#1691: Look into GCC -flto and -fwhole-program link options
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Reporter: petdance | Owner:
Type: experimental | Status: new
Priority: normal | Milestone:
Component: none | Version: 2.5.0
Severity: medium | Keywords:
Lang: | Patch:
Platform: |
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-flto is link-time optimization.
-fwhole-program is optimization for the entire program.
From http://lwn.net/Articles/387122/
When source files are compiled and linked using -flto, GCC applies
optimizations as if all the source code were in a single file. This allows
GCC to perform more aggressive optimizations across files, such as
inlining the body of a function from one file that is called from a
different file, and propagating constants across files. In general, the
LTO framework enables all the usual optimizations that work at a higher
level than a single function to also work across files that are
independently compiled.
The LTO option works almost like any other optimization flag. First, one
needs to use optimization (using one of the -O{1,2,3,s} options). In cases
where compilation and linking are done in a single step, adding the option
-flto is sufficient
gcc -o myprog -flto -O2 foo.c bar.c
This effectively deprecates the old -combine option, which was too slow in
practice and only supported for C.
With independent compilation steps, the option -flto must be specified at
all steps of the process:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
An interesting possibility is to combine the options -flto and -fwhole-
program. The latter assumes that the current compilation unit represents
the whole program being compiled. This means that most functions and
variables are optimized more aggressively. Adding -fwhole-program in the
final link step in the example above, makes LTO even more powerful.
When using multiple steps, it is strongly recommended to use exactly the
same optimization and machine-dependent options in all commands, because
conflicting options during compilation and link-time may lead to strange
errors. In the best case, the options used during compilation will be
silently overridden by those used at link-time. In the worst case, the
different options may introduce subtle inconsistencies leading to
unpredictable results at runtime. This, of course, is far from ideal, and,
hence, in the next minor release, GCC will identify such conflicting
options and provide appropriate diagnostics. Meanwhile, some extra care
should be taken when using LTO.
--
Ticket URL: <https://trac.parrot.org/parrot/ticket/1691>
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