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digitalmars.D.announce - Re: DMD 1.034 and 2.018 releases - Align

reply bearophile <bearophileHUGS lycos.com> writes:
Aligned memory for array ops:

The MMX (etc), SSE, SSE2, SSE3, SSSE3, SSE4, SSE4.1, SSE4.2, SSE4a, SSE5, AVE
instructions require data aligned to 8/16 bytes (and maybe more in the future,
the D language has to be flexible for things present in the CPUs 3 years from
now), the current array ops of D contain code that manages such problems in the
alignment, but I presume properly aligned data may lead to better performance.

If the static/dynamic pointers in D aren't guaranteed to be aligned to 8/16
(and in the future maybe 32 bytes) then a syntax may be added to ensure it. 

I presume that something like this may solve the problem with dynamically
allocated pointers, but it's not nice looking, it's error-prone, and you have
to keep both pointers around in your program:

memPtr = malloc(sizeInBytes + alignmentInBytes - 1);
alignedPtr = (T*)( ((int)memPtr + alignmentInBytes - 1) & ~(alignmentInBytes -
1) );

A first possible syntax (mostly by LeoD):

align(16) int[10] a; // static
auto a = align(32) new int[100]; // dynamic pointer

Bye,
bearophile
Aug 11 2008
next sibling parent Pete <example example.com> writes:
bearophile Wrote:

 Aligned memory for array ops:
 
 The MMX (etc), SSE, SSE2, SSE3, SSSE3, SSE4, SSE4.1, SSE4.2, SSE4a, SSE5, AVE
instructions require data aligned to 8/16 bytes (and maybe more in the future,
the D language has to be flexible for things present in the CPUs 3 years from
now), the current array ops of D contain code that manages such problems in the
alignment, but I presume properly aligned data may lead to better performance.
 
 If the static/dynamic pointers in D aren't guaranteed to be aligned to 8/16
(and in the future maybe 32 bytes) then a syntax may be added to ensure it. 
 
 I presume that something like this may solve the problem with dynamically
allocated pointers, but it's not nice looking, it's error-prone, and you have
to keep both pointers around in your program:
 
 memPtr = malloc(sizeInBytes + alignmentInBytes - 1);
 alignedPtr = (T*)( ((int)memPtr + alignmentInBytes - 1) & ~(alignmentInBytes -
1) );
 
 A first possible syntax (mostly by LeoD):
 
 align(16) int[10] a; // static
 auto a = align(32) new int[100]; // dynamic pointer
 
 Bye,
 bearophile

Ah. You got there before me :-)
Aug 11 2008
prev sibling parent reply Don <nospam nospam.com.au> writes:
bearophile wrote:
 Aligned memory for array ops:
 
 The MMX (etc), SSE, SSE2, SSE3, SSSE3, SSE4, SSE4.1, SSE4.2, SSE4a, SSE5, AVE
instructions require data aligned to 8/16 bytes (and maybe more in the future,
the D language has to be flexible for things present in the CPUs 3 years from
now), the current array ops of D contain code that manages such problems in the
alignment, but I presume properly aligned data may lead to better performance.
 
 If the static/dynamic pointers in D aren't guaranteed to be aligned to 8/16
(and in the future maybe 32 bytes) then a syntax may be added to ensure it. 

Dynamic and static memory allocation of arrays is guaranteed to be aligned to 16 since D1.023. Stack-allocated arrays aren't yet aligned. I've added a possible solution in bugzilla #2278. There'll always be a problem with slicing, though -- if you start a slice from an odd index, it's going to be misaligned.
Aug 11 2008
next sibling parent Wayne Anderson <wanderon comcast.net> writes:
Don Wrote:

 bearophile wrote:
 Aligned memory for array ops:
 
 The MMX (etc), SSE, SSE2, SSE3, SSSE3, SSE4, SSE4.1, SSE4.2, SSE4a, SSE5, AVE
instructions require data aligned to 8/16 bytes (and maybe more in the future,
the D language has to be flexible for things present in the CPUs 3 years from
now), the current array ops of D contain code that manages such problems in the
alignment, but I presume properly aligned data may lead to better performance.
 
 If the static/dynamic pointers in D aren't guaranteed to be aligned to 8/16
(and in the future maybe 32 bytes) then a syntax may be added to ensure it. 

Dynamic and static memory allocation of arrays is guaranteed to be aligned to 16 since D1.023. Stack-allocated arrays aren't yet aligned. I've added a possible solution in bugzilla #2278. There'll always be a problem with slicing, though -- if you start a slice from an odd index, it's going to be misaligned.

Aug 11 2008
prev sibling parent "Koroskin Denis" <2korden gmail.com> writes:
On Mon, 11 Aug 2008 22:13:03 +0400, Wayne Anderson <wanderon comcast.net>  
wrote:

 Don Wrote:

 bearophile wrote:
 Aligned memory for array ops:

 The MMX (etc), SSE, SSE2, SSE3, SSSE3, SSE4, SSE4.1, SSE4.2, SSE4a,  

more in the future, the D language has to be flexible for things present in the CPUs 3 years from now), the current array ops of D contain code that manages such problems in the alignment, but I presume properly aligned data may lead to better performance.
 If the static/dynamic pointers in D aren't guaranteed to be aligned  

to ensure it. Dynamic and static memory allocation of arrays is guaranteed to be aligned to 16 since D1.023. Stack-allocated arrays aren't yet aligned. I've added a possible solution in bugzilla #2278. There'll always be a problem with slicing, though -- if you start a slice from an odd index, it's going to be misaligned.

being stored in the array. If the size of the type being stored is a multiple of 16 shouldn't any slice still be aligned?

I think he was talking about ints, floats and doubles since SSE# operate with those types.
Aug 11 2008