Hello, > > Paul Brook wrote: > > >> Compiler can optimize it any way it wants, > > >> as long as result is the same as unoptimized one. > > > > > > We have an option for that. It's called -O0. > > > > > > Pretty much all optimization will change the behavior of your program. > > > > Now that's a bit TOO strong a statement, critical optimizations like > > register allocation and instruction scheduling will generally not change > > the behavior of the program (though the basic decision to put something > > in a register will, and *surely* no one suggests avoiding this critical > > optimization). > > Actually they will with multi threaded program, since you can have a case > where it works and now it is broken because one thread has speed up so much it > writes to a variable which had a copy on another thread's stack. So the > argument > about it being too strong is wrong because timming matters now a days. > Instruction > scheduling can cause the same issue as it forces a write too early for > another thread > to act on.
actually, you do not even need (invalid) multithreaded programs to realize that register allocation may change behavior of a program. If the size of the stack is bounded, register allocation may cause or prevent program from running out of stack, thus turning a crashing program to non-crashing one or vice versa. Zdenek
