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

Question

Is this hardcoded in the gcc source or is it somehow programmed?

Answer 1

I think it is hardcoded in a special folder, for example. 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, for example. 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 during the build process.

So, alignments are compiled by default, but they can be changed by manipulating objects like TREE from gcc code.

UPDATE: x86 config has the best 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 arches of hands, mixes, sparks and others (which limits non-attached memory access), the required alignment of any machine command can be written in the arch.md file (for example, in sparc.md)