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Rearrange the pipeline of pow to gain efficiency #74367

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Jul 24, 2020
Merged

Rearrange the pipeline of pow to gain efficiency #74367

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87 changes: 44 additions & 43 deletions src/libcore/num/mod.rs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1103,6 +1103,9 @@ $EndFeature, "
without modifying the original"]
#[inline]
pub const fn checked_pow(self, mut exp: u32) -> Option<Self> {
if exp == 0 {
return Some(1);
}
let mut base = self;
let mut acc: Self = 1;

Expand All @@ -1113,15 +1116,11 @@ $EndFeature, "
exp /= 2;
base = try_opt!(base.checked_mul(base));
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
acc = try_opt!(acc.checked_mul(base));
}

Some(acc)
Some(try_opt!(acc.checked_mul(base)))
}
}

Expand Down Expand Up @@ -1631,6 +1630,9 @@ $EndFeature, "
without modifying the original"]
#[inline]
pub const fn wrapping_pow(self, mut exp: u32) -> Self {
if exp == 0 {
return 1;
}
let mut base = self;
let mut acc: Self = 1;

Expand All @@ -1642,14 +1644,11 @@ $EndFeature, "
base = base.wrapping_mul(base);
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
acc = acc.wrapping_mul(base);
}

acc
acc.wrapping_mul(base)
}
}

Expand Down Expand Up @@ -1999,6 +1998,9 @@ $EndFeature, "
without modifying the original"]
#[inline]
pub const fn overflowing_pow(self, mut exp: u32) -> (Self, bool) {
if exp == 0 {
return (1,false);
}
let mut base = self;
let mut acc: Self = 1;
let mut overflown = false;
Expand All @@ -2017,16 +2019,13 @@ $EndFeature, "
overflown |= r.1;
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
r = acc.overflowing_mul(base);
acc = r.0;
overflown |= r.1;
}

(acc, overflown)
r = acc.overflowing_mul(base);
r.1 |= overflown;
r
}
}

Expand All @@ -2050,6 +2049,9 @@ $EndFeature, "
#[inline]
#[rustc_inherit_overflow_checks]
pub const fn pow(self, mut exp: u32) -> Self {
if exp == 0 {
return 1;
}
let mut base = self;
let mut acc = 1;

Expand All @@ -2061,14 +2063,11 @@ $EndFeature, "
base = base * base;
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
acc = acc * base;
}

acc
acc * base
}
}

Expand Down Expand Up @@ -3306,6 +3305,9 @@ assert_eq!(", stringify!($SelfT), "::MAX.checked_pow(2), None);", $EndFeature, "
without modifying the original"]
#[inline]
pub const fn checked_pow(self, mut exp: u32) -> Option<Self> {
if exp == 0 {
return Some(1);
}
let mut base = self;
let mut acc: Self = 1;

Expand All @@ -3317,14 +3319,12 @@ assert_eq!(", stringify!($SelfT), "::MAX.checked_pow(2), None);", $EndFeature, "
base = try_opt!(base.checked_mul(base));
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
acc = try_opt!(acc.checked_mul(base));
}

Some(acc)
Some(try_opt!(acc.checked_mul(base)))
}
}

Expand Down Expand Up @@ -3715,6 +3715,9 @@ assert_eq!(3u8.wrapping_pow(6), 217);", $EndFeature, "
without modifying the original"]
#[inline]
pub const fn wrapping_pow(self, mut exp: u32) -> Self {
if exp == 0 {
return 1;
}
let mut base = self;
let mut acc: Self = 1;

Expand All @@ -3726,14 +3729,11 @@ assert_eq!(3u8.wrapping_pow(6), 217);", $EndFeature, "
base = base.wrapping_mul(base);
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
acc = acc.wrapping_mul(base);
}

acc
acc.wrapping_mul(base)
}
}

Expand Down Expand Up @@ -4040,6 +4040,9 @@ assert_eq!(3u8.overflowing_pow(6), (217, true));", $EndFeature, "
without modifying the original"]
#[inline]
pub const fn overflowing_pow(self, mut exp: u32) -> (Self, bool) {
if exp == 0{
return (1,false);
}
let mut base = self;
let mut acc: Self = 1;
let mut overflown = false;
Expand All @@ -4058,16 +4061,14 @@ assert_eq!(3u8.overflowing_pow(6), (217, true));", $EndFeature, "
overflown |= r.1;
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
r = acc.overflowing_mul(base);
acc = r.0;
overflown |= r.1;
}
r = acc.overflowing_mul(base);
r.1 |= overflown;

(acc, overflown)
r
}
}

Expand All @@ -4088,6 +4089,9 @@ Basic usage:
#[inline]
#[rustc_inherit_overflow_checks]
pub const fn pow(self, mut exp: u32) -> Self {
if exp == 0 {
return 1;
}
let mut base = self;
let mut acc = 1;

Expand All @@ -4099,14 +4103,11 @@ Basic usage:
base = base * base;
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
acc = acc * base;
}

acc
acc * base
}
}

Expand Down
33 changes: 32 additions & 1 deletion src/libcore/tests/num/int_macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -255,12 +255,43 @@ macro_rules! int_module {
#[test]
fn test_pow() {
let mut r = 2 as $T;

assert_eq!(r.pow(2), 4 as $T);
assert_eq!(r.pow(0), 1 as $T);
assert_eq!(r.wrapping_pow(2), 4 as $T);
assert_eq!(r.wrapping_pow(0), 1 as $T);
assert_eq!(r.checked_pow(2), Some(4 as $T));
assert_eq!(r.checked_pow(0), Some(1 as $T));
assert_eq!(r.overflowing_pow(2), (4 as $T, false));
assert_eq!(r.overflowing_pow(0), (1 as $T, false));
assert_eq!(r.saturating_pow(2), 4 as $T);
assert_eq!(r.saturating_pow(0), 1 as $T);

r = MAX;
// use `^` to represent .pow() with no overflow.
// if itest::MAX == 2^j-1, then itest is a `j` bit int,
// so that `itest::MAX*itest::MAX == 2^(2*j)-2^(j+1)+1`,
// thussaturating_pow the overflowing result is exactly 1.
assert_eq!(r.wrapping_pow(2), 1 as $T);
assert_eq!(r.checked_pow(2), None);
assert_eq!(r.overflowing_pow(2), (1 as $T, true));
assert_eq!(r.saturating_pow(2), MAX);
//test for negative exponent.
r = -2 as $T;
assert_eq!(r.pow(2), 4 as $T);
assert_eq!(r.pow(3), -8 as $T);
assert_eq!(r.pow(0), 1 as $T);
assert_eq!(r.wrapping_pow(2), 4 as $T);
assert_eq!(r.wrapping_pow(3), -8 as $T);
assert_eq!(r.wrapping_pow(0), 1 as $T);
assert_eq!(r.checked_pow(2), Some(4 as $T));
assert_eq!(r.checked_pow(3), Some(-8 as $T));
assert_eq!(r.checked_pow(0), Some(1 as $T));
assert_eq!(r.overflowing_pow(2), (4 as $T, false));
assert_eq!(r.overflowing_pow(3), (-8 as $T, false));
assert_eq!(r.overflowing_pow(0), (1 as $T, false));
assert_eq!(r.saturating_pow(2), 4 as $T);
assert_eq!(r.saturating_pow(3), -8 as $T);
assert_eq!(r.saturating_pow(0), 1 as $T);
}
}
};
Expand Down
25 changes: 25 additions & 0 deletions src/libcore/tests/num/uint_macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -184,6 +184,31 @@ macro_rules! uint_module {
assert_eq!($T::from_str_radix("Z", 10).ok(), None::<$T>);
assert_eq!($T::from_str_radix("_", 2).ok(), None::<$T>);
}

#[test]
fn test_pow() {
let mut r = 2 as $T;
assert_eq!(r.pow(2), 4 as $T);
assert_eq!(r.pow(0), 1 as $T);
assert_eq!(r.wrapping_pow(2), 4 as $T);
assert_eq!(r.wrapping_pow(0), 1 as $T);
assert_eq!(r.checked_pow(2), Some(4 as $T));
assert_eq!(r.checked_pow(0), Some(1 as $T));
assert_eq!(r.overflowing_pow(2), (4 as $T, false));
assert_eq!(r.overflowing_pow(0), (1 as $T, false));
assert_eq!(r.saturating_pow(2), 4 as $T);
assert_eq!(r.saturating_pow(0), 1 as $T);

r = MAX;
// use `^` to represent .pow() with no overflow.
// if itest::MAX == 2^j-1, then itest is a `j` bit int,
// so that `itest::MAX*itest::MAX == 2^(2*j)-2^(j+1)+1`,
// thussaturating_pow the overflowing result is exactly 1.
assert_eq!(r.wrapping_pow(2), 1 as $T);
assert_eq!(r.checked_pow(2), None);
assert_eq!(r.overflowing_pow(2), (1 as $T, true));
assert_eq!(r.saturating_pow(2), MAX);
}
}
};
}