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Warning: this log message is not for the weak. Read at your own
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The problem was that we had several variables in Fortran common blocks
of various types, but their C counterparts were all of a type
equivalent to a fortran double complex. This didn't seem to matter
for the compilers that we tested, but we never tested static builds
(which is where this problem seems to occur, at least with the Intel
compiler: the linker compilains that the variable in the common block
in the user's .o file was of one size/alignment but the one in the C
library was a different size/alignment).
So this patch fixes the sizes/types of the Fortran common block
variables and their corresponding C instantiations to be of the same
sizes/types.
But wait, there's more.
We recently introduced a fix for the OSX linker where some C versions
of the fortran common block variables (e.g.,
_ompi_fortran_status_ignore) were not being found when linking
ompi_info (!). Further research shows that the code path for
ompi_info to require ompi_fortran_status_ignore is, unfortunately,
necessary (a quirk of how various components pull in different
portions of the code base -- nothing in ompi_info itself requires
fortran or MPI knowledge, of course).
Hence, the real problem was that there was no code path from ompi_info
to the portion of the code base where the C globals corresponding to
the Fortran common block variables were instantiated. This is because
the OSX linker does not automatically pull in .o files that only
contain unintialized global variables; the OSX linker typically only
pulls in a .o file from a library if it either has a function that is
used or have a global variable that is initialized (that's the short
version; lots of details and corner cases omitted). Hence, we changed
the global C variables corresponding to the fortran common blocks to
be initialized, thereby causing the OSX linker to pull them in
automatically -- problem solved. At the same time, we moved the
constants to another .c file with a function, just for good measure.
However, this didn't really solve the problem:
1. The function in the file with the C versions of the fortran common
block variables (ompi/mpi/f77/test_constants_f.c) did not have a
code path that was reachable from ompi_info, so the only reason
that the constants were found (on OSX) was because they were
initialized in the global scope (i.e., causing the OSX compiler to
pull in that .o file).
2. Initializing these variable in the global scope causes problems for
some linkers where -- once all the size/type problems mentioned
above were fixed -- the alignments of fortran common blocks and C
global variables do not match (even though the types of the Fortran
and C variables match -- wow!). Hence, initializing the C
variables would not necessarily match the alignment of what Fortran
expected, and the linker would issue a warning (i.e., the alignment
warnings referenced in the original post).
The solution is two-fold:
1. Move the Fortran variables from test_constants_f.c to
ompi/mpi/runtime/ompi_mpi_init.c where there are other global
constants that *are* initialized (that had nothing to do with
fortran, so the alignment issues described above are not a factor),
and therefore all linkers (including the OSX linker) will pull in
this .o file and find all the symbols that it needs.
2. Do not initialize the C variables corresponding to the Fortran
common blocks in the global scope. Indeed, never initialize them
at all (because we never need their *values* - we only check for
their *locations*). Since nothing is ever written to these
variables (particularly in the global scope), the linker does not
see any alignment differences during initialization, but does make
both the C and Fortran variables have the same addresses (this
method has been working in LAM/MPI for over a decade).
There were some comments here in the OMPI code base and in the LAM
code base that stated/implied that C variables corresponding to
Fortran common blocks had to have the same alignment as the Fortran
common blocks (i.e., 16). There were attempts in both code bases to
ensure that this was true. However, the attempts were wrong (in both
code bases), and I have now read enough Fortran compiler documentation
to convince myself that matching alignments is not required (indeed,
it's beyond our control). As long as C variables corresponding to
Fortran common blocks are not initialized in the global scope, the
linker will "figure it out" and adjust the alignment to whatever is
required (i.e., the greater of the alignments). Specifically (to
counter comments that no longer exist in the OMPI code base but still
exist in the LAM code base):
- there is no need to make attempts to specially align C variables
corresponding to Fortran common blocks
- the types and sizes of C variables corresponding to Fortran common
blocks should match, but do not need to be on any particular
alignment
Finally, as a side effect of this effort, I found a bunch of
inconsistencies with the intent of status/array_of_statuses
parameters. For all the functions that I modified they should be
"out" (not inout).
This commit was SVN r11057.
520147f209