XL Fortran Enterprise Edition for AIX : User's Guide - IBM

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If your primary concern is execution performance, you may see some speedup if
you specify the appropriate -qarch suboption and perhaps also specify the
-qtune and -qcache options. In this case, you may need to produce different
versions of the executable file for different machines, which might complicate
configuration management. You will need to test the performance gain to see if
the additional effort is justified.
It is usually better to target a specific architecture so your program can take
advantage of the targeted machine’s characteristics. For example, specifying
-qarch=pwr4 when targeting a POWER4 machine will benefit those programs
that are floating-point intensive or have integer multiplies. On PowerPC
systems, programs that process mainly unpromoted single-precision variables
are more efficient when you specify -qarch=ppc. On POWER2 and POWER3
systems, programs that process mainly double-precision variables (or
single-precision variables promoted to double by one of the -qautodbl options)
become more efficient with -qarch=pwr2, -qarch=pwr3, -qarch=pwr4 and
-qarch=pwr5. The -qautodbl=dblpad4 option will improve POWER and
POWER2, but not POWER3, POWER4, and POWER5, which are PowerPC
processors.
Other Considerations
The PowerPC instruction set includes two optional instruction groups that may be
implemented by a particular hardware platform, but are not required. These two
groups are the graphics instruction group and the sqrt instruction group. Code
compiled with specific -qarch options (all of which refer to specific PowerPC
machines) will run on any equivalent PowerPC machine that has an identical
instruction group. The following table illustrates the instruction groups that are
included for the various PowerPC machines.
Table 14. Instruction groups for PowerPC platforms
Processor Graphics group sqrt group 64-bit
601 no no no
603 yes no no
604 yes no no
ppc no no no
ppcgr yes no no
ppc64 no no yes
ppc64gr yes no yes
ppc64grsq yes yes yes
rs64a no no yes
rs64b yes yes yes
rs64c yes yes yes
pwr3 yes yes yes
pwr4 yes yes yes
pwr5 yes yes yes
ppc970 yes yes yes
If you compile code using the -qarch=pwr3 option, the code will run on an RS64B
hardware platform but may not run on an RS64A platform because the instruction
groups are not identical. Similarily, code compiled with the -qarch=603 option will
run on a POWER3 machine, but may not run on a RS64A machine.
130 XL Fortran Enterprise Edition for AIX : User’s Guide