slnc <s@xxxxxxx> writes: > I have tried disabling all -O1 flags just to check if I could select > only one by disabling all the others but: > > gcc -c -O1 -fno-defer-pop -fno-delayed-branch > -fno-guess-branch-probability -fno-cprop-registers -fno-if-conversion > -fno-if-conversion2 -fno-tree-ccp -fno-tree-dce > -fno-tree-dominator-opts -fno-tree-dse -fno-tree-ter -fno-tree-lrs > -fno-tree-sra -fno-tree-copyrename -fno-tree-fre -fno-tree-ch > -fno-unit-at-a-time -fno-merge-constants main.c > objdump -d dce.o > > generates an optimized version. So why are the flags there if they > cannot be selected individually? (I feel like I am missing something > completely obvious..) One view is that they can be selected individually. However, there are many optimizations which are not controlled by any flag. And if you don't turn on optimization, then gcc won't build the datastructures needed for most of the optimization passes, so they won't be run even if you explicitly ask for them. Another view is that optimizations are not independent. There is basic work that has to be done for every optimization, like build the CFG, go into SSA form, build the dataflow information, etc. The main difference between -O0 and -O1 is whether or not this basic work is done. Ian
