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Avoid fmod when rounding layout results to the pixel grid #2041

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Merged
intergalacticspacehighway merged 1 commit into from
Oct 16, 2026
Merged

Avoid fmod when rounding layout results to the pixel grid #2041

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10 changes: 9 additions & 1 deletion yoga/algorithm/PixelGrid.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,8 @@
#include <yoga/algorithm/PixelGrid.h>
#include <yoga/numeric/Comparison.h>

#include <cmath>

namespace facebook::yoga {

namespace {
Expand All @@ -23,7 +25,13 @@ double roundValueToPixelGridDouble(
// We want to calculate `fractional` such that `floor(scaledValue) =
// scaledValue
// - fractional`.
double fractional = fmod(scaledValue, 1.0);
//
// `copysign(x - trunc(x), x)` is bit-identical to `fmod(x, 1.0)` for every
// double (including -0.0, infinities and NaN) but avoids the libm `fmod`
// loop, which is slow on some platforms (e.g. Android's bionic). This
// function runs six times per node on every layout pass.
double fractional =
std::copysign(scaledValue - std::trunc(scaledValue), scaledValue);
if (fractional < 0) {
// This branch is for handling negative numbers for `value`.
//
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