gcc link time optimization and the Linux kernel Andi Kleen Intel OTC

29 Pages·2013·0.22 MB·English

by  Andi Kleen

gcc link time optimization and the Linux kernel
Andi Kleen
Intel OTC
Apr 2013
[e-mail]
Acknowledgments
Lots of people helped/contributed
●
Ralf Baechle, Richard Biener, Tim Bird, Honza
●
Hubicka, H.J. Lu, Joe Mario, Markus
Trippelsdorf, Changlong Xie, others
Why LTO?
Optimize over the whole binary
●
– Not just function (3.0) or file (<4.5)
Avoid inline dependency hell in header files
●
Without changing Makefiles significantly
●
gcc 4.7+ LTO WHOPR crash course
Compiler parses files, writes GIMPLE to object files
●
(LGEN)
– Function optimization summaries are computed
Linker calls lto1 with all files for sequential whole
●
program analysis (WPA)
– Merges types, Generates global callgraph, IPA optimization
summaries, writes partitions
Generate code per partition in parallel (LTRANS)
●
– Inline inside partition, run IPA optimizations for real, run per
function optimizations, generate object code
LTO IPA optimizations (4.8)
Inlining between files De-virtualization
● ●
Function cloning for
● Change ABI (SSE)
●
specific arguments
Constant propagation
●
Remove unused code
●
Scalar replacement of
(fwhole-program) ●
aggregates
Increase alignment
●
Constructor /
●
Discover pure/const
●
destructor merging
Keep globals/statics
●
alive over calls
green: does not benefit kernel today
Build time
Build time small
User time
800
small config
600
4000
)
400 s
e ( 2000
200 m
0 er ti 0
s
u Gcc 4.7 Gcc 4.8 4.7 -lto 4.8 -lto
Gcc 4.7 Gcc 4.8 4.7 -lto 4.8 -lto
Faults small config
Parallelism
60.00
e 15.00
m
al ti 10.00 M) 40.00
e / re 5.00 ults ( 20.00
m a
r ti 0.00 F
e 0.00
Us Gcc 4.7 Gcc 4.8 4.7 -lto 4.8 -lto
Gcc 4.7 Gcc 4.8 4.7 -lto 4.8 -lto
LTO is slow and not parallel enough
Parallelism small build 4.7
Object file generation
Parsing / LGEN
Modules without
Runnable processes
job server
LTO kernel build small config
60
50
LTRANS
40
code generation
p) r
e ( 30
bl
a
n
n
u 20
r
10
WPA + real linker
Type merging
0
11 31 51 71 91111131151171191211231251271291311331351371391411431451471491511531551 4 times
1 21 41 61 81101121141161181201221241261281301321341361381401421441461481501521541
time (s)
Small config parallelism
User time / Runtime
15
10
) WHOPR still has poor parallelism
s
e ( 5
m
ti 0
Gcc 4.7 Gcc 4.7 no lto
gcc 4.8 Gcc 4.8 no lto
Multilink
vmlinux links 2-4x: runs LTO that often
●
Generates integrated symbol table (kallsyms)
●
So far not fixed
●
– KALLSYMS can be disabled
One of those unexpected quirks of real build
●
systems
6000.00
4000.00
2000.00
0.00
Gcc 4.8 w/o kallsyms
Gc c 4.8 no lto Gcc 4.7 with kallsyms
Memory usage 4.7 small build
Active memory
Kernel LTO build small config
8
7
6
5
)
B
G
( 4
m
e
m
e 3
v
cti
a 2
1
0
9 25 41 57 73 89105121137153169185201217233249265281297313329345361377393409425441457473489505521537553
1 17 33 49 65 81 97113129145161177193209225241257273289305321337353369385401417433449465481497513529545
time (s)
Faults small config
Even small build swaps in 4GB system
60.00
Memory peaks all in WPA
M) 40.00
(
s
ult 2 0.00
a
F
0.00
Gcc 4.7 Gcc 4.8 4.7 -lto 4.8 -lto
Memory consumption
Temporary data can be a problem, together
●
with WPA
– Early many swap storms with /tmp = tmpfs
– Partitioning algorithm was improved
– Use TMPDIR=objdir
– With modules need to avoid too large -j* for
parallel WPA
– Jobserver has to be disabled, makes it worse