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strconv: add comment re extFloat errorscale

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Change-Id: I6f006ba72e1711ba2a24cd71552855ad88284eec
Reviewed-on: https://go-review.googlesource.com/c/go/+/227797
Reviewed-by: Rémy Oudompheng <remyoudompheng@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
This commit is contained in:
Nigel Tao 2020-04-10 11:01:14 +10:00
parent 64dcef3045
commit b10849fbb9
1 changed files with 23 additions and 1 deletions
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24
src/strconv/extfloat.go
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@ -231,8 +231,30 @@ var uint64pow10 = [...]uint64{
// float32 depending on flt.
func (f *extFloat) AssignDecimal(mantissa uint64, exp10 int, neg bool, trunc bool, flt *floatInfo) (ok bool) {
const uint64digits = 19
// Errors (in the "numerical approximation" sense, not the "Go's error
// type" sense) in this function are measured as multiples of 1/8 of a ULP,
// so that "1/2 of a ULP" can be represented in integer arithmetic.
//
// The C++ double-conversion library also uses this 8x scaling factor:
// https://github.com/google/double-conversion/blob/f4cb2384/double-conversion/strtod.cc#L291
// but this Go implementation has a bug, where it forgets to scale other
// calculations (further below in this function) by the same number. The
// C++ implementation does not forget:
// https://github.com/google/double-conversion/blob/f4cb2384/double-conversion/strtod.cc#L366
//
// Scaling the "errors" in the "is mant_extra in the range (halfway ±
// errors)" check, but not scaling the other values, means that we return
// ok=false (and fall back to a slower atof code path) more often than we
// could. This affects performance but not correctness.
//
// Longer term, we could fix the forgot-to-scale bug (and look carefully
// for correctness regressions; https://codereview.appspot.com/5494068
// landed in 2011), or replace this atof algorithm with a faster one (e.g.
// Ryu). Shorter term, this comment will suffice.
const errorscale = 8
errors := 0 // An upper bound for error, computed in errorscale*ulp.
errors := 0 // An upper bound for error, computed in ULP/errorscale.
if trunc {
// the decimal number was truncated.
errors += errorscale / 2