Re: C++ virtual function calls

genew@mindlink.bc.ca (Gene Wirchenko) writes:


> On hoisting function calls out of a loop:
>
> It seems to me that this could (read as "would") cause trouble where
> the hoisted routine had static variables or (shudder) diddled globals.
> Would this not be enough to result in compiler designers not taking the
> chance or can checking for these cases be done efficiently (and
> accurately)?


Certainly the compiler must do it "correctly". Code can be hoisted
if it's loop-invariant - the value produced by the code doesn't
change with the loop interations. The _value_ produced by a
subroutine call is the return value, AND all side-effects, like
modifications to static/global memory and I/O. To hoist a call
requires you to know it doesn't modify memory or do I/O (correct
exception handling behavior is another tricky question).


Cliff
--
Cliff Click Compiler Researcher & Designer
RISC Software, Motorola PowerPC Compilers
cliffc@risc.sps.mot.com (512) 891-7240
--

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