Alexey Salmin <alexey.salmin@xxxxxxxxx> writes: > On 10/12/09, Kevin P. Fleming <kpfleming@xxxxxxxxxx> wrote: >> >> This is frequently asked on this list, and the answer is always the >> same: yes, the -O<N> flag does more than just enable a series of -f<foo> >> flags. Said another way, there are optimizations that do *not* have >> -f<foo> flags that control them, they are enabled by directly checking >> the value supplied to the -O flag, and they cannot be directly controlled. > > And what's the purpose of that? Simplicity. There are many dependencies between different optimizations. Rather than represent all of these dependencies in a data structure, many passes are simply run at certain optimization levels. The compiler could be organized differently, but making that continue to work would impose a significant testing burden: it would be necessary to test many combinations of options to make sure that they remained effective. Also, frankly, the compiler already has too many command line options. Adding a separate command line option for each separate optimization, above and beyond -O1, -O2, etc., would increase complexity for little gain. Which is to say: it would not be impossible, but it would be an ongoing maintenance burden, and there would be no benefit for the average user of gcc. Given that gcc is free software, this is something that will only be done if some contributor takes a serious an ongoing interest in making it work. Ian
