[PATCH,fortran]: fix bind(c) common block alignment

[Christopher D. Rickett]

hi Tobias,

currently, the patch only changes the alignment if the common block is 
bind(c), which means the compiler should have already verified that the 
common block fields interop with C.

as to why we weren't seeing it before, i do not know.  the 
bind_c_coms.f90/bind_c_coms_driver.c illustrates on x86 linux that there 
is a difference in the alignment.  the generated assembly shows it, but 
the linker didn't complain.

if it is acceptable for all common blocks to choose a C compatible 
alignment, then the patch is trivial to change to do this.  is that what 
people prefer?

Chris

On Mon, 6 Aug 2007, Tobias Schlüter wrote:

> Christopher D. Rickett wrote:
>> as mentioned previously on the list by Jack Howarth, there is a mismatch in 
>> alignment with bind(c) common blocks.  the Fortran compiler simply sets the 
>> alignment of all common blocks to BIGGEST_ALIGN, while the C compiler tries 
>> to align based on the size of the fields in the interoperable C struct. 
>> the attached patch should fix the alignment difference by changing the 
>> alignment setting of bind(c) common blocks.
>
> How often does this change something?  It is a fairly widespread assumption 
> that COMMON blocks can be mapped to C structs (e.g. I recall there being an 
> example of doing this in the ROOT manual, and a quick websearch reveals lots 
> of pages that describe this as a fact).  I.e. that
>  COMMON /X/A,B,I(7),C
> maps to
>  struct { float a, b; int i[7]; float c; }
> etc.
> Since this seems to be really common, I'm surprised that we weren't hit by 
> this before.
>
> I think we should choose C compatible alignments everywhere possible.
>
> A testcase that triggers on i86 would also be a good idea.
>
> Cheers,
> - Tobi
>
>> bootstrapped and regtested on x86 linux with no new failures.
>> 
>> ChangeLog entry:
>> 
>> 2007-08-06  Christopher D. Rickett  <crickett@lanl.gov>
>>
>>     * trans-common.c (build_common_decl): Fix the alignment for
>>     BIND(C) common blocks.
>