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c-b: Preserve the NaN sign bit in soft-float addition #1239

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c-b: Preserve the NaN sign bit in soft-float addition #1239

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c-b: Preserve the NaN sign bit in soft-float addition
HuzaifaAbdulRehman Jul 26, 2026
41f7526
test: fold NaN sign checks into float_sum
HuzaifaAbdulRehman Sep 2, 2026
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19 changes: 19 additions & 0 deletions builtins-test/tests/addsub.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -108,6 +108,25 @@ macro_rules! float_sum {
);
}
});

// Quiet sNaN by setting the top significand bit. `sub(x, nan)`
// flips that sign because sub is add with b's sign bit inverted.
let one = <$f as Float>::ONE;
for input in [
<$f as Float>::SNAN,
<$f as Float>::NAN,
<$f as Float>::NEG_SNAN,
<$f as Float>::NEG_NAN,
] {
let expected =
<$f>::from_bits(input.to_bits() | <$f as Float>::SIG_TOP_BIT);
assert_eq!($fn_add(input, one).to_bits(), expected.to_bits());
assert_eq!($fn_add(one, input).to_bits(), expected.to_bits());
assert_eq!($fn_sub(input, one).to_bits(), expected.to_bits());
let flipped =
<$f>::from_bits(expected.to_bits() ^ <$f as Float>::SIGN_MASK);
assert_eq!($fn_sub(one, input).to_bits(), flipped.to_bits());
Comment on lines +112 to +128

@tgross35 tgross35 Sep 2, 2026 •
edited by rustbot
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May as well check the entire matrix I suppose:

let qnan = <$f>::NAN;
let snan = <$f>::SNAN;
let qsnan = <$f>::QSNAN;
let neg_qnan = <$f>::NEG_NAN;
let neg_snan = <$f>::NEG_SNAN;
let neg_qsnan = <$f>::NEG_QSNAN;
let one = <$f>::ONE;

let nan_cases = [
    (qnan, qnan, qnan),
    (qnan, snan, qnan),
    (qnan, neg_qnan, qnan),
    (qnan, neg_snan, qnan),
    (qnan, one, qnan),

    (snan, qnan, qsnan),
    (snan, snan, qsnan),
    (snan, neg_qnan, qsnan),
    (snan, neg_snan, qsnan),
    (snan, one, qsnan),

    (neg_qnan, qnan, neg_qnan),
    (neg_qnan, snan, neg_qnan),
    (neg_qnan, neg_qnan, neg_qnan),
    (neg_qnan, neg_snan, neg_qnan),
    (neg_qnan, one, neg_qnan),

    (neg_snan, qnan, neg_qsnan),
    (neg_snan, snan, neg_qsnan),
    (neg_snan, neg_qnan, neg_qsnan),
    (neg_snan, neg_snan, neg_qsnan),
    (neg_snan, one, neg_qsnan),
];

for (x, y, expected) in add_nan_cases {
    // ...
}

for (x, y, expected) in sub_cases {
    // ...
}

QSNAN doesn't exist yet, adding it in #1314

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Comment on lines +121 to +128

@tgross35 tgross35 Sep 2, 2026 •
edited by rustbot
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Use assert_biteq!, #1314 makes that possible

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}
}
)*
}
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4 changes: 2 additions & 2 deletions compiler-builtins/src/float/add.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -33,11 +33,11 @@ where
if a_abs.wrapping_sub(one) >= inf_rep - one || b_abs.wrapping_sub(one) >= inf_rep - one {
// NaN + anything = qNaN
if a_abs > inf_rep {
return F::from_bits(a_abs | quiet_bit);
return F::from_bits(a_rep | quiet_bit);
}
// anything + NaN = qNaN
if b_abs > inf_rep {
return F::from_bits(b_abs | quiet_bit);
return F::from_bits(b_rep | quiet_bit);
}

if a_abs == inf_rep {
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