gcc - gcc 如何在特定平台上获得每种类型的对齐方式？

Question

它是硬编码到 gcc 的源代码中还是以某种方式以编程方式获取？

Answer 1

我认为它是硬编码在特定于 arch 的文件夹中的，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 在同一文件夹中。

一些基本的对齐规则在 gcc/tree.c 中定义，例如对于无效:

  /* 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成 gcc。

因此，默认对齐方式已编译，但可以通过操作 gcc 代码中的 TREE 类型对象来更改。

更新:x86 配置有更好的注释:

/* 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arch(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)