Re: Fortran's Equivalence

[mfb@mbunix.mitre.org (Michael F Brenner)]

The requirements for poratability and speed might be opposing each other
a little, and some balance might be required. One possible compromise is
to use the Ada 95 SHIFT capababilities. To get at a bit string, shift it
and AND it, similar to what the FORTRAN generated code is doing in this
case. You are right, however, that address overlays are almost always
faster in execution time than any operation in a HOL, unless the HOL
optimizes the operation away. The portability problem comes in because
the Ada-95 manual did not standardize a bit numbering in the word, nor
does it standardize a pragma to tell which bit is zero in a portable
manner. Had it done so, then about half of all bit field applications
would have been portable using address overlays. As it is, true
portability (without a changing global flag telling where bit one is), is
difficult to accomplish without using specific operations (like SHIFT),
which have effects on performance (both speed and logically overspecified
algorithms).