Re: while loop code generation

Andrew Tucker (ast@halcyon.com) wrote:
> On page 227 of Fraser and Hanson's _A Retargetable C Compiler_,
> they claim that the code


> goto L+1
> L: statement
> L+1: if expression != 0 goto L
> L+2:


> generates n+2 goto's for a loop executing statement n times while
> the code


> L:
> L+1: if expression == 0 goto L+2
> statement
> goto L
> L+2:


This is processor dependent, and depends on such things as how well
the processor supports pipelining and branch prediction. But if you
take a simple architecture that does neither, then the time it takes
to fall through a conditional jump is not insignificant compared to
the time it takes to take the branch. On many processors the times
are approximately equal. On a processor where such is the case, the
extra jump that has to be executed for the second case costs time, and
the first is preferable to optimize that extra branch out.


Even on later processors that do simple pipelineing, the processor may
not be up to predicting an unconditional branch and in that case the
first case is preferable as well. Even on some of the latest
processors that do hefty branch predictions and pipelining there is a
little bit of time lost when the jumps are fectched from the pipeline
and executed, but of course it won't be nearly as much on such
processors.


David




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