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The two main functions of the back-end
are: To optimize your program for faster
performance To generate the code that goes into the object file
Optimization is performed by two subcomponents
of the compiler's back end: The High-Level Optimizer
(HLO), which performs large-scale, high-semantic-level
analyses and transformations to increase performance. The low-level optimizer, which performs traditional
optimizations (such as common subexpression elimination and dead-code
removal) as well as machine-specific optimizations.
Options for controlling optimization form the largest group
of the compile-line options. These options enable you to do the
following: To set the level of optimization that
is applied to your program To apply a package of optimizations that meet certain
requirements of your application—for example, optimizations
that favor compile-time speed over performance To apply specific optimization technologies to your
program, or to specific parts of your program, for fine-tuning performance
Table 1-4 “Options
for controlling optimization” lists (in summary
form) the options that control optimization. For information about
how to use these options, see “Using options to control optimization”. The optimization technology of HP compilers
is the subject of a white paper: see the HP
PA-RISC Compiler Optimization Technology White Paper. Table 1-4 Options
for controlling optimization | Option | Function |
|---|
| +DC7200 | Perform memory hierarchy optimizations
for the PA7200 processor. | | -O[optlevel] | Optimize program, where optlevel
is 0 (no optimization), 1, 2, or 3 (the highest level). If optlevel
is not specified, the program is optimized at level 2 (-O2). | | +Ooptlevel | This option has the same meaning as the
-O[optlevel]
option, except that optlevel must be specified. It is provided
for compatibility with makefiles. | | +O[no]info | Provide [do not provide]
feedback information about the optimization process. This option
is most useful at optimization level 3 and higher. The default
is +Onoinfo. | | +O[no]optimization | Enable [disable] optimization,
a predefined string that indicates a category of optimizations (for
example, those that do not increase code size) or a specific optimization
technology (for example, inlining). See the HP Fortran
90 Programmer's Reference, for the different values
for optimization. |
The other component of the back end is the code generator
(CodeGen), which you can control by using the compile-line options
in Table 1-5 “Options
for controlling code generation”. These options allow
you to specify (among other things) that the output file include
debugging or profiling information or that local variables be saved
in static memory. Table 1-5 Options
for controlling code generation | Option | Function |
|---|
| +[no]asm | Compile the named source files and leave
[do not leave] the assembly language output in
corresponding files whose names are suffixed with .s.
The default is +noasm. | | +DAmodel | Generate code for a specific version
of the PA-RISC architecture. model can
be one of the following: PA-RISC
version number (1.1 or 2.0) A model number (for example, 750
or 870). One of the PA-RISC processor names (for example,
PA7000, PA7100,
or PA8000). The word portable
to generate code compatible across all PA-RISC 1.1 and 2.0 workstations
and servers.
For information
about using this option, see “Compiling for different PA-RISC machines”. | | +DSmodel | Perform instruction scheduling appropriate
for a specific implementation of the PA-RISC architecture. model
can be one of the following: PA-RISC
version number (1.1 or 2.0) A model number (for example, 750
or 870). One of the PA-RISC processor names (for example,
PA7000, PA7100,
or PA8000).
For
information about using this option, see “Compiling for different PA-RISC machines”. | | -g | Generate debugging information needed
by the debugger. This option is compatible with optimization levels
0, 1, and 2. If you compile and link separately and specify -g
on the compile line, you must also specify it on the link line. For
information about using this option to prepare programs for the
debugger, see “Using the HP DDE debugger”. | | +[no]gprof | Prepare [do not prepare]
object files for profiling with gprof;
see the gprof(1) man page. The default
is +nogprof. If you compile and
link separately and specify +gprof
on the compile line, you must also specify it on the link line. For
information about using this option to profile programs with gprof,
see “gprof”. | | +k | Generate code for programs that reference
a very large number of shared data items. The linker will issue
a diagnostic message in the rare cases when this option is needed. | | +pic={short|long|no} | Generate Position Independent Code (PIC)
with short displacements (+pic=short)
or long displacements (+pic=long)
for use in shared libraries. The default is +pic=no. For
information about using this option when creating shared libraries,
see “Compiling with +pic”. | | +[no]prof | Prepare [do not prepare]
object files for profiling with prof;
see the prof(1) man page. The default
is +noprof. If you compile and
link separately and specify +prof
on the compile line, you must also specify it on the link line. For
information about using this option to profile programs with prof,
see “prof”. | | +[no]save | Save [do not save]
all local variables in all subprograms. For information
about using this option when porting, see “Uninitialized variables”. |
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