diff options
Diffstat (limited to 'gcc/doc')
| -rw-r--r-- | gcc/doc/extend.texi | 28 | ||||
| -rw-r--r-- | gcc/doc/tm.texi | 5 |
2 files changed, 22 insertions, 11 deletions
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index f131a4da2ba..5a7e2d83e29 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -3697,21 +3697,27 @@ that forces the union to be double-word aligned. As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor -and just ask the compiler to align a variable or field to the maximum -useful alignment for the target machine you are compiling for. For -example, you could write: +and just ask the compiler to align a variable or field to the +default alignment for the target architecture you are compiling for. +The default alignment is sufficient for all scalar types, but may not be +enough for all vector types on a target which supports vector operations. +The default alignment is fixed for a particular target ABI. + +Gcc also provides a target specific macro @code{__BIGGEST_ALIGNMENT__}, +which is the largest alignment ever used for any data type on the +target machine you are compiling for. For example, you could write: @smallexample -short array[3] __attribute__ ((aligned)); +short array[3] __attribute__ ((aligned (__BIGGEST_ALIGNMENT__))); @end smallexample -Whenever you leave out the alignment factor in an @code{aligned} attribute -specification, the compiler automatically sets the alignment for the declared -variable or field to the largest alignment which is ever used for any data -type on the target machine you are compiling for. Doing this can often make -copy operations more efficient, because the compiler can use whatever -instructions copy the biggest chunks of memory when performing copies to -or from the variables or fields that you have aligned this way. +The compiler automatically sets the alignment for the declared +variable or field to @code{__BIGGEST_ALIGNMENT__}. Doing this can +often make copy operations more efficient, because the compiler can +use whatever instructions copy the biggest chunks of memory when +performing copies to or from the variables or fields that you have +aligned this way. Note that the value of @code{__BIGGEST_ALIGNMENT__} +may change depending on command line options. When used on a struct, or struct member, the @code{aligned} attribute can only increase the alignment; in order to decrease it, the @code{packed} diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi index d748a754e2d..92de6e5b8ac 100644 --- a/gcc/doc/tm.texi +++ b/gcc/doc/tm.texi @@ -1105,6 +1105,11 @@ Alignment, in bits, a C conformant malloc implementation has to provide. If not defined, the default value is @code{BITS_PER_WORD}. @end defmac +@defmac ATTRIBUTE_ALIGNED_VALUE +Alignment used by the @code{__attribute__ ((aligned))} construct. If +not defined, the default value is @code{BIGGEST_ALIGNMENT}. +@end defmac + @defmac MINIMUM_ATOMIC_ALIGNMENT If defined, the smallest alignment, in bits, that can be given to an object that can be referenced in one operation, without disturbing any |