Section D.2. Porting Considerations

D.2. Porting Considerations

Software professionals porting applications from UNIX or Linux on other platforms to Linux on POWER must remember the following:

• Linux on POWER is a big-endian platform.

• Linux on POWER supports two programming models: ILP32 and LP64.

• The current IBM POWER microprocessor is a 64-bit architecture running a 64-bit Linux OS. However, Linux on POWER provides full binary compatibility with existing 32-bit applications and supports new 64-bit applications.

• Linux on POWER uses the ELF object file format.

• The basic compiler flags used in GCC and XL compilers are generally the same. However, a number of the flags differ. For example, Table D-1 compares the commonly used GNU and XL compiler flags.

Table D-1. GNU and XL Compiler Switches

GCC	XL C/C++	Description
-v	-v, -V, -#	Turn on verbose mode.
-p/-profile	-p	Set up the object files produced by the compiler for profiling.
N/A	-q32, -q64, or set the OBJECT_MODE environment variable	Create 32- or 64-bit objects. GCC 64-bit compilers are located at /opt/cross/bin.
-fsyntax-only	-qsyntaxonly	Perform syntax checking without generating object files.
-fpic	-qpic=small	Generate position-independent code for use in shared libraries. In XL C/C++, the size of the Global Offset Table is no larger than 64K. If -qpic is specified without any suboptions, -qpic=small is assumed. The -qpic option is enabled if the -qmkshrobj compiler option is specified.
-fPIC	-qpic=large	Allows the Global Offset Table to be larger than 64K in size.
-pthread	-qthreaded or _r invocation mode	Create programs running in a multithreaded environment.
-fno-rtti	-qnortti	Disable generation of runtime type -qrtti identification (RTTI) for exception handling and for use by typeid and dynamic_cast operators. On XL C/C++, the default is -qnortti.
-static	-qstaticlink	Objects generated with this option do not link with shared libraries.
-static-libgcc	-qstaticlink = libgcc	Instruct the compiler to link with the static version of libgcc.
-shared	-qnostaticlink or -qmkshrobj	Instruct the compiler to produce a shared object.
-shared-libgcc	-qnostaticlink =libgcc	Instruct the compiler to link with the shared version of libgcc.
-Wl, -rpath	-Wl, -rpath, or -R	Pass a colon-separated list of directories used to specify directories searched by the runtime linker.
-fno-implicit-templates, -frepo	-qtempinc, -qtemplateregistry, -qtemplaterecompile	Template instantiation.
-w	-w	Suppress warning messages.
	-warn64	Enables checking for long-to-integer truncation.
	-qinfo=<…>	Produces informational messages.
-fpack-struct	-qalign= bit_packed	Use bit_packed alignment rules.
	-qalign= linuxppc	Uses default GCC alignment rules to maintain compatibility with GCC objects. This is the default.
-O, -O2, -O3	-O, -O2, -O3, -O4, -O5	Optimization levels.
	-qarch, -qtune, -qcache	Optimization options for a particular processor.