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bpo-41513: Make steps after the loop consistent with the loop body #22315

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Merged
rhettinger merged 1 commit into from
Sep 19, 2020
Merged

bpo-41513: Make steps after the loop consistent with the loop body #22315

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Make fractional value accumulation consistent inside and outside the … 
…loop.
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@rhettinger
rhettinger committed Sep 18, 2020
commit d38e1ac95060bd83a93007748dd988ae559f1bd8
9 changes: 4 additions & 5 deletions Modules/mathmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2550,8 +2550,7 @@ vector_norm(Py_ssize_t n, double *vec, double max, int found_nan)
assert(csum + lo * lo == csum);
frac_lo += lo * lo;
}
frac += frac_lo + frac_mid;
h = sqrt(csum - 1.0 + frac);
h = sqrt(csum - 1.0 + (frac_lo + frac_mid + frac));

x = h;
t = x * T27;
Expand All @@ -2569,15 +2568,15 @@ vector_norm(Py_ssize_t n, double *vec, double max, int found_nan)
assert(fabs(csum) >= fabs(x));
oldcsum = csum;
csum += x;
frac += (oldcsum - csum) + x;
frac_mid += (oldcsum - csum) + x;

x = -lo * lo;
assert(fabs(csum) >= fabs(x));
oldcsum = csum;
csum += x;
frac += (oldcsum - csum) + x;
frac_lo += (oldcsum - csum) + x;

x = csum - 1.0 + frac;
x = csum - 1.0 + (frac_lo + frac_mid + frac);
return (h + x / (2.0 * h)) / scale;
}
/* When max_e < -1023, ldexp(1.0, -max_e) overflows.
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