• d-bugmail puremagic.com (42/42) Jan 29 2014 https://d.puremagic.com/issues/show_bug.cgi?id=12033

d-bugmail puremagic.com writes:

https://d.puremagic.com/issues/show_bug.cgi?id=12033

           Summary: align() for array arguments of functions
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



At the moment this enhancement proposal is not fully fleshed out, but it's a
starting point for future discussions.

Perhaps it can be useful to support a syntax like:


void foo(align(4) ubyte[] buf) {}
void bar(align(16) double[] arr) {}


In a debug build it performs a run-time test that the .ptr field is aligned to
4 or 16 bytes. This is very handy for many situations, including functions that
need to work with the CPU SIMD registers.

It's equivalent to code like:

void bar(double[] arr)
in {
    assert(arr.ptr % 16 == 0);
} body {}


But ideally the D type system should carry around the "align(4)" information at
compile-time as an optional part of the type of an array (and avoid some of
those compile-time tests). Once this information is an optional part of the
type of an array, a templated function can tell apart the two cases:

void bar(double[] arr) if (__traits(alignof, arr) == 16) {}
void bar(double[] arr) if (__traits(alignof, arr) != 16) {}

If an array type doesn't have a specified alignment, it is assumed to have the
natural alignment of the type it contains.

I think that in a statically type system language a compile-time knowledge of
the alignment of a pointer or array is a very valuable information in many
cases. I find it strange that in D you can use int[5] to express the idea of a
compile-time knowledge of an array of 5 ints, but you can't express the
alignment of the data using the built-in type system of D.

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