Hari Sandanagobalane <harihar...@picochip.com> writes: > Can anyone familiar with backend explain to me when we use > VIRTUAL_STACK_VARS_REGNUM (or) VIRTUAL_STACK_DYNAMIC_REGNUM in gcc? > Rather, when does the compiler decide to allocate a variable to > stack_vars region and when to stack_dynamic?
See calls to allocate_dynamic_stack_space. It's used for, e.g., allocate, variable sized local arrays, etc. > I encountered some trouble with virtual-stack-vars. Since it gets > translated to FRAME_POINTER_REGNUM and we don't actually have a > dedicated frame pointer register in picochip, i got a compiler ICE. I > have set ELIMINABLE_REGS to eliminate FRAME_POINTER in preference to > STACK_POINTER, but it doesn't work in this case (presumably > because FRAME_POINTER is used in a memory reference and elimination > does not work well with memory) gcc can not always eliminate the frame pointer. A frame pointer is required for functions that allocate stack space dynamically. You don't need to have a dedicated frame pointer register, but you do need to have a register which can serve as the frame pointer in a function which requires a frame pointer. > In the attached C code, at expand it saves the result from each call > to getPort function to the virtual_stack_vars region and then copies > it from there into virtual_stack_dynamic region. Why can it not copy > straight into virtual_stack_dynamic region? In the example, if i were > to write to the "int32_elements" instead of cmplx_elements, it does > work fine by writing it directly into virtual_stack_dynamic region. > > I have attached the compiler dumps from expand, sched and ira for reference. > > I tried this on GCC 4.6.0 release compiler. The port has had some > local changes for vector support, which i will submit into mainline > after a while. It looks like the port is requiring the local variables to have a specific alignment, but that the stack pointer is not guaranteed to be aligned. If you do expect the stack pointer to be aligned, make sure you express that in STACK_BOUNDARY and PREFERRED_STACK_BOUNDARY. If you do not expect the stack pointer to be aligned, but the local variables have to be aligned, then I don't see what else gcc can do. Ian
