On Wed, Jun 25, 2014 at 4:00 PM, Vladimir Makarov <vmaka...@redhat.com> wrote: > On 2014-06-25, 5:32 AM, Renato Golin wrote: >> >> On 25 June 2014 10:26, Bingfeng Mei <b...@broadcom.com> wrote: >>> >>> Why is GCC code size so much bigger than LLVM? Does -Ofast have more >>> unrolling >>> on GCC? It doesn't seem increasing code size help performance (164.gzip & >>> 197.parser) >>> Is there comparisons for O2? I guess that is more useful for typical >>> mobile/embedded programmers. >> >> >> Hi Bingfeng, >> >> My analysis wasn't as thorough as Vladimir's, but I found that GCC >> wasn't eliminating some large blocks of dead code or inlining as much >> as LLVM was. > > > That might be a consequence of difference in aliasing I wrote about. I > looked at the code generated by LLVM and GCC of an interpreter and saw > bigger code generated by GCC too. > > A sequence of bytecodes execution and each bytecode checks types of > variables (small structure in memory) and set up values and types of results > variables. So GCC was worse to propagate the variable type info (e.g. int) > in the bytecode sequence execution where it would be possible and remove > unnecessary code (cases where other types, e.g. fp, is processed). LLVM was > more successful with this task.
Maybe LLVM is just too aggressive here (but is lucky to not miscompile this case). Richard. > > I haven't dug deeper, though. Some of the differences >> >> were quite big, I'd be surprised if it all can be explained by >> unrolling loops and vectorization... >> >
