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Further simplify float-to-int range checks 

We don't actually need to compute the `trunc()` value, as long as we can
figure out the right values for the exclusive range `(MIN-1, MAX+1)` to
measure the same truncation effect.
This commit is contained in:
Josh Stone 2018-03-13 13:38:17 -07:00
parent f0ed42b3bc
commit a4d234c253
1 changed files with 30 additions and 23 deletions
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49
src/cast.rs
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@ -220,18 +220,23 @@ macro_rules! impl_to_primitive_float_to_signed_int {
($f:ident : $( fn $method:ident -> $i:ident ; )*) => {$( ($f:ident : $( fn $method:ident -> $i:ident ; )*) => {$(
#[inline] #[inline]
fn $method(&self) -> Option<$i> { fn $method(&self) -> Option<$i> {
let t = self.trunc(); // round toward zero. // Float as int truncates toward zero, so we want to allow values
// MIN is a power of two, which we can cast and compare directly. // in the exclusive range `(MIN-1, MAX+1)`.
if t >= $i::MIN as $f { if size_of::<$f>() > size_of::<$i>() {
// The mantissa might not be able to represent all digits of MAX. // With a larger size, we can represent the range exactly.
let sig_bits = size_of::<$i>() as u32 * 8 - 1; const MIN_M1: $f = $i::MIN as $f - 1.0;
let max = if sig_bits > $f::MANTISSA_DIGITS { const MAX_P1: $f = $i::MAX as $f + 1.0;
let lost_bits = sig_bits - $f::MANTISSA_DIGITS; if *self > MIN_M1 && *self < MAX_P1 {
$i::MAX & !((1 << lost_bits) - 1) return Some(*self as $i);
}
} else { } else {
$i::MAX // We can't represent `MIN-1` exactly, but there's no fractional part
}; // at this magnitude, so we can just use a `MIN` inclusive boundary.
if t <= max as $f { const MIN: $f = $i::MIN as $f;
// We can't represent `MAX` exactly, but it will round up to exactly
// `MAX+1` (a power of two) when we cast it.
const MAX_P1: $f = $i::MAX as $f;
if *self >= MIN && *self < MAX_P1 {
return Some(*self as $i); return Some(*self as $i);
} }
} }
@ -244,17 +249,19 @@ macro_rules! impl_to_primitive_float_to_unsigned_int {
($f:ident : $( fn $method:ident -> $u:ident ; )*) => {$( ($f:ident : $( fn $method:ident -> $u:ident ; )*) => {$(
#[inline] #[inline]
fn $method(&self) -> Option<$u> { fn $method(&self) -> Option<$u> {
let t = self.trunc(); // round toward zero. // Float as int truncates toward zero, so we want to allow values
if t >= 0.0 { // in the exclusive range `(-1, MAX+1)`.
// The mantissa might not be able to represent all digits of MAX. if size_of::<$f>() > size_of::<$u>() {
let sig_bits = size_of::<$u>() as u32 * 8; // With a larger size, we can represent the range exactly.
let max = if sig_bits > $f::MANTISSA_DIGITS { const MAX_P1: $f = $u::MAX as $f + 1.0;
let lost_bits = sig_bits - $f::MANTISSA_DIGITS; if *self > -1.0 && *self < MAX_P1 {
$u::MAX & !((1 << lost_bits) - 1) return Some(*self as $u);
}
} else { } else {
$u::MAX // We can't represent `MAX` exactly, but it will round up to exactly
}; // `MAX+1` (a power of two) when we cast it.
if t <= max as $f { const MAX_P1: $f = $u::MAX as $f;
if *self > -1.0 && *self < MAX_P1 {
return Some(*self as $u); return Some(*self as $u);
} }
} }