UNIX to Linux Porting: A Comprehensive Reference

2017-07-07 02:10:07

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.

D.2. Porting Considerations

Table D-1. GNU and XL Compiler Switches

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