[Rd] Optimization bug when byte compiling with gcc 5.2.0 on windows

[luke-tierney at uiowa.edu]

I believe the issue is that on Windows the sqrt function when called
with a NaN does not return the same NaN, as it does on other platforms.
We have

#if (defined(_WIN32) || defined(_WIN64)) && defined(__GNUC__) && \
     __GNUC__ <= 4
# define R_sqrt(x) (ISNAN(x) ? x : sqrt(x))
#else
# define R_sqrt sqrt
#endif

for implementing the SQRT opcode. I suspect this came from Duncan
Murdoch; I don't know the reason for restricting to __GNUC__ <= 4. I
seem to recall that there are other places in the code where we have
similar workarounds but I can't find them at the moment.

The builtin sqrt always pretests with ISNAN.

Best,

luke



On Mon, 14 Sep 2015, Jeroen Ooms wrote:

> When building R-devel with gcc 5.2.0 (mingw-w64 v4) on Windows, make
> check fails reg-tests-1b.R at the following check:
>
>  x <- c(1:2, NA)
>  sx <- sd(x)
>  !is.nan(sx)
>
> Here 'sx' should be 'NA' but it is 'NaN'. It turns out this problem
> only appears when the function is byte compiled with optimization
> level 3:
>
>  mysd <- function (x, na.rm = FALSE)
>    sqrt(var(if (is.vector(x)) x else as.double(x), na.rm = na.rm))
>
>  mysd(x)
>  # [1] NA
>  compiler::cmpfun(mysd, list(optimize = 1L))(x)
>  # [1] NA
>  compiler::cmpfun(mysd, list(optimize = 2L))(x)
>  # [1] NA
>  compiler::cmpfun(mysd, list(optimize = 3L))(x)
>  # [1] NaN
>
> The problem does not appear with gcc 5.2.0 on Debian, and also not
> with gcc 4.9.3 on Windows. Where do I start debugging this? The
> disassembled output from the compiled functions is here:
> https://gist.github.com/jeroenooms/3206945a6db6680a9c5c
>
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>

-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu