How does gcc get the alignment for each type on a specific platform?

Is it hard coded into gcc's source or fetched somehow progra开发者_如何转开发matically?

I think it is hardcoded in arch-specific folder, e.g. for 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))

and sparc.c in the same folder.

Some basic alignment rules are defined in gcc/tree.c e.g. for void:

  /* 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;

It will be compiled into gcc in the build process.

So, default aligns are compiled in, but can be changed by manipulating TREE type objects from the gcc code.

UPDATE: x86 config has better comments:

/* 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

For arm, mips, sparc, and others archs (which limits unaligned access to memory) the alignment requires of any machine instruction may be recorded in arch.md file (e.g. in sparc.md)