parseFloat drops powers of 10 for 17+ digit significands with exponents 23..37 #3060

Bug description

parseFloat, Number.parseFloat, and f64.parse are off by up to 10^15 when the parsed integer significand exceeds 2^53 - 1 (17+ significant digits) and the remaining decimal exponent is in 23..37. This also breaks parseFloat(x.toString()) round-tripping in that range.

In scientific, when exp > 22 && exp <= 22 + 15, significandf is scaled and exp is set to 22 before checking significand <= 9007199254740991, so the fallback scaleup(significand, exp) receives the unscaled integer significand with exp = 22.

Steps to reproduce

export function a(): f64 { return parseFloat("1.2345678901234567e+40"); }
export function b(): f64 { return parseFloat("12345678901234567e24"); }
export function c(): f64 { return f64.parse("4.9999999999999998e+39"); }
export function d(): f64 { return parseFloat("100000000000000000000000000000000000000000"); }

Actual return values:

• a(): 1.2345678901234567e+38
• b(): 1.2345678901234567e+38
• c(): 5e+38
• d(): 1e+40

Expected return values (matching JavaScript parseFloat):

• a(): 1.2345678901234566e+40
• b(): 1.2345678901234566e+40
• c(): 4.9999999999999995e+39
• d(): 1e+41

AssemblyScript version

v0.28.20

Credit: https://ucsbplse.github.io/

Playground

https://www.assemblyscript.org/editor.html#IyFvcHRpbWl6ZT1zcGVlZCZydW50aW1lPWluY3JlbWVudGFsCmV4cG9ydCBmdW5jdGlvbiBhKCk6IGY2NCB7IHJldHVybiBwYXJzZUZsb2F0KCIxLjIzNDU2Nzg5MDEyMzQ1NjdlKzQwIik7IH0KZXhwb3J0IGZ1bmN0aW9uIGIoKTogZjY0IHsgcmV0dXJuIHBhcnNlRmxvYXQoIjEyMzQ1Njc4OTAxMjM0NTY3ZTI0Iik7IH0KZXhwb3J0IGZ1bmN0aW9uIGMoKTogZjY0IHsgcmV0dXJuIGY2NC5wYXJzZSgiNC45OTk5OTk5OTk5OTk5OTk4ZSszOSIpOyB9CmV4cG9ydCBmdW5jdGlvbiBkKCk6IGY2NCB7IHJldHVybiBwYXJzZUZsb2F0KCIxMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAiKTsgfQojIWh0bWwKPHByZSBpZD0ib3V0Ij48L3ByZT4KPHNjcmlwdCB0eXBlPSJtb2R1bGUiPgpjb25zdCBlID0gYXdhaXQgaW5zdGFudGlhdGUoYXdhaXQgY29tcGlsZSgpLCB7fSk7CmRvY3VtZW50LmdldEVsZW1lbnRCeUlkKCJvdXQiKS50ZXh0Q29udGVudCA9IFsKICAiYSgpID0gIiArIGUuYSgpICsgIiAgKGV4cGVjdGVkIDEuMjM0NTY3ODkwMTIzNDU2NmUrNDApIiwKICAiYigpID0gIiArIGUuYigpICsgIiAgKGV4cGVjdGVkIDEuMjM0NTY3ODkwMTIzNDU2NmUrNDApIiwKICAiYygpID0gIiArIGUuYygpICsgIiAgKGV4cGVjdGVkIDQuOTk5OTk5OTk5OTk5OTk5NWUrMzkpIiwKICAiZCgpID0gIiArIGUuZCgpICsgIiAgKGV4cGVjdGVkIDFlKzQxKSIKXS5qb2luKCJcbiIpOwo8L3NjcmlwdD4=

Click Run: each line shows the actual value next to the expected one; all four are off by a power of 10.