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@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,

@c 1999, 2000, 2001, 2002, 2004 Free Software Foundation, Inc.

@c This is part of the GCC manual.

@c For copying conditions, see the file gcc.texi.

@node Portability

@chapter GCC and Portability

@cindex portability

@cindex GCC and portability

GCC itself aims to be portable to any machine where @code{int} is at least

a 32-bit type.  It aims to target machines with a flat (non-segmented) byte

addressed data address space (the code address space can be separate).

Target ABIs may have 8, 16, 32 or 64-bit @code{int} type.  @code{char}

can be wider than 8 bits.

GCC gets most of the information about the target machine from a machine

description which gives an algebraic formula for each of the machine's

instructions.  This is a very clean way to describe the target.  But when

the compiler needs information that is difficult to express in this

fashion, ad-hoc parameters have been defined for machine descriptions.

The purpose of portability is to reduce the total work needed on the

compiler; it was not of interest for its own sake.

@cindex endianness

@cindex autoincrement addressing, availability

@findex abort

GCC does not contain machine dependent code, but it does contain code

that depends on machine parameters such as endianness (whether the most

significant byte has the highest or lowest address of the bytes in a word)

and the availability of autoincrement addressing.  In the RTL-generation

pass, it is often necessary to have multiple strategies for generating code

for a particular kind of syntax tree, strategies that are usable for different

combinations of parameters.  Often, not all possible cases have been

addressed, but only the common ones or only the ones that have been

encountered.  As a result, a new target may require additional

strategies.  You will know

if this happens because the compiler will call @code{abort}.  Fortunately,

the new strategies can be added in a machine-independent fashion, and will

affect only the target machines that need them.