GCC and Portability
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GCC itself aims to be portable to any machine where int is at least a 32-bit type.  It aims to target machines with a flat (non-segmented) byte addressed data address space (the code address space can be separate). Target ABIs may have 8, 16, 32 or 64-bit int type. char can be wider than 8 bits.

GCC gets most of the information about the target machine from a machine description which gives an algebraic formula for each of the machine's instructions.  This is a very clean way to describe the target.  But when the compiler needs information that is difficult to express in this fashion, ad-hoc parameters have been defined for machine descriptions. The purpose of portability is to reduce the total work needed on the compiler; it was not of interest for its own sake.

GCC does not contain machine dependent code, but it does contain code that depends on machine parameters such as endianness (whether the most significant byte has the highest or lowest address of the bytes in a word) and the availability of autoincrement addressing.  In the RTL-generation pass, it is often necessary to have multiple strategies for generating code for a particular kind of syntax tree, strategies that are usable for different combinations of parameters.  Often, not all possible cases have been addressed, but only the common ones or only the ones that have been encountered.  As a result, a new target may require additional strategies.  You will know if this happens because the compiler will call abort.  Fortunately, the new strategies can be added in a machine-independent fashion, and will affect only the target machines that need them.