39 lines
2.0 KiB
Plaintext
39 lines
2.0 KiB
Plaintext
@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
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@c 1999, 2000, 2001 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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The main goal of GCC was to make a good, fast compiler for machines in
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the class that the GNU system aims to run on: 32-bit machines that address
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8-bit bytes and have several general registers. Elegance, theoretical
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power and simplicity are only secondary.
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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, I have not hesitated to define an ad-hoc parameter to the machine
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description. The purpose of portability is to reduce the total work needed
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on the 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 I have not tried to address all possible
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cases, but only the common ones or only the ones that I have encountered.
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As a result, a new target may require additional 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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