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Make sNaN removal code tolerate different sNaN encodings #43025

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Jul 4, 2017
Merged

Make sNaN removal code tolerate different sNaN encodings #43025

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24 changes: 16 additions & 8 deletions src/libstd/f32.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1131,13 +1131,16 @@ impl f32 {
#[inline]
pub fn from_bits(mut v: u32) -> Self {
const EXP_MASK: u32 = 0x7F800000;
const QNAN_MASK: u32 = 0x00400000;
const FRACT_MASK: u32 = 0x007FFFFF;
if v & EXP_MASK == EXP_MASK && v & FRACT_MASK != 0 {
// If we have a NaN value, we
// convert signaling NaN values to quiet NaN
// by setting the the highest bit of the fraction
v |= QNAN_MASK;
// While IEEE 754-2008 specifies encodings for quiet NaNs
// and signaling ones, certain MIPS and PA-RISC
// CPUs treat signaling NaNs differently.
// Therefore to be safe, we pass a known quiet NaN
// if v is any kind of NaN.
// The check above only assumes IEEE 754-1985 to be
// valid.
v = unsafe { ::mem::transmute(NAN) };
}
unsafe { ::mem::transmute(v) }
}
Expand Down Expand Up @@ -1732,8 +1735,15 @@ mod tests {
}
#[test]
fn test_snan_masking() {
// NOTE: this test assumes that our current platform
// implements IEEE 754-2008 that specifies the difference
// in encoding of quiet and signaling NaNs.
// If you are porting Rust to a platform that does not
// implement IEEE 754-2008 (but e.g. IEEE 754-1985, which
// only says that "Signaling NaNs shall be reserved operands"
// but doesn't specify the actual setup), feel free to
// cfg out this test.
let snan: u32 = 0x7F801337;
const PAYLOAD_MASK: u32 = 0x003FFFFF;
const QNAN_MASK: u32 = 0x00400000;
let nan_masked_fl = f32::from_bits(snan);
let nan_masked = nan_masked_fl.to_bits();
Expand All @@ -1742,7 +1752,5 @@ mod tests {
// Ensure that we have a quiet NaN
assert_ne!(nan_masked & QNAN_MASK, 0);
assert!(nan_masked_fl.is_nan());
// Ensure the payload wasn't touched during conversion
assert_eq!(nan_masked & PAYLOAD_MASK, snan & PAYLOAD_MASK);
}
}
13 changes: 8 additions & 5 deletions src/libstd/f64.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1046,13 +1046,16 @@ impl f64 {
#[inline]
pub fn from_bits(mut v: u64) -> Self {
const EXP_MASK: u64 = 0x7FF0000000000000;
const QNAN_MASK: u64 = 0x0001000000000000;
const FRACT_MASK: u64 = 0x000FFFFFFFFFFFFF;
if v & EXP_MASK == EXP_MASK && v & FRACT_MASK != 0 {
// If we have a NaN value, we
// convert signaling NaN values to quiet NaN
// by setting the the highest bit of the fraction
v |= QNAN_MASK;
// While IEEE 754-2008 specifies encodings for quiet NaNs
// and signaling ones, certain MIPS and PA-RISC
// CPUs treat signaling NaNs differently.
// Therefore to be safe, we pass a known quiet NaN
// if v is any kind of NaN.
// The check above only assumes IEEE 754-1985 to be
// valid.
v = unsafe { ::mem::transmute(NAN) };
}
unsafe { ::mem::transmute(v) }
}
Expand Down