Hello, I am sorry for wrong reply address in previously sent thread. After working for quite a long time, I would like to introduce new IPA pass. Goal of the pass is to merge semantically equivalent functions and read-only variables. If we prove that a function A is an equivalent to a function B, depending on circumstances, an alias, thunk or a function redirection is created.
The optimization is inspired by Microsoft /OPT:ICF optimization (http://msdn.microsoft.com/en-us/library/bxwfs976.aspx) that merges COMDAT sections with each function reside in a separate section. Apart from that, GOLD linker implemented equivalent implementation for GNU toolchain: SAFE ICF (http://research.google.com/pubs/pub36912.html). Both implementations suffer from a collection of functions where an address was taken for comparison purpose. GOLD linker adds more conservative --icf=safe option. You may ask, why the GNU GCC does need such a new optimization. The compiler, having simply better knowledge of a compiled source file, is capable of reaching better results, especially if Link-Time optimization is enabled. Apart from that, GCC implementation adds support for read-only variables like construction vtables (mentioned in: http://hubicka.blogspot.cz/2014/02/devirtualization-in-c-part-3-building.html). The pass is capable of building GIMP, Inkscape and Firefox with enabled LTO. Majority of issues connected to testsuite has been fixed, there is a few of IPA tests I need to consult with Jan, how to fix it correctly. Statistics about the pass: Inkscape: 11.95 MB -> 11.44 MB (-4.27%) Firefox: 70.12 MB -> 70.12 MB (-3.07%) SPEC 2K6 statistics show how many equivalent functions (and variables) are proved by GOLD and GCC ICF: +----------------+----------+---------+------------+------------------+--------------+--------------+-------------+ | SPEC | GOLD ICF | IPA ICF | difference | difference perc. | intersection | Just in GOLD | Just in ICF | +----------------+----------+---------+------------+------------------+--------------+--------------+-------------+ | 400.perlbench | 26 | 53 | 27 | 203.85% | 26 | 0 | 27 | | 401.bzip2 | 0 | 1 | 1 | 0.00% | 0 | 0 | 1 | | 403.gcc | 88 | 223 | 135 | 253.41% | 81 | 7 | 142 | | 410.bwaves | 0 | 0 | 0 | 0.00% | 0 | 0 | 0 | | 416.gamess | 9 | 23 | 14 | 255.56% | 9 | 0 | 14 | | 429.mcf | 0 | 0 | 0 | 0.00% | 0 | 0 | 0 | | 433.milc | 0 | 8 | 8 | 0.00% | 0 | 0 | 8 | | 434.zeusmp | 0 | 1 | 1 | 0.00% | 0 | 0 | 1 | | 436.cactusADM | 19 | 55 | 36 | 289.47% | 14 | 5 | 41 | | 437.leslie3d | 0 | 0 | 0 | 0.00% | 0 | 0 | 0 | | 444.namd | 0 | 0 | 0 | 0.00% | 0 | 0 | 0 | | 445.gobmk | 0 | 332 | 332 | 0.00% | 0 | 0 | 332 | | 453.povray | 73 | 93 | 20 | 127.40% | 60 | 13 | 33 | | 454.calculix | 3 | 6 | 3 | 200.00% | 3 | 0 | 3 | | 456.hmmer | 0 | 1 | 1 | 0.00% | 0 | 0 | 1 | | 458.sjeng | 0 | 1 | 1 | 0.00% | 0 | 0 | 1 | | 459.GemsFDTD | 0 | 0 | 0 | 0.00% | 0 | 0 | 0 | | 462.libquantum | 0 | 0 | 0 | 0.00% | 0 | 0 | 0 | | 464.h264ref | 0 | 4 | 4 | 0.00% | 0 | 0 | 4 | | 470.lbm | 0 | 0 | 0 | 0.00% | 0 | 0 | 0 | | 471.omnetpp | 134 | 179 | 45 | 133.58% | 127 | 7 | 52 | | 473.astar | 0 | 1 | 1 | 0.00% | 0 | 0 | 1 | | 481.wrf | 51 | 75 | 24 | 147.06% | 50 | 1 | 25 | | 482.sphinx3 | 0 | 1 | 1 | 0.00% | 0 | 0 | 1 | +----------------+----------+---------+------------+------------------+--------------+--------------+-------------+ | TOTAL | 403 | 1057 | 654 | 262.28% | 370 | 33 | 687 | +----------------+----------+---------+------------+------------------+--------------+--------------+-------------+ I've been testing the pass on a QEMU Gentoo virtual machine and I will bring stats as soon as possible. There are still unresolved issues connected to correct debugging. I am not familiar with DWARF extensions, but it looks there is a solution that can be applied to the pass: http://wiki.dwarfstd.org/index.php?title=ICF ? I tried to create equivalent functions foo and bar, merged by ICF GOLD. Function bar is removed from .text section, but gdb can put a breakpoint to the function, where content of function foo is displayed. I think this is acceptable behavior. I expect many comments related to pass, but I hope effort invested by me a Honza will be transformed to acceptation to trunk. Martin (please ignore this diff) diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 3ddd98c..4f9418e 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,4 @@ + 2014-06-13 Thomas Preud'homme <thomas.preudho...@arm.com> PR tree-optimization/61375 -- 1.8.4.5
