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1 @c Copyright (C) 1988-2020 Free Software Foundation, Inc.
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2 @c This is part of the GCC manual.
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3 @c For copying conditions, see the file gcc.texi.
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4
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5 @node Portability
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6 @chapter GCC and Portability
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7 @cindex portability
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8 @cindex GCC and portability
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9
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10 GCC itself aims to be portable to any machine where @code{int} is at least
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11 a 32-bit type. It aims to target machines with a flat (non-segmented) byte
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12 addressed data address space (the code address space can be separate).
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13 Target ABIs may have 8, 16, 32 or 64-bit @code{int} type. @code{char}
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14 can be wider than 8 bits.
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15
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16 GCC gets most of the information about the target machine from a machine
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17 description which gives an algebraic formula for each of the machine's
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18 instructions. This is a very clean way to describe the target. But when
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19 the compiler needs information that is difficult to express in this
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20 fashion, ad-hoc parameters have been defined for machine descriptions.
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21 The purpose of portability is to reduce the total work needed on the
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22 compiler; it was not of interest for its own sake.
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23
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24 @cindex endianness
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25 @cindex autoincrement addressing, availability
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26 @findex abort
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27 GCC does not contain machine dependent code, but it does contain code
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28 that depends on machine parameters such as endianness (whether the most
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29 significant byte has the highest or lowest address of the bytes in a word)
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30 and the availability of autoincrement addressing. In the RTL-generation
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31 pass, it is often necessary to have multiple strategies for generating code
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32 for a particular kind of syntax tree, strategies that are usable for different
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33 combinations of parameters. Often, not all possible cases have been
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34 addressed, but only the common ones or only the ones that have been
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35 encountered. As a result, a new target may require additional
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36 strategies. You will know
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37 if this happens because the compiler will call @code{abort}. Fortunately,
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38 the new strategies can be added in a machine-independent fashion, and will
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39 affect only the target machines that need them.
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