TITLE: Compilers and register variables


PROBLEM: ded@aplcen.apl.jhu.edu (nod sivad)

In the past, to speed up a C function's performance, one would
declare frequently used variables as "register."  
But what happens with a C++ inline function?  Does declaring
the variables as "register" offer any hope of greater speed?

P.S.  Supposedly, compilers are so sophisticated nowadays that
the "register" keyword is unnecessary.  Would any compiler writers
care to comment on this?


PROBLEM: steve@taumet.com (Steve Clamage), 9 Dec 92

Declaring a variable as "register" has no defined semantics, except
that you can't then take its address.  Using register variables in
an inline function doesn't change that.

You have to read your compiler documentation to see what it says
about register variables, the effect of register declarations,
and when to use (and not to use) the declarations.  Compilers differ.

Most modern compilers ignore register declarations when deciding how
to allocate variables.  A good compiler will make better decisions
about allocation than the programmer can.  If you force a variable
into a register, you may force less efficient code generation for
other aspects of the function.  A good compiler evaluates all the
costs and benefits and assigns variables accordingly.  It might put
a variable into a register for part of a function, and use the
register for something else in another part.


RESPONSE: p j l @cs.uiuc.edu (Paul Lucas)
	  AT&T Bell Laboratories


*****>	Actually, in C++ you can take the address of a register
	variable.  (This will _force_ the variable to not be put into
	a register, however.)