Register variables...

[maxwell at speedy.DEC]

It's a  shame  that  we're  all  too  often  hampered  by  the  lack of good
optimizing compilers.

The only  reason that K&R's C *has* register variables in the first place is
because their  original  C  compiler  (and  most  later ones) can't generate
reasonable  code without (and often even with) them. These compilers require
'hints'  about  variable  use.  The trouble is, many compilers take the hint
indiscriminatly,  without  regard  for  the  program itself. (The variety of
assignment operators are there for the same reason).

The 'register'  storage  class  qualifier should generally be *ignored* by a
good  compiler:  the  compiler  should  place values (and not just variables
necessarily, but common subexpressions as well) in registers as it sees fit.
E.g. a  compiler  can  realize  that  a  variable  is used inside a loop and
generate code accordingly, etc.

Not to  blow  DEC's  horn  too  loudly,  but  the VAX C compiler *is* a good
compiler,  at  least  by  this  definition.  Declaring a variable 'register'
increases  the likelyhood of that variable getting a register (increases its
priority); but  generally  the compiler keeps track of variable usage (up to
32  of them) and generates code according to their use. 'register' variables
are otherwise treated *exactly* like regular 'auto' ones.

For those who might be interested, the compiler technology used by the VAX C
compiler is described in "Engineering a Compiler: VAX-11 Code Generation and
Optimization",  Digital  Press.  Perhaps PCC (et al) users should read books
like this and rewrite a few compiler backends.

-+- Sid  "some of my best friends write compilers" Maxwell, DEC @ Spit Brook
Rd, Nashua NH

"I seriously  doubt that the opinion, as expressed in the preceding message,
is necessarily shared by anyone else, including my employer. So there."