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73a8ed7e | 1 | @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
962e6e00 | 2 | @c 1999, 2000, 2001, 2002, 2004 Free Software Foundation, Inc. |
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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 | ||
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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. | |
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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 | |
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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. | |
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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 | |
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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 | |
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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. |