它是硬编码到gcc的源代码还是以某种方式以编程方式获取？
答案 0 :(得分：5)
我认为它在特定于arch的文件夹中是硬编码的，例如对于sparc
http://www.google.com/codesearch#Yj7Hz1ZInUg/trunk/gcc-4.2.1/gcc/config/sparc/sparc.h
/* No data type wants to be aligned rounder than this. */
#define BIGGEST_ALIGNMENT (TARGET_ARCH64 ? 128 : 64)
/* The best alignment to use in cases where we have a choice. */
#define FASTEST_ALIGNMENT 64
...
/* Make strings word-aligned so strcpy from constants will be faster. */
#define CONSTANT_ALIGNMENT(EXP, ALIGN) \
((TREE_CODE (EXP) == STRING_CST \
&& (ALIGN) < FASTEST_ALIGNMENT) \
? FASTEST_ALIGNMENT : (ALIGN))
/* Make arrays of chars word-aligned for the same reasons. */
#define DATA_ALIGNMENT(TYPE, ALIGN) \
(TREE_CODE (TYPE) == ARRAY_TYPE \
&& TYPE_MODE (TREE_TYPE (TYPE)) == QImode \
&& (ALIGN) < FASTEST_ALIGNMENT ? FASTEST_ALIGNMENT : (ALIGN))
和sparc.c在同一个文件夹中。
一些基本的对齐规则在gcc / tree.c中定义，例如无效：
/* We are not going to have real types in C with less than byte alignment,
so we might as well not have any types that claim to have it. */
TYPE_ALIGN (void_type_node) = BITS_PER_UNIT;
TYPE_USER_ALIGN (void_type_node) = 0;
它将在构建过程中编译为gcc。
因此，编译默认对齐，但可以通过从gcc代码中操作TREE类型对象来更改。
更新：x86配置有更好的评论：
/* Minimum size in bits of the largest boundary to which any
and all fundamental data types supported by the hardware
might need to be aligned. No data type wants to be aligned
rounder than this.
Pentium+ prefers DFmode values to be aligned to 64 bit boundary
and Pentium Pro XFmode values at 128 bit boundaries. */
#define BIGGEST_ALIGNMENT 128
/* Decide whether a variable of mode MODE should be 128 bit aligned. */
#define ALIGN_MODE_128(MODE) \
((MODE) == XFmode || SSE_REG_MODE_P (MODE))
/* The published ABIs say that doubles should be aligned on word
boundaries, so lower the alignment for structure fields unless
-malign-double is set. */
/* ??? Blah -- this macro is used directly by libobjc. Since it
supports no vector modes, cut out the complexity and fall back
on BIGGEST_FIELD_ALIGNMENT. */
...
#define BIGGEST_FIELD_ALIGNMENT 32
...
/* If defined, a C expression to compute the alignment given to a
constant that is being placed in memory. EXP is the constant
and ALIGN is the alignment that the object would ordinarily have.
The value of this macro is used instead of that alignment to align
the object.
If this macro is not defined, then ALIGN is used.
The typical use of this macro is to increase alignment for string
constants to be word aligned so that `strcpy' calls that copy
constants can be done inline. */
#define CONSTANT_ALIGNMENT(EXP, ALIGN) ix86_constant_alignment ((EXP), (ALIGN))
/* If defined, a C expression to compute the alignment for a static
variable. TYPE is the data type, and ALIGN is the alignment that
the object would ordinarily have. The value of this macro is used
instead of that alignment to align the object.
If this macro is not defined, then ALIGN is used.
One use of this macro is to increase alignment of medium-size
data to make it all fit in fewer cache lines. Another is to
cause character arrays to be word-aligned so that `strcpy' calls
that copy constants to character arrays can be done inline. */
#define DATA_ALIGNMENT(TYPE, ALIGN) ix86_data_alignment ((TYPE), (ALIGN))
/* If defined, a C expression to compute the alignment for a local
variable. TYPE is the data type, and ALIGN is the alignment that
the object would ordinarily have. The value of this macro is used
instead of that alignment to align the object.
If this macro is not defined, then ALIGN is used.
One use of this macro is to increase alignment of medium-size
data to make it all fit in fewer cache lines. */
#define LOCAL_ALIGNMENT(TYPE, ALIGN) ix86_local_alignment ((TYPE), (ALIGN))
...
/* Set this nonzero if move instructions will actually fail to work
when given unaligned data. */
#define STRICT_ALIGNMENT 0
对于arm，mips，sparc和其他archs（限制对内存的未对齐访问），任何机器指令的对齐要求都可以记录在arch.md文件中（例如在sparc.md中）