On Tue, Aug 01, 2017 at 11:12:12AM -0400, Eric Gallager wrote: > On 8/1/17, Jakub Jelinek <ja...@redhat.com> wrote: > > On Tue, Aug 01, 2017 at 07:08:41AM -0400, Eric Gallager wrote: > >> > Heh. I suspect -Os would benefit from a separate compilation pipeline > >> > such as -Og. Nowadays the early optimization pipeline is what you > >> > want (mostly simple CSE & jump optimizations, focused on code > >> > size improvements). That doesn't get you any loop optimizations but > >> > loop optimizations always have the chance to increase code size > >> > or register pressure. > >> > > >> > >> Maybe in addition to the -Os optimization level, GCC mainline could > >> also add the -Oz optimization level like Apple's GCC had, and clang > >> still has? Basically -Os is -O2 with additional code size focus, > >> whereas -Oz is -O0 with the same code size focus. Adding it to the > >> FSF's GCC, too, could help reduce code size even further than -Os > >> currently does. > > > > No, lack of optimizations certainly doesn't reduce the code size. > > For small code, you need lots of optimizations, but preferrably code-size > > aware ones. For RTL that is usually easier, because you can often compare > > the sizes of the old and new sequences and choose smaller, for GIMPLE > > optimizations it is often just a wild guess on what optimizations generally > > result in smaller and what optimizations generally result in larger code. > > There are too many following passes to know for sure, and finding the right > > heuristics is hard. > > > > Jakub > > > > Upon rereading of the relevant docs, I guess it was a mistake to > compare -Oz to -O0. Let me quote from the apple-gcc "Optimize Options" > page: > > -Oz > (APPLE ONLY) Optimize for size, regardless of performance. -Oz > enables the same optimization flags that -Os uses, but -Oz also > enables other optimizations intended solely to reduce code size. > In particular, instructions that encode into fewer bytes are > preferred over longer instructions that execute in fewer cycles. > -Oz on Darwin is very similar to -Os in FSF distributions of GCC. > -Oz employs the same inlining limits and avoids string instructions > just like -Os. > > Meanwhile, their description of -Os as contrasted to -Oz reads: > > -Os > Optimize for size, but not at the expense of speed. -Os enables all > -O2 optimizations that do not typically increase code size. > However, instructions are chosen for best performance, regardless > of size. To optimize solely for size on Darwin, use -Oz (APPLE > ONLY). > > And the clang docs for -Oz say: > > -Oz Like -Os (and thus -O2), but reduces code size further. > > So -Oz does actually still optimize, so it's more like -O2 than -O0 > after all, just even more size-focused than -Os.
The relationship between -Os and -Oz is like the relationship between -O2 and -O3. If -O3 says, try everything you can to increase performance even at the expense of code-size and compile time, then -Oz says, try everything you can to reduce the code size, even at the expense of performance and compile time. Thanks, James
