On Fri, Sep 10, 2010 at 10:44:24AM +0200, Steven Bosscher wrote: > On Thu, Sep 9, 2010 at 6:43 PM, DJ Delorie <d...@redhat.com> wrote: > > > I guess the most important missing optimizations are various forms of > code unification, such as the sequence abstraction code that GCC used > to have (http://gcc.gnu.org/projects/cfo.html, but it never worked > properly and it was way too slow), or some suffix-tree based sequence > finding code. Various algorithms can be found in the academic > literature about code size optimizations via abstraction (see e.g. > "Procedural Abstraction with Reverse Prefix Trees", > http://portal.acm.org/citation.cfm?id=1545074).
Was CFO finally merged to mainline? At least, I can't find it in current gcc. > Personally, I had hoped that the ARM folks (Linaro, or what's it > called?) would work on -Os. While I've never actually used it, a web > search suggests that the RealView compilers generate code that is as > much as 20% smaller than GCC at -Os (for unnamed benchmarks), so > apparently there is a lot of room for improvement in GCC and the ARM > people should know where. > We, Linaro Toolchain Working Group, are doing the investigation on code size improvements on thumb-2. As you said, there would be a lot of room for improvement, and here is the report we got, fyi. http://lists.linaro.org/pipermail/linaro-toolchain/2010-September/000202.html > Finally, of course there are just various issues with instruction > selection in GCC that result in larger-than-necessary code. It seems > that this doesn't hurt code speed so much, but for code size GCC > doesn't always select the shortes sequence possible. Some Google folks > (Carrot Wei in particular) have filed bugs and patches for a couple of > cases for ARM, but there is no target-independent frame work for > selecting the shortest insn or sequence. During the investigation, I feel that all the potential improvements are identified by ARM experts or by reading asm code manually. This mode doesn't scale very well. IMO, it is necessary to have a target-independent framework for code size optimization. I have no idea to do that framework though. -- Yao Qi CodeSourcery y...@codesourcery.com (650) 331-3385 x739
