|
0
|
1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
|
|
|
2 @c 1999, 2000, 2001, 2002, 2004 Free Software Foundation, Inc.
|
|
|
3 @c This is part of the GCC manual.
|
|
|
4 @c For copying conditions, see the file gcc.texi.
|
|
|
5
|
|
|
6 @node Portability
|
|
|
7 @chapter GCC and Portability
|
|
|
8 @cindex portability
|
|
|
9 @cindex GCC and portability
|
|
|
10
|
|
|
11 GCC itself aims to be portable to any machine where @code{int} is at least
|
|
|
12 a 32-bit type. It aims to target machines with a flat (non-segmented) byte
|
|
|
13 addressed data address space (the code address space can be separate).
|
|
|
14 Target ABIs may have 8, 16, 32 or 64-bit @code{int} type. @code{char}
|
|
|
15 can be wider than 8 bits.
|
|
|
16
|
|
|
17 GCC gets most of the information about the target machine from a machine
|
|
|
18 description which gives an algebraic formula for each of the machine's
|
|
|
19 instructions. This is a very clean way to describe the target. But when
|
|
|
20 the compiler needs information that is difficult to express in this
|
|
|
21 fashion, ad-hoc parameters have been defined for machine descriptions.
|
|
|
22 The purpose of portability is to reduce the total work needed on the
|
|
|
23 compiler; it was not of interest for its own sake.
|
|
|
24
|
|
|
25 @cindex endianness
|
|
|
26 @cindex autoincrement addressing, availability
|
|
|
27 @findex abort
|
|
|
28 GCC does not contain machine dependent code, but it does contain code
|
|
|
29 that depends on machine parameters such as endianness (whether the most
|
|
|
30 significant byte has the highest or lowest address of the bytes in a word)
|
|
|
31 and the availability of autoincrement addressing. In the RTL-generation
|
|
|
32 pass, it is often necessary to have multiple strategies for generating code
|
|
|
33 for a particular kind of syntax tree, strategies that are usable for different
|
|
|
34 combinations of parameters. Often, not all possible cases have been
|
|
|
35 addressed, but only the common ones or only the ones that have been
|
|
|
36 encountered. As a result, a new target may require additional
|
|
|
37 strategies. You will know
|
|
|
38 if this happens because the compiler will call @code{abort}. Fortunately,
|
|
|
39 the new strategies can be added in a machine-independent fashion, and will
|
|
|
40 affect only the target machines that need them.
|
