Re: compiler smarts: register variables and catching exceptions
andrew_nuss@yahoo.com wrote:
Hi,
I have a long running loop that uses 4 stacks whose pointers are
declared as register
variables and which catches a SpecialException. The question is
whether the
introduction of the catch block will destroy register optimizations,
and force the
compiler to ignore using registers to hold the stack pointers.
---------------------------------------------------------------------------------------------------------------
do {
register int* sp1 = ...;
register int* sp2 = ...;
register int* sp3 = ...;
register int* sp4 = ...;
try {
do {
register int op = ...;
switch (op) {
// all of the cases do brief manipulations of the
various sp1,...,sp4
// some of the cases call functions which throw
SpecialException
}
} while (true);
} catch (SpecialException& e) {
// code to cleanup the values currently "held" by the
sp1,...,sp4
// and then re-enter the outer loop
}
} while (true);
In a word, probably. The compiler might choose to ignore the register
keyword (depending on the implementation) and is more likely to when
something can't be inlined anyway. Exception handlers add function call
overhead. The performance hits of function calls and exception handling
in the switch block will overwhelm the advantage of declaring something
register (which the compiler will do automatically if it can optimize
things).
I'm not exactly sure what it's doing that it tracks four stacks, so
much of this is speculation...
If you wish to optimize something like the code above (assuming there's
an empirical or other justification), avoid exception and function
calls that can't be inlined. If the functions aren't yours, e.g., they
cross an API boundary, then optimization probably won't help. If
SpecialException is your own, then you can probably control some of the
code. Can those functions be rewritten not to throw exceptions?
To cleanup the values of sp1..4, consider using RAII. You could write
some lightweight class(es) whose destructors do that for you, if you
have multiple code points that need to reset the values.
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