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[IPA ICF,RFC 1/5] New Identical Code Folding IPA pass
- From: mliska <mliska at suse dot cz>
- To: gcc-patches at gnu dot gcc dot org
- Cc: hubicka at ucw dot cz
- Date: Fri, 13 Jun 2014 12:24:47 +0200
- Subject: [IPA ICF,RFC 1/5] New Identical Code Folding IPA pass
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Hello,
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)
---
gcc/ChangeLog | 1 +
1 file changed, 1 insertion(+)
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.preudhomme@arm.com>
PR tree-optimization/61375
--
1.8.4.5