rsz: Reduce error in lerp formula #7604
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Signed-off-by: Martin Povišer <[email protected]>
Signed-off-by: Martin Povišer <[email protected]>
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clang-tidy review says "All clean, LGTM! 👍" |
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Secure CI came back green |
maliberty
reviewed
Jun 17, 2025
| return slack - (ref_slack_ - slack) * 2e-6; | ||
| } | ||
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| float lerp(float a, float b, float t) |
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I purposely open-coded it so we can see the formula and reason about the rounding errors (though I'm not fully sure compiler cannot rewrite it in optimization)
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Note the std version handles more corner cases but perhaps they don't arise here.
template<typename _Fp>
constexpr _Fp
__lerp(_Fp __a, _Fp __b, _Fp __t) noexcept
{
if ((__a <= 0 && __b >= 0) || (__a >= 0 && __b <= 0))
return __t * __b + (1 - __t) * __a;
if (__t == 1)
return __b; // exact
// Exact at __t=0, monotonic except near __t=1,
// bounded, determinate, and consistent:
const _Fp __x = __a + __t * (__b - __a);
return (__t > 1) == (__b > __a)
? (__b < __x ? __x : __b)
: (__b > __x ? __x : __b); // monotonic near __t=1
}
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Interesting they have different cases. I assume that's just one way it may be implemented.
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They handle overflow for one. The core formula is similar.
maliberty
approved these changes
Jun 17, 2025
maliberty
approved these changes
Jun 17, 2025
maliberty
merged commit c394f51
into
The-OpenROAD-Project:master
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Adjust lerp formula to reduce the introduced rounding error and fix the failure seen at The-OpenROAD-Project/OpenROAD-flow-scripts#3242 (comment)