On Tue, May 9, 2017 at 2:10 PM, Ed Vielmetti <e...@packet.net> wrote: > > Consider the following code snippet, at https://play.golang.org/p/hUUF9O-p-k > > Using go 1.8.1 (same thing with current master branch) > > package main > func main() { > x := float64(-1) > println(uint64(x)) > println(int64(x)) > println(uint64(int64(x))) > } > > On ARM64: > 0 > -1 > 18446744073709551615 > > On x86: > > 18446744073709551615 > -1 > 18446744073709551615 > > > The comment from the ARM engineer: "It seems on arm we are using FCVTZU, for > rounding to unsigned, which is what the code example is doing. Except on x86 > they are rounding to signed int, and then dropping the sign." > > I'd like to know what the results are on other non-ARM, non-x86 platforms > (Power, Z, MIPS) to know what level of variety exists in this result and > then adjust the code path everywhere to do the right thing.
For the record, the spec says, in https://golang.org/ref/spec#Conversions: "In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent." That is the case that applies here: you are converting a negative floating point number to uint64, which can not represent a negative value, so the result is implementation-dependent. The conversion to int64 works, of course. And the conversion to int64 and then to uint64 succeeds in converting to int64, and when converting to uint64 follows a different rule: "When converting between integer types, if the value is a signed integer, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type's size." So, basically, don't convert a negative floating point number to an unsigned integer type. Ian -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
