[thirdparty/gcc.git] / gcc / doc / portability.texi

@c Copyright (C) 1988-2021 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Portability
@chapter GCC and Portability
@cindex portability
@cindex GCC and portability

GCC itself aims to be portable to any machine where @code{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 @code{int} type.  @code{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.

@cindex endianness
@cindex autoincrement addressing, availability
@findex abort
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 @code{abort}.  Fortunately,
the new strategies can be added in a machine-independent fashion, and will
affect only the target machines that need them.
Commit	Line	Data
99dee823	1	@c Copyright (C) 1988-2021 Free Software Foundation, Inc.
73a8ed7e JM	2	@c This is part of the GCC manual.
	3	@c For copying conditions, see the file gcc.texi.
	4
	5	@node Portability
	6	@chapter GCC and Portability
	7	@cindex portability
	8	@cindex GCC and portability
	9
0c685f12 NS	10	GCC itself aims to be portable to any machine where @code{int} is at least
	11	a 32-bit type. It aims to target machines with a flat (non-segmented) byte
	12	addressed data address space (the code address space can be separate).
8a36672b	13	Target ABIs may have 8, 16, 32 or 64-bit @code{int} type. @code{char}
0c685f12	14	can be wider than 8 bits.
73a8ed7e JM	15
	16	GCC gets most of the information about the target machine from a machine
	17	description which gives an algebraic formula for each of the machine's
	18	instructions. This is a very clean way to describe the target. But when
	19	the compiler needs information that is difficult to express in this
962e6e00 JM	20	fashion, ad-hoc parameters have been defined for machine descriptions.
	21	The purpose of portability is to reduce the total work needed on the
	22	compiler; it was not of interest for its own sake.
73a8ed7e JM	23
	24	@cindex endianness
	25	@cindex autoincrement addressing, availability
	26	@findex abort
	27	GCC does not contain machine dependent code, but it does contain code
	28	that depends on machine parameters such as endianness (whether the most
	29	significant byte has the highest or lowest address of the bytes in a word)
	30	and the availability of autoincrement addressing. In the RTL-generation
	31	pass, it is often necessary to have multiple strategies for generating code
	32	for a particular kind of syntax tree, strategies that are usable for different
962e6e00 JM	33	combinations of parameters. Often, not all possible cases have been
	34	addressed, but only the common ones or only the ones that have been
	35	encountered. As a result, a new target may require additional
	36	strategies. You will know
73a8ed7e JM	37	if this happens because the compiler will call @code{abort}. Fortunately,
	38	the new strategies can be added in a machine-independent fashion, and will
	39	affect only the target machines that need them.