Perry Smith writes: > I wrote a class that switches the stack to a new area. This is for > the power PC. > > In the text below, I'll use main, testit, and newStack. main is the > main program, testit is a function that main calls, and newStack is > the method that switches the stack to the new space. main calls > testit which calls s.newStack. (s is an instance of the class that > switches the stack). > > The purpose of separating main and testit is so I can verify that > returning from testit works properly. > > newStack gets the current value of r1 (the stack pointer) and copies > the last two stack frames (which would be the stack frame for testit > and newStack) to the top of some allocated memory. It alters r1(0) > (the previous stack value for newStack) in the new memory to point to > the address of testit's new stack frame. It sets r1 up to the base > of this new area and returns. OK, before we go any further. Did you write and test DWARF unwinder information for newStack? > With g++ and no optimization, this works. When newStack returns, > it consumes its stack frame in the new memory leaving only testit's > new stack frame and r1 pointing to the base of the new stack from > for testit. When testit returns, it loads r1 with r1(0) and > returns. This properly puts r1 back to main's stack frame. > > If I put -O3, then at the return of testit, instead of loading r1 > with r1(0), just adds in the size of the stack frame (and assumes > that r1 has not been munged with). I presume this is faster. I > know that xlc does the same thing. As a result, when we return > back to main, the stack pointer is off in the weeds somewhere. I get the feeling I'm not understanding something here. As long as newStack is correct and handles all registers according to the ABI, there shouldn't be any trouble. Andrew.
