XS rounds some numbers incorrectly

[gibson042]

Environment: XS 13.0.0 32 4

Description
Converting to numbers that require rounding from a literal or string is inconsistent with the ECMAScript specification.

Steps to Reproduce

[9007199254740993, +"9007199254740993"]

Actual behavior

[9007199254740994, 9007199254740994]

Expected behavior

[9007199254740992, 9007199254740992]

When rounding, ties should resolve to the even IEEE 754 significand: https://tc39.es/ecma262/multipage/ecmascript-data-types-and-values.html#number-value-for (for NumericLiteral via Evaluation → NumericValue → RoundMVResult → [because the decimal representation has 20 or fewer significant digits] 𝔽 → the Number value for x and for string via StringToNumber → StringNumericValue → RoundMVResult).

[phoddie]

We took a quick look into this issue. The conversion from string to number is done using strtod from the venerable dtoa library. Given the complexity of implementing that correctly, you'll understand that we try to avoid modifying that code. Some possibilities:

• A mismatch between the behavior required by Ecma-262 and what is implemented by strtod
• XS is using strtod incorrectly
• XS is configuring strtod incorrectly for this case

At one point other engines also used the same library. I'm not sure if that's still the case. If they do, that would be a good reference point to see how they are handling this case.