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Fix incorrect rounding with subnormal/zero results of float multiplication

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beetrees 2024-06-30 22:02:41 +01:00
parent cea216b195
commit ffb31aee13
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GPG key ID: 8791BD754191EBD6
2 changed files with 12 additions and 20 deletions
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19
library/compiler-builtins/src/float/mul.rs
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@ -149,18 +149,13 @@ where
}
// Otherwise, shift the significand of the result so that the round
// bit is the high bit of productLo.
if shift < bits {
let sticky = product_low << (bits - shift);
product_low = product_high << (bits - shift) | product_low >> shift | sticky;
product_high >>= shift;
} else if shift < (2 * bits) {
let sticky = product_high << (2 * bits - shift) | product_low;
product_low = product_high >> (shift - bits) | sticky;
product_high = zero;
} else {
product_high = zero;
}
// bit is the high bit of `product_low`.
// Ensure one of the non-highest bits in `product_low` is set if the shifted out bit are
// not all zero so that the result is correctly rounded below.
let sticky = product_low << (bits - shift) != zero;
product_low =
product_high << (bits - shift) | product_low >> shift | (sticky as u32).cast();
product_high >>= shift;
} else {
// Result is normal before rounding; insert the exponent.
product_high &= significand_mask;

13
library/compiler-builtins/testcrate/tests/mul.rs
View file

@ -107,14 +107,11 @@ macro_rules! float_mul {
fuzz_float_2(N, |x: $f, y: $f| {
let mul0 = apfloat_fallback!($f, $apfloat_ty, $sys_available, Mul::mul, x, y);
let mul1: $f = $fn(x, y);
// multiplication of subnormals is not currently handled
if !(Float::is_subnormal(mul0) || Float::is_subnormal(mul1)) {
if !Float::eq_repr(mul0, mul1) {
panic!(
"{}({:?}, {:?}): std: {:?}, builtins: {:?}",
stringify!($fn), x, y, mul0, mul1
);
}
if !Float::eq_repr(mul0, mul1) {
panic!(
"{}({:?}, {:?}): std: {:?}, builtins: {:?}",
stringify!($fn), x, y, mul0, mul1
);
}
});
}