Re: register variables in C.

[with respect to "register" declarations and related optimizer hints]


just my 3 cents worth:


$.01 : declare the variables that are most important to put into registers
                first; compiler might try to keep n variables in registers, but when
                the n+1 st declaration for a register variable is seen, it is stored
                in memory.


$.02 : declare variables in the innermost scope in which they are active,
                and use {} frequently to reduce the scope of temporaries. Doing
                so will reduce optimization time drastically, can only increase
                performance of less-than-optimal optimizers, especially if the
                optimizer has a "time-out" switch (like many chess programs), where,
                if iterative optimization becomes too complex, the optimizer will,
                iterate until a time/count limit. Doing so may also force a compiler
                to spill a register variable in an outer scope to memory while the
                newly-declared register variable in the inner scope occupies the
                register, which is usually what one would want.


$.03 : if you know a machine has, say 3, registers for local temporaries,
then put the following in a header file:


                #define register1 register
                #define register2 register
                #define register3 register
                #define register4
                #define register5
                        .
                        .
                        .


                then declare the local var that is most important to put in a
register with register1, the second most important with register2,
etc.




On my wish list:


                there would be a construct registerof(local) or locationof(local)
                that you could use within asm stmts to access the storage location
                of <local>, and not have to disable optimizations for the entire
                subroutine when you use asm's, just because the optimizer allocates
                registers in an unpredictable manner.


bb
[Most of the C compilers I know have some sort of hack to let you splice
register or local names into asm's. -John]
--

Return to the comp.compilers page.
Search the comp.compilers archives again.