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Fix potential infinite loop in S/D ROTMG when the update factor computes to exact zero #6032

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Fix potential infinite loop in S/D ROTMG when the update factor computes to exact zero #6032

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22 changes: 21 additions & 1 deletion interface/rotmg.c
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Original file line number Diff line number Diff line change
Expand Up @@ -109,11 +109,31 @@ void CNAME(FLOAT *dd1, FLOAT *dd2, FLOAT *dx1, FLOAT dy1, FLOAT *dparam){
dh12 = dp2 / dp1;

du = ONE - dh12 * dh21;
if(du <= ZERO)
{
/* This can only happen through rounding: the
branch was chosen on ABS(dq1) > ABS(dq2),
which in exact arithmetic makes du positive.
Matches the reference BLAS srotmg.f and the
gonum implementation. */
dflag = -ONE;

dh11 = ZERO;
dh12 = ZERO;
dh21 = ZERO;
dh22 = ZERO;

*dd1 = ZERO;
*dd2 = ZERO;
*dx1 = ZERO;
}
else
{
dflag = ZERO;
*dd1 = *dd1 / du;
*dd2 = *dd2 / du;
*dx1 = *dx1 * du;

}
}
else
{
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