> Some text size measurement. > > Summary: > 1) LTO with -O3 bloats up code considerably; Yes, you need either -fwhole-program or -fuse-linker-plugin to make it behave sanely.
For Mozilla I have best experience with -fuse-linker-plugin --param inline-unit-growth=5 That gives me about 16% code size savings (so LTO -O3 is same size as -Os). This means that it is smaller than -O2+LTO+-fwhole-program. > 2) LTO with -O2 reduces text size compared with -O2 An more so with -fwhole-program as you noticed ;) > 3) Google 4.4.3 based compiler is really effective in reducing C++ > program size -- this is where the focus of the tuning was done. > Witnessed by eon in SPEC2k and all C++ apps in SPEC06 Again, I would be very interested to see the patches (and sooner than later given tham I am re-tunning inliner for 4.6 now). Looking at Mozilla I also concluded that we have a lot of room for improvement. How old is the tree you use for testing? I recently improved code size somewhat at mainline. > > SPEC06 C++ program Data (the first data column is the TOT O2 base number) > > 1. TOT O3 vs TOT O2 ( 3.35% total increase) > > 471.omnetpp/ 853708 867988 1.67% > 450.soplex/ 643273 656349 2.03% > 483.xalancbmk/ 3634416 3777600 3.94% > 444.namd/ 393142 402038 2.26% > 473.astar/ 102182 111038 8.67% > size_sum 5626721 5815013 3.35% > > 2. TOT LTO+whole program + O3 vs TOT O2 (0.35% total increase) > > 471.omnetpp/ 853708 937728 9.84% > 450.soplex/ 643273 654057 1.68% > 483.xalancbmk/ 3634416 3540646 -2.58% > 444.namd/ 393142 401318 2.08% > 473.astar/ 102182 112538 10.13% > size_sum 5626721 5646287 0.35% > > 3. TOT LTO+whole program + O2 vs TOT O2 (8.10% total reduction) > > 471.omnetpp/ 853708 822868 -3.61% > 450.soplex/ 643273 611653 -4.92% > 483.xalancbmk/ 3634416 3245157 -10.71% > 444.namd/ 393142 391698 -0.37% > 473.astar/ 102182 99586 -2.54% > size_sum 5626721 5170962 -8.10% > > 4. google 4.4.3 compiler O2 vs TOT O2 (13.95% total reduction) > > 471.omnetpp/ 853708 545840 -36.06% > 450.soplex/ 643273 374674 -41.76% > 483.xalancbmk/ 3634416 3556306 -2.15% > 444.namd/ 393142 329897 -16.09% > 473.astar/ 102182 35301 -65.45% > size_sum 5626721 4842018 -13.95% Hmm, this really seems interesting. Why the changes was not contributed this stage1? Honza
