[Rcpp-devel] Report of CRAN compilation problem and solution with architecture arm64

[Juan Domingo Esteve]

Keywords: compiler warning, package check, arm64 architectures

This is the first of two reports with CRAN check problems that I found in my package and
that affect only some particular architectures (in this case, arm64)

Problem description:

  When compiling a package with C++ source code using Rcpp in a Linux system,
  kernel 5.19.16-100, distribution Fedora 35, the generated package passed
  R CMD check --as-cran test, giving no compilation warnings.

  Nevertheless, the compilation in the CRAN server provoked a warning exclusively
  for the arm64 architecture (found mostly in recent Apple Macs):

  jmatrix.cpp:50:8: runtime error: load of misaligned address 0x7ffdb159b8c2 for type 'indextype', which requires 4 byte alignment
0x7ffdb159b8c2: note: pointer points here
  00 00  00 fa 06 00 00 00 08 00  00 00 07 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00

  This seems to be a particularity of the compiler used by the CRAN sever for this architecture, which does not like
  to have 4-byte variables (indextype is a typedef I use for unsigned long) aligned to non-multiple of 4 addresses.

  The original code which provoked this error was:

  // header was a pointer to unsigned char and header+2 points to safely allocated memory
  indextype *t;
  t=(indextype *)(header+2);

The solution was:

  Assuming that in all platforms the compiler will align properly simple variable declarations,
  we will use a variable instead of a pointer:

  indextype t=0;              // This should be aligned, I hope..
  unsigned char *t1;
  t1=header+2;
  memcpy((void *)&t,(void *)t1,size_t(sizeof(indextype)));

  Another possible solution would have involved the use of the libc function posix_memalign, but after
  a web search it seems that it is not widely available in Windows and OSX.

  Even this is a really rare situation, I have preferred to document it, just in case anyone else may benefit of the information.

     Juan

-- 
________________________________________________________________
Juan Domingo Esteve
Dept. of Informatics, School of Engineering
University of Valencia
Avda. de la Universidad, s/n.
         46100-Burjasot (Valencia)
            SPAIN

Telephone:      +34-963543572
Fax:            +34-963543550
email:  Juan.Domingo at uv.es
________________________________________________________________