Does anyone know of a gcc compiler flag or option that would enable us to compile and successfully run 'C' code that will access mis-aligned integers or dioubles which runs on a SPARC workstation running Solaris.
The Sun compiler achieves this by means of the -misalign flag
The Sun -misalign flag was originally added for people migrating from Sun3 (m68k) systems to Sun4 (sparc) systems. m68k systems only align data to 2 bytes, whereas sparc systems align data to 4 bytes. So if you have existing data created on a m68k system, you might not be able to read it on a sparc system. -misalign adds the assumption that all pointers point to unaligned data, and thus we use byte loads and shift to access all data. This allows use of m68k created data on a sparc system.
The m68k to sparc switchover happened so long ago that people really shouldn't need the -misalign flag anymore. Use of this option will seriously hurt performance. You would be much better off if you use attribute packed and/or attribute aligned to make your data structures match your data.
I did once add an equivalent option to gcc at Cygnus a long time ago, but I never cleaned it up to the point where I felt confident that it would be accepted into the FSF gcc sources. It wasn't hard to add it. You just add another flags to MEM to indicate that they are unaligned. You set this flag when expanding an INDIRECT_REF in expand_expr. In emit_move_insn, if the source is a MEM with this flag set, then you call extract_bitfield. If the dest is a MEM with this flag set, then you call store_bitfield.
As a bonus, one can use this to implement unaligned pointers. Just add an attribute unaligned, and when we deference a pointer with this attribute set, we set the MEM unaligned flag.
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Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
