gccは特定のプラットフォーム上の各タイプの配置をどのように取得しますか？

Question

gccのソースにハードコード化されているのか、何らかの形でプログラマチックにフェッチされていますか？

Answer 1

アーチ専用のフォルダにハードコードされていると思います。 http://www.google.com/codesearch#Yj7Hz1ZInUg/trunk/gcc-4.2.1/gcc/config/sparc/sparc.h

SPARC

/* 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)) 

と同じフォルダにsparc.cため。

いくつかの基本的な配置ルールは、gcc/tree.cで定義されています。 voidの場合：

/* 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; 

ビルドプロセスでgccにコンパイルされます。

したがって、デフォルトのアライメントはコンパイルされますが、gccコードからTREEタイプのオブジェクトを操作することで変更できます。

UPDATE：アーム、MIPS、SPARCについて

/* 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 

、アライメントはアーチに記録することができる任意のマシン命令の必要（メモリへの非整列アクセスを制限する）他のarchsを：x86の設定ファイルは、より良いコメントを有します。 （例えば、sparc.md）