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Make abs, wrapping_abs, overflowing_abs const functions #63786

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merged 2 commits into from
Sep 10, 2019
Merged

Make abs, wrapping_abs, overflowing_abs const functions #63786

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39260d9
make abs, wrapping_abs, and overflowing_abs const functions
tspiteri Aug 21, 2019
adee559
test const abs, wrapping_abs, and overflowing_abs
tspiteri Aug 21, 2019
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30 changes: 9 additions & 21 deletions src/libcore/num/mod.rs
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Expand Up @@ -1401,12 +1401,8 @@ $EndFeature, "
```"),
#[stable(feature = "no_panic_abs", since = "1.13.0")]
#[inline]
pub fn wrapping_abs(self) -> Self {
if self.is_negative() {
self.wrapping_neg()
} else {
self
}
pub const fn wrapping_abs(self) -> Self {
(self ^ (self >> ($BITS - 1))).wrapping_sub(self >> ($BITS - 1))
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I am not a big fan of making code less readable just to please our const qualification. :/

Cc @oli-obk @eddyb

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We've had variables for a while, please use them.

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Has it been verified that this optimizes to identical LLVM IR?

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@eddyb Since this has already passed final comment period and been merged, shall I open a new PR to use variables?

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@nikic I can't read LLVM IR; what I did do before submitting the PR was to use llvm-mca, and although the assembly is not identical, the performance measure from llvm-mca was the same, or if I remember well, in some tests the reverse throughput improved by something tiny like 0.1, but not regressed. I only used llvm-mca with i686 or x86-64 instructions, and I don't think I tested all types comprehensively.

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@nikic Hmm, using variables as @eddyb suggested seems to hit two birds with one stone. If I insert a variable for the sign, I get one implementation and an alias: https://godbolt.org/z/m-seT0

Does that mean that the performance will not regress?

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@tspiteri Oops, I simply made a typo in my test and that's why the result is different, duh :) It actually does produce the same IR, both with variable and without. So forget everything I said, this change is fine from the optimization perspective.

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@nikic While abs and wrapping_abs seem to be fine performance-wise, overflowing_abs from this PR actually does suffer from the problem discussed, so I will not forget everything you said! :) It's easily fixed by making it return (self.wrapping_abs(), self == Self::min_value()).

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And also, the code with the sign factored out is more readable. ;)

That said, I still think we should rather wait for CTFE to support conditionals than compromise code readability, and even the let sign version is still arcane magic. But that call is up to T-libs.

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No, we should definitely make as many of these small ops into const as soon as we can.

}
}

Expand Down Expand Up @@ -1764,12 +1760,8 @@ $EndFeature, "
```"),
#[stable(feature = "no_panic_abs", since = "1.13.0")]
#[inline]
pub fn overflowing_abs(self) -> (Self, bool) {
if self.is_negative() {
self.overflowing_neg()
} else {
(self, false)
}
pub const fn overflowing_abs(self) -> (Self, bool) {
(self ^ (self >> ($BITS - 1))).overflowing_sub(self >> ($BITS - 1))
}
}

Expand Down Expand Up @@ -1973,15 +1965,11 @@ $EndFeature, "
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
#[rustc_inherit_overflow_checks]
pub fn abs(self) -> Self {
if self.is_negative() {
// Note that the #[inline] above means that the overflow
// semantics of this negation depend on the crate we're being
// inlined into.
-self
} else {
self
}
pub const fn abs(self) -> Self {
// Note that the #[inline] above means that the overflow
// semantics of the subtraction depend on the crate we're being
// inlined into.
(self ^ (self >> ($BITS - 1))) - (self >> ($BITS - 1))
}
}

Expand Down
8 changes: 8 additions & 0 deletions src/test/ui/consts/const-int-overflowing-rpass.rs
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Expand Up @@ -18,6 +18,10 @@ const SHR_B: (u32, bool) = 0x10u32.overflowing_shr(132);
const NEG_A: (u32, bool) = 0u32.overflowing_neg();
const NEG_B: (u32, bool) = core::u32::MAX.overflowing_neg();

const ABS_POS: (i32, bool) = 10i32.overflowing_abs();
const ABS_NEG: (i32, bool) = (-10i32).overflowing_abs();
const ABS_MIN: (i32, bool) = i32::min_value().overflowing_abs();

fn main() {
assert_eq!(ADD_A, (7, false));
assert_eq!(ADD_B, (0, true));
Expand All @@ -36,4 +40,8 @@ fn main() {

assert_eq!(NEG_A, (0, false));
assert_eq!(NEG_B, (1, true));

assert_eq!(ABS_POS, (10, false));
assert_eq!(ABS_NEG, (10, false));
assert_eq!(ABS_MIN, (i32::min_value(), true));
}
6 changes: 6 additions & 0 deletions src/test/ui/consts/const-int-sign-rpass.rs
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Expand Up @@ -11,6 +11,9 @@ const SIGNUM_POS: i32 = 10i32.signum();
const SIGNUM_NIL: i32 = 0i32.signum();
const SIGNUM_NEG: i32 = (-42i32).signum();

const ABS_A: i32 = 10i32.abs();
const ABS_B: i32 = (-10i32).abs();

fn main() {
assert!(NEGATIVE_A);
assert!(!NEGATIVE_B);
Expand All @@ -20,4 +23,7 @@ fn main() {
assert_eq!(SIGNUM_POS, 1);
assert_eq!(SIGNUM_NIL, 0);
assert_eq!(SIGNUM_NEG, -1);

assert_eq!(ABS_A, 10);
assert_eq!(ABS_B, 10);
}
8 changes: 8 additions & 0 deletions src/test/ui/consts/const-int-wrapping-rpass.rs
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Expand Up @@ -18,6 +18,10 @@ const SHR_B: u32 = 128u32.wrapping_shr(128);
const NEG_A: u32 = 5u32.wrapping_neg();
const NEG_B: u32 = 1234567890u32.wrapping_neg();

const ABS_POS: i32 = 10i32.wrapping_abs();
const ABS_NEG: i32 = (-10i32).wrapping_abs();
const ABS_MIN: i32 = i32::min_value().wrapping_abs();

fn main() {
assert_eq!(ADD_A, 255);
assert_eq!(ADD_B, 199);
Expand All @@ -36,4 +40,8 @@ fn main() {

assert_eq!(NEG_A, 4294967291);
assert_eq!(NEG_B, 3060399406);

assert_eq!(ABS_POS, 10);
assert_eq!(ABS_NEG, 10);
assert_eq!(ABS_MIN, i32::min_value());
}