[Rd] configure.ac clock_gettime (and R_CHECK_FUNCS) incorrect?

[Martin Morgan]

When R is configured with CFLAGS=-O2, clock_gettime is included

$ nm src/main/libR.a | grep clock_gettime
                  U clock_gettime

whereas when configured with -O0 it is not

$ nm src/main/libR.a | grep clock_gettime
$

Similarly when built as a shared library the linker flags include or not -lrt, 
and ldd indicates dependency on librt or not.

This thread

   http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2012-December/005022.html

lead to the conclusion that the test for clock_gettime from configure.ac:1838 is 
flawed, because

R_CHECK_FUNCS([clock_gettime timespec_get], [#include <time.h>])
if test "${ac_cv_have_decl_clock_gettime}" = "yes"; then
AC_CHECK_LIB(rt, clock_gettime)
fi

generates test code, from config.log, which looks like

| int
| main ()
| {
| #ifndef clock_gettime
|   char *p = (char *) clock_gettime;
| #endif
|
|   ;
|   return 0;
| }

when compiled with CFLAGS=-O2, the assignment to p is optimized out and the test 
succeeds whether clock_gettime is present or not; the autoconf macro may add 
-lrt to the compile flags.

When compiled with CFLAGS=-O0, the test fails and autoconf does not try to add 
-lrt, even though that might be (is) necessary.

It seems that R_CHECK_FUNCS (R_CHECK_DECLS) in general and the logic of this 
specific test need revisiting?

$ bin/R --version
R Under development (unstable) (2013-03-16 r62282) -- "Unsuffered Consequences"
...

on

$ gcc --version
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

or

$ clang --version
Ubuntu clang version 3.0-6ubuntu3 (tags/RELEASE_30/final) (based on LLVM 3.0)
Target: x86_64-pc-linux-gnu
Thread model: posix

Martin
-- 
Computational Biology / Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N.
PO Box 19024 Seattle, WA 98109

Location: Arnold Building M1 B861
Phone: (206) 667-2793