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fixes and changes the recently introduced 'alignas' to be 'align' (#12666)

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* fixes and changes the recently introduced 'alignas' to be 'align'

* more improvements
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Andreas Rumpf 2019-11-15 20:24:14 +01:00 • committed by GitHub
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doc/manual.rst
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@ -6433,38 +6433,39 @@ generates:
unsigned int flag:1;
};
Alignas pragma
--------------
The ``alignas`` pragma is for variables and object field members. It
modifies the alignment requirement of the thing being declared. The
argument must be a constant power of 2 or 0. Valid non-zero
alignments that are weaker than another alignas pragmas on the same
declaration are ignored. Alignments that are weaker that the
alignment requirement of the type are ignored. ``alignas(0)`` is
always ignored.
Align pragma
------------
The `align`:idx: pragma is for variables and object field members. It
modifies the alignment requirement of the entity being declared. The
argument must be a constant power of 2. Valid non-zero
alignments that are weaker than nother align pragmas on the same
declaration are ignored. Alignments that are weaker that the
alignment requirement of the type are ignored.
.. code-block:: Nim
type
sseType = object
sseData {.alignas(16).}: array[4,float32]
sseData {.align(16).}: array[4,float32]
# every object will be aligned to 128-byte boundary
Data = object
x: char
cacheline {.alignas(128).}: array[128, char] # over-aligned array of char,
cacheline {.align(128).}: array[128, char] # over-aligned array of char,
proc main() =
echo "sizeof(Data) = ", sizeof(Data), " (1 byte + 127 bytes padding + 128-byte array)"
# output: sizeof(Data) = 256 (1 byte + 127 bytes padding + 128-byte array)
echo "alignment of sseType is ", alignof(sseType)
# output: alignment of sseType is 16
var d {.alignas(2048).}: Data # this instance of data is aligned even stricter
var d {.align(2048).}: Data # this instance of data is aligned even stricter
main()
This pragma has no effect on nimvm or the js backend.
This pragma has no effect for the JS backend.
Volatile pragma
---------------