
a.checked_shr(b)
}

53
tests/mir-opt/pre-codegen/checked_ops.checked_shl.PreCodegen.after.panic-abort.mir
View File

@ -5,60 +5,33 @@ fn checked_shl(_1: u32, _2: u32) -> Option<u32> {
debug rhs => _2;
let mut _0: std::option::Option<u32>;
scope 1 (inlined core::num::<impl u32>::checked_shl) {
debug self => _1;
debug rhs => _2;
let mut _6: bool;
scope 2 {
debug a => _4;
debug b => _5;
}
scope 3 (inlined core::num::<impl u32>::overflowing_shl) {
debug self => _1;
debug rhs => _2;
let mut _4: u32;
let mut _5: bool;
scope 4 (inlined core::num::<impl u32>::wrapping_shl) {
debug self => _1;
debug rhs => _2;
let mut _3: u32;
scope 5 (inlined core::num::<impl u32>::unchecked_shl) {
debug self => _1;
debug rhs => _3;
}
}
let mut _3: bool;
let mut _4: u32;
scope 2 (inlined core::num::<impl u32>::unchecked_shl) {
}
}
bb0: {
StorageLive(_4);
StorageLive(_5);
StorageLive(_3);
_3 = BitAnd(_2, const 31_u32);
_4 = ShlUnchecked(_1, _3);
StorageDead(_3);
_5 = Ge(_2, const core::num::<impl u32>::BITS);
StorageLive(_6);
_6 = unlikely(move _5) -> [return: bb1, unwind unreachable];
_3 = Lt(_2, const core::num::<impl u32>::BITS);
switchInt(move _3) -> [0: bb1, otherwise: bb2];
}
bb1: {
switchInt(move _6) -> [0: bb2, otherwise: bb3];
_0 = const Option::<u32>::None;
goto -> bb3;
}
bb2: {
_0 = Option::<u32>::Some(_4);
goto -> bb4;
StorageLive(_4);
_4 = ShlUnchecked(_1, _2);
_0 = Option::<u32>::Some(move _4);
StorageDead(_4);
goto -> bb3;
}
bb3: {
_0 = const Option::<u32>::None;
goto -> bb4;
}
bb4: {
StorageDead(_6);
StorageDead(_5);
StorageDead(_4);
StorageDead(_3);
return;
}
}

53
tests/mir-opt/pre-codegen/checked_ops.checked_shl.PreCodegen.after.panic-unwind.mir
View File

@ -5,60 +5,33 @@ fn checked_shl(_1: u32, _2: u32) -> Option<u32> {
debug rhs => _2;
let mut _0: std::option::Option<u32>;
scope 1 (inlined core::num::<impl u32>::checked_shl) {
debug self => _1;
debug rhs => _2;
let mut _6: bool;
scope 2 {
debug a => _4;
debug b => _5;
}
scope 3 (inlined core::num::<impl u32>::overflowing_shl) {
debug self => _1;
debug rhs => _2;
let mut _4: u32;
let mut _5: bool;
scope 4 (inlined core::num::<impl u32>::wrapping_shl) {
debug self => _1;
debug rhs => _2;
let mut _3: u32;
scope 5 (inlined core::num::<impl u32>::unchecked_shl) {
debug self => _1;
debug rhs => _3;
}
}
let mut _3: bool;
let mut _4: u32;
scope 2 (inlined core::num::<impl u32>::unchecked_shl) {
}
}
bb0: {
StorageLive(_4);
StorageLive(_5);
StorageLive(_3);
_3 = BitAnd(_2, const 31_u32);
_4 = ShlUnchecked(_1, _3);
StorageDead(_3);
_5 = Ge(_2, const core::num::<impl u32>::BITS);
StorageLive(_6);
_6 = unlikely(move _5) -> [return: bb1, unwind unreachable];
_3 = Lt(_2, const core::num::<impl u32>::BITS);
switchInt(move _3) -> [0: bb1, otherwise: bb2];
}
bb1: {
switchInt(move _6) -> [0: bb2, otherwise: bb3];
_0 = const Option::<u32>::None;
goto -> bb3;
}
bb2: {
_0 = Option::<u32>::Some(_4);
goto -> bb4;
StorageLive(_4);
_4 = ShlUnchecked(_1, _2);
_0 = Option::<u32>::Some(move _4);
StorageDead(_4);
goto -> bb3;
}
bb3: {
_0 = const Option::<u32>::None;
goto -> bb4;
}
bb4: {
StorageDead(_6);
StorageDead(_5);
StorageDead(_4);
StorageDead(_3);
return;
}
}

2
tests/mir-opt/pre-codegen/checked_ops.rs
View File

@ -1,5 +1,5 @@
// skip-filecheck
//@ compile-flags: -O -Zmir-opt-level=2 -Cdebuginfo=2
//@ compile-flags: -O -Zmir-opt-level=2
// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
#![crate_type = "lib"]