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[libc][math] Fix sqrtf128 implicit conversions. #127154

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Feb 15, 2025
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[libc][math] Fix sqrtf128 implicit conversions. #127154

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13 changes: 7 additions & 6 deletions libc/src/math/generic/sqrtf128.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -383,25 +383,26 @@ LLVM_LIBC_FUNCTION(float128, sqrtf128, (float128 x)) {
// 1 so just need to add shifted m and 1.
Int128 t1 = t0;
Int128 sgn = t0 >> 127; // sign of the difference
t1 -= (m << 1) ^ sgn;
t1 += 1 + sgn;
Int128 m_xor_sgn = static_cast<Int128>(m << 1) ^ sgn;
t1 -= m_xor_sgn;
t1 += Int128(1) + sgn;

Int128 sgn1 = t1 >> 127;
if (LIBC_UNLIKELY(sgn == sgn1)) {
t0 = t1;
v -= sgn << 15;
t1 -= (m << 1) ^ sgn;
t1 += 1 + sgn;
t1 -= m_xor_sgn;
t1 += Int128(1) + sgn;
}

if (t1 == 0) {
// 1 ulp offset brings again an exact root
v = (m - (2 * sgn + 1)) << 15;
v = (m - static_cast<UInt128>((sgn << 1) + 1)) << 15;
} else {
t1 += t0;
Int128 side = t1 >> 127; // select what is closer m or m+-1
v &= ~UInt128(0) << 15; // wipe the fractional bits
v -= ((sgn & side) | (~sgn & 1)) << (15 + side);
v -= ((sgn & side) | (~sgn & 1)) << (15 + static_cast<int>(side));
v |= 1; // add sticky bit since we cannot have an exact mid-point
// situation
}
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