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