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Revert to original loop for const pow exponents
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Give LLVM the for original, optimizable loop in pow and wrapped_pow
functions in the case when the exponent is statically known.
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@mzabaluev
mzabaluev committed Aug 13, 2024
1 parent 2f23534 commit ac88b33
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Showing 2 changed files with 110 additions and 160 deletions.
135 changes: 55 additions & 80 deletions library/core/src/num/int_macros.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2174,54 +2174,41 @@ macro_rules! int_impl {
#[inline]
#[rustc_allow_const_fn_unstable(is_val_statically_known)]
pub const fn wrapping_pow(self, mut exp: u32) -> Self {
if exp == 0 {
return 1;
}
let mut base = self;
let mut acc: Self = 1;

if intrinsics::is_val_statically_known(exp) {
// Unroll multiplications for small exponent values.
// This gives the optimizer a way to efficiently inline call sites
// for the most common use cases with constant exponents.
// Currently, LLVM is unable to unroll the loop below.
match exp {
0 => return 1,
1 => return base,
2 => return base.wrapping_mul(base),
3 => {
let squared = base.wrapping_mul(base);
return squared.wrapping_mul(base);
}
4 => {
let squared = base.wrapping_mul(base);
return squared.wrapping_mul(squared);
while exp > 1 {
if (exp & 1) == 1 {
acc = acc.wrapping_mul(base);
}
5 => {
let squared = base.wrapping_mul(base);
return squared.wrapping_mul(squared).wrapping_mul(base);
}
6 => {
let cubed = base.wrapping_mul(base).wrapping_mul(base);
return cubed.wrapping_mul(cubed);
}
_ => {}
exp /= 2;
base = base.wrapping_mul(base);
}
} else {
if exp == 0 {
return 1;
}
}
debug_assert!(exp != 0);

let mut acc: Self = 1;

loop {
if (exp & 1) == 1 {
acc = acc.wrapping_mul(base);
// since exp!=0, finally the exp must be 1.
if exp == 1 {
return acc;
// 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.
acc.wrapping_mul(base)
} else {
// This is faster than the above when the exponent is not known
// at compile time. We can't use the same code for the constant
// exponent case because LLVM is currently unable to unroll
// this loop.
loop {
if (exp & 1) == 1 {
acc = acc.wrapping_mul(base);
// since exp!=0, finally the exp must be 1.
if exp == 1 {
return acc;
}
}
exp /= 2;
base = base.wrapping_mul(base);
}
exp /= 2;
base = base.wrapping_mul(base);
}
}

Expand Down Expand Up @@ -2753,54 +2740,42 @@ macro_rules! int_impl {
#[rustc_inherit_overflow_checks]
#[rustc_allow_const_fn_unstable(is_val_statically_known)]
pub const fn pow(self, mut exp: u32) -> Self {
if exp == 0 {
return 1;
}
let mut base = self;
let mut acc = 1;

if intrinsics::is_val_statically_known(exp) {
// Unroll multiplications for small exponent values.
// This gives the optimizer a way to efficiently inline call sites
// for the most common use cases with constant exponents.
// Currently, LLVM is unable to unroll the loop below.
match exp {
0 => return 1,
1 => return base,
2 => return base * base,
3 => {
let squared = base * base;
return squared * base;
}
4 => {
let squared = base * base;
return squared * squared;
while exp > 1 {
if (exp & 1) == 1 {
acc = acc * base;
}
5 => {
let squared = base * base;
return squared * squared * base;
}
6 => {
let cubed = base * base * base;
return cubed * cubed;
}
_ => {}
exp /= 2;
base = base * base;
}
} else {
if exp == 0 {
return 1;
}
}
debug_assert!(exp != 0);

let mut acc = 1;

loop {
if (exp & 1) == 1 {
acc = acc * base;
// since exp!=0, finally the exp must be 1.
if exp == 1 {
return acc;
// 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.
acc * base
} else {
// This is faster than the above when the exponent is not known
// at compile time. We can't use the same code for the constant
// exponent case because LLVM is currently unable to unroll
// this loop.
loop {
if (exp & 1) == 1 {
acc = acc * base;
// since exp!=0, finally the exp must be 1.
if exp == 1 {
return acc;
}
}
exp /= 2;
base = base * base;
}
exp /= 2;
base = base * base;
}
}

Expand Down
135 changes: 55 additions & 80 deletions library/core/src/num/uint_macros.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2050,54 +2050,41 @@ macro_rules! uint_impl {
#[inline]
#[rustc_allow_const_fn_unstable(is_val_statically_known)]
pub const fn wrapping_pow(self, mut exp: u32) -> Self {
if exp == 0 {
return 1;
}
let mut base = self;
let mut acc: Self = 1;

if intrinsics::is_val_statically_known(exp) {
// Unroll multiplications for small exponent values.
// This gives the optimizer a way to efficiently inline call sites
// for the most common use cases with constant exponents.
// Currently, LLVM is unable to unroll the loop below.
match exp {
0 => return 1,
1 => return base,
2 => return base.wrapping_mul(base),
3 => {
let squared = base.wrapping_mul(base);
return squared.wrapping_mul(base);
}
4 => {
let squared = base.wrapping_mul(base);
return squared.wrapping_mul(squared);
while exp > 1 {
if (exp & 1) == 1 {
acc = acc.wrapping_mul(base);
}
5 => {
let squared = base.wrapping_mul(base);
return squared.wrapping_mul(squared).wrapping_mul(base);
}
6 => {
let cubed = base.wrapping_mul(base).wrapping_mul(base);
return cubed.wrapping_mul(cubed);
}
_ => {}
exp /= 2;
base = base.wrapping_mul(base);
}
} else {
if exp == 0 {
return 1;
}
}
debug_assert!(exp != 0);

let mut acc: Self = 1;

loop {
if (exp & 1) == 1 {
acc = acc.wrapping_mul(base);
// since exp!=0, finally the exp must be 1.
if exp == 1 {
return acc;
// 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.
acc.wrapping_mul(base)
} else {
// This is faster than the above when the exponent is not known
// at compile time. We can't use the same code for the constant
// exponent case because LLVM is currently unable to unroll
// this loop.
loop {
if (exp & 1) == 1 {
acc = acc.wrapping_mul(base);
// since exp!=0, finally the exp must be 1.
if exp == 1 {
return acc;
}
}
exp /= 2;
base = base.wrapping_mul(base);
}
exp /= 2;
base = base.wrapping_mul(base);
}
}

Expand Down Expand Up @@ -2578,54 +2565,42 @@ macro_rules! uint_impl {
#[rustc_inherit_overflow_checks]
#[rustc_allow_const_fn_unstable(is_val_statically_known)]
pub const fn pow(self, mut exp: u32) -> Self {
if exp == 0 {
return 1;
}
let mut base = self;
let mut acc = 1;

if intrinsics::is_val_statically_known(exp) {
// Unroll multiplications for small exponent values.
// This gives the optimizer a way to efficiently inline call sites
// for the most common use cases with constant exponents.
// Currently, LLVM is unable to unroll the loop below.
match exp {
0 => return 1,
1 => return base,
2 => return base * base,
3 => {
let squared = base * base;
return squared * base;
}
4 => {
let squared = base * base;
return squared * squared;
while exp > 1 {
if (exp & 1) == 1 {
acc = acc * base;
}
5 => {
let squared = base * base;
return squared * squared * base;
}
6 => {
let cubed = base * base * base;
return cubed * cubed;
}
_ => {}
exp /= 2;
base = base * base;
}
} else {
if exp == 0 {
return 1;
}
}
debug_assert!(exp != 0);

let mut acc = 1;

loop {
if (exp & 1) == 1 {
acc = acc * base;
// since exp!=0, finally the exp must be 1.
if exp == 1 {
return acc;
// 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.
acc * base
} else {
// This is faster than the above when the exponent is not known
// at compile time. We can't use the same code for the constant
// exponent case because LLVM is currently unable to unroll
// this loop.
loop {
if (exp & 1) == 1 {
acc = acc * base;
// since exp!=0, finally the exp must be 1.
if exp == 1 {
return acc;
}
}
exp /= 2;
base = base * base;
}
exp /= 2;
base = base * base;
}
}

Expand Down

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