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Optimizing and Parallelizing
individual optimizations can sometimes cause slowdowns, and must be used carefully to ensure
performance improvements.
There are other useful command line options related to optimization and parallelization, such as –
help, –Minfo, –Mneginfo, –dryrun, and –v.
3.2.1. –help
As described in Help with Command–Line Options, you can see a specification of any
command–line option by invoking any of the PGI compilers with –help in combination with the
option in question, without specifying any input files.
For example, you might want information on –O:
$ pgfortran -help -O
The resulting output is similar to this:
-O Set opt level. All -O1 optimizations plus traditional scheduling and
global scalar optimizations performed
Or you can see the full functionality of –help itself, which can return information on either an
individual option or groups of options:
$ pgfortran -help -help
The resulting output is similar to this:
-help[=groups|asm|debug|language|linker|opt|other|overall|
phase|prepro|suffix|switch|target|variable]
Show compiler switches
3.2.2. –Minfo
You can use the –Minfo option to display compile–time optimization listings. When this option
is used, the PGI compilers issue informational messages to standard error (stderr) as compilation
proceeds. From these messages, you can determine which loops are optimized using unrolling,
SSE instructions, vectorization, parallelization, interprocedural optimizations and various
miscellaneous optimizations. You can also see where and whether functions are inlined.
For more information on –Minfo, refer to Optimization Controls section of the PGI Compiler
Reference Manual.
3.2.3. –Mneginfo
You can use the –Mneginfo option to display informational messages to standard error (stderr)
that explain why certain optimizations are inhibited.
For more information on –Mneginfo, refer to Optimization Controls section of the PGI
Compiler Reference Manual.
3.2.4. –dryrun
The –dryrun option can be useful as a diagnostic tool if you need to see the steps used by
the compiler driver to preprocess, compile, assemble and link in the presence of a given set of
command line inputs. When you specify the –dryrun option, these steps are printed to standard
error (stderr) but are not actually performed. For example, you can use this option to inspect the
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