> If you fix all but 4 registers, then the compiler will only use the > remaining 4. Extra credit if your code compiles with just 2 registers > left!
Fixing registers causes the compiler to fight for its life to put those values into the remaining registers => code bloat, and I guess you are referring to reload failing if too many registers are marked as fixed. I threw in a #if 0 in local-alloc.c + global.c and now it appears as if registers are not allocated, except as needed by reload. Code appears to compile & execute correctly. It did reduce code size significantly in one case(10% ish). However, it seems like -fomit-frame-pointer stopped working as well as it used to, so many references to pseudo registers are via the frame pointer instead of the stack pointer, which causes code bloat(+20% ish in one case). Here is somewhat dated, but interesting discussion on omitting frame pointers: http://gcc.gnu.org/ml/gcc/2002-04/msg00009.html Another factor is that pseudo variables seem to be allocated before the frame(is this correct? I just looked at the machinecode), so the distance between the SP and the pseudo variables is quite big. Here is an interesting article on this issue: http://gcc.fyxm.net/summit/2003/Optimal%20Stack%20Slot%20Assignment.pdf The 10% code saving is interesting, but obviously with a system as complex(necessarily so) as GCC it would be rather optimistic to expect turning off register allocation to give a net saving in the general case. -- Øyvind Harboe http://www.zylin.com
