cmd/gc: compiler must not accept p-notation floats in regular Go code

Question

cmd/gc: compiler must not accept p-notation floats in regular Go code

griesemer opened this issue 11 years ago · 12 comments

gc accepts the p-notation for floating point numbers:

mantissa "p" exponent

such as 12p-2, etc.

http://play.golang.org/p/xOixorUxUD

The spec doesn't mention them. gccgo and go/types don't support them.

a) we should either disallow them, or
b) we should support them with consistent syntax and document them.

Answer 1 · 2014-10-31T23:48:53.000Z

Comment 1:

I can go either way but I would be ok supporting them.
If we do, I propose one of these productions:
pfloat_lit = int_lit ( "p" | "P" ) [ "+" | "-" ] decimals .
or
pfloat_lit = decimal_lit ( "p" | "P" ) [ "+" | "-" ] decimals .
i.e., if we permit non-decimal mantissa, we should permit any integer notation.

Answer 2 · 2014-10-31T23:50:53.000Z

Comment 2:

Why does gc accept this? Is it needed for export data only?

Answer 3 · 2014-11-01T00:06:39.000Z

Comment 3:

It's plausible that it's an export data relict - I don't know of std lib code that needs
this.

Answer 4 · 2014-11-03T15:28:03.000Z

Comment 4:

It's for export data. It's not supposed to be generally available.

Answer 5 · 2014-11-03T15:29:27.000Z

Comment 5:

Will fix for Go 1.5. It's been there basically forever; no harm in leaving it in for Go
1.4.

Answer 6 · 2014-11-03T15:29:39.000Z

Comment 6:

Labels changed: removed documentation, languagechange.

Answer 7 · 2015-04-03T22:51:07.000Z

There are several related errors:

http://play.golang.org/p/VVK5tF24e2

p exponent
hex mantissa and p exponent
hex mantissa and e exponent interpreted as int

Answer 8 · 2015-05-27T18:50:46.000Z

What is this "export data"? I've been trying to figure out the conditions under which 'p' and 'P' etc should not be rejected?

Answer 9 · 2015-05-27T18:55:39.000Z

The export data is what compiler read from already compiled Go packages. They are mostly regular Go declarations with some extensions (hex float, argument escape tagging, etc) Open a Go .a file under pkg directory with a text editor and you will see.

Answer 10 · 2015-05-27T18:55:47.000Z

"Export data" is the textual (go-syntax like) representation of the package "objects" that are exported from the package. The export data of a package A is parsed when another package B imports A. The export data can be found in the object/archive file.

The p-format is used to read and write floating-point numbers quickly and accurately without the need to convert to a decimal exponent.

Answer 11 · 2015-05-27T19:11:36.000Z

@dr2chase see also the go/internal/gcimporter package and cmd/compile/internal/gc/export.go

Answer 12 · 2015-05-27T22:00:35.000Z

CL https://golang.org/cl/10450 mentions this issue.