This is the LL code generated using the -output-ll switch of
ldc2 (it's a kind of nearly universal bytecode for llvm):
@_D5test51ayG16i = global [16 x i32] [i32 1, i32 2, i32 3, i32
4, i32 5, i32 6, i32 7, i32 8, i32 9, i32 10, i32 11, i32 12,
i32 13, i32 14, i32 15, i32 16]
@_D5test51byG16i = global [16 x i32] [i32 16, i32 15, i32 14,
i32 13, i32 12, i32 11, i32 10, i32 9, i32 8, i32 7, i32 6, i32
5, i32 4, i32 3, i32 2, i32 1]
@_D5test51cG16i = global [16 x i32] zeroinitializer
If I add a "align(16)" annotation to a, b, c it adds the align
16 annotation in the LL code too:
@_D5test51ayG16i = global [16 x i32] [i32 1, i32 2, i32 3, i32
4, i32 5, i32 6, i32 7, i32 8, i32 9, i32 10, i32 11, i32 12,
i32 13, i32 14, i32 15, i32 16], align 16
@_D5test51byG16i = global [16 x i32] [i32 16, i32 15, i32 14,
i32 13, i32 12, i32 11, i32 10, i32 9, i32 8, i32 7, i32 6, i32
5, i32 4, i32 3, i32 2, i32 1], align 16
@_D5test51cG16i = global [16 x i32] zeroinitializer, align 16
I didn't think to also take a look at the asm produced. Even if I
don't add align(16) in the code, and even if the LLVM IR doesn't
have an alignment annotation, the data in the asm has such
alignment:
.data
.globl __D4test1ayG16i
.align 16
__D4test1ayG16i:
.long 1
.long 2
.long 3
.long 4
.long 5
.long 6
.long 7
.long 8
.long 9
.long 10
.long 11
.long 12
.long 13
.long 14
.long 15
.long 16
.globl __D4test1byG16i
.align 16
__D4test1byG16i:
.long 16
.long 15
.long 14
.long 13
.long 12
.long 11
.long 10
.long 9
.long 8
.long 7
.long 6
.long 5
.long 4
.long 3
.long 2
.long 1
Regarding adding the alignment to the D type system, to assure
sliced data is aligned, a possible first step is to support:
void foo(align(16) double[] a1,
align(16) double[] a2) {}
That annotation is dynamically tested at entry point. So it's
similar to this:
void foo(double[] a1, double[] a2)
in {
assert(isAligned(a1, 16) && isAligned(a2, 16));
} body {}
But inside the body of foo() the compiler is allowed to use
aligned SIMD instructions (in theory this should be true for the
version with asserts too).
A more elegant (and maybe better) solution is to verify the
alignment statically. This means foo() accepts only 16-aligned
arrays. Array cast(), new array, dup, idup, and little else is
statically known to produce/return aligned arrays. And for the
other slices you need something like makeAligned!N() that returns
a statically known alignment.
Bye,
bearophile
