[Rd] formatting issue with gcc 9.3.0 on Ubuntu on WSL2

[Dirk Eddelbuettel]

On 17 November 2020 at 12:34, Bill Dunlap wrote:
| I just got a new Windows laptop (i7, 10th generation CPU), installed
| 'Windows Subsystem for Linux 2' and then installed Ubuntu 20.04 and
| used 'apt-get install' to install packages that the R build seems
| to require.  In particular, I am using gcc version 9.3.0.   The
| build went without a hitch but the tests showed that deparse(1e-16)
| produced "1.00000000000000e-16" instead of the expected "1e-16".
| 
| It looks like the problem is in src/main/format.c:scientific().  The
| lowest two+ bytes in the fractional part of the long double (80-bit)
| return value of powl(10.0L, -30L), seem to be corrupted.  I made a
| standalong program to test powl and saw no problem - it gives the
| same results for the fractional part as bc does.
| 
|         bc: A2425FF7 5E14FC31 A125...
| standalone: 22425FF7 5E14FC32
|          R: 22425FF7 5E151800
| 
| There are lots of other small numbers with the same problem:
| 
| 
|                              > grep(value=TRUE, "0e",
| vapply((1+(0:10000)/1000)*1e-15, deparse, ""))
| [1] "8.56000000000000e-15" "8.71700000000000e-15" "8.77800000000000e-15"
| [4] "8.93500000000000e-15" "9.50800000000000e-15" "9.83800000000000e-15"
| [7] "9.89900000000000e-15" "9.93400000000000e-15" "9.99500000000000e-15"
| > str(grep(value=TRUE, "0e", vapply((1+(0:10000)/1000)*1e-14, deparse, "")))
|  chr [1:295] "8.00200000000000e-14" "8.00500000000000e-14" ...
| 
| Has anyone else seen this?  I am wondering if this is an oddity in WSL2
| 
|                              or Ubuntu's gcc-9.3.0.

Plain Ubuntu 20.04.1 here, current. No issue:

  > str(grep(value=TRUE, "0e", vapply((1+(0:10000)/1000)*1e-14, deparse, "")))
   chr(0) 
  > 

I made sure to start R as `R --vanilla` to not have anything in my dotfiles
affect printing.

Dirk

-- 
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