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Add bit twiddling

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This commit is contained in:
Pyry Kontio 2020-05-26 04:50:53 +09:00
parent bd68de89b5
commit 6973fd716b
2 changed files with 19 additions and 15 deletions
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18
src/libcore/num/f32.rs
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@ -857,7 +857,7 @@ impl f32 {
let mut left = self.to_bits() as i32;
let mut right = other.to_bits() as i32;
// In case of negatives, flip all the bits except the sign
// In case of negatives, flip all the bits expect the sign
// to achieve a similar layout as two's complement integers
//
// Why does this work? IEEE 754 floats consist of three fields:
@ -872,13 +872,15 @@ impl f32 {
// To easily compare the floats as signed integers, we need to
// flip the exponent and mantissa bits in case of negative numbers.
// We effectively convert the numbers to "two's complement" form.
if left < 0 {
// i32::MAX corresponds the bit pattern of "all ones except for the sign bit"
left ^= i32::MAX
};
if right < 0 {
right ^= i32::MAX
};
//
// To do the flipping, we construct a mask and XOR against it.
// We branchlessly calculate an "all-ones expect for the sign bit"
// mask from negative-signed values: right shifting sign-extends
// the integer, so we "fill" the mask with sign bits, and then
// convert to unsigned to push one more zero bit.
// On positive values, the mask is all zeros, so it's a no-op.
left ^= (((left >> 31) as u32) >> 1) as i32;
right ^= (((right >> 31) as u32) >> 1) as i32;
left.cmp(&right)
}

16
src/libcore/num/f64.rs
View file

@ -886,13 +886,15 @@ impl f64 {
// To easily compare the floats as signed integers, we need to
// flip the exponent and mantissa bits in case of negative numbers.
// We effectively convert the numbers to "two's complement" form.
if left < 0 {
// i64::MAX corresponds the bit pattern of "all ones expect for the sign bit"
left ^= i64::MAX
};
if right < 0 {
right ^= i64::MAX
};
//
// To do the flipping, we construct a mask and XOR against it.
// We branchlessly calculate an "all-ones expect for the sign bit"
// mask from negative-signed values: right shifting sign-extends
// the integer, so we "fill" the mask with sign bits, and then
// convert to unsigned to push one more zero bit.
// On positive values, the mask is all zeros, so it's a no-op.
left ^= (((left >> 63) as u64) >> 1) as i64;
right ^= (((right >> 63) as u64) >> 1) as i64;
left.cmp(&right)
}