Andrei Alexandrescu:
http://youtube.com/watch?v=q_39RnxtkgM
Very nice.
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Slide 3:
In practise, say we have iterative code like this:
int data[100];
for(int i = 0; i < data.length; ++i) {
data[i] += 10; }
For code like that in D we have vector ops:
int[100] data;
data[] += 10;
Regarding vector ops: currently they are written with handwritten
asm that uses SIMD where possible. Once std.simd is in good shape
I think the array ops can be rewritten (and completed in their
missing parts) using a higher level style of coding.
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Slide 22:
Comparisons:
Full suite of comparisons Can produce bit-masks, or boolean
'any'/'all' logic.
Maybe a little of compiler support (for the syntax) will help
here.
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Slide 26:
Always pass vectors by value.
Unfortunately it seems a bad idea to give a warning if you pass
one of those by reference.
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Slide 27:
3. Use ‘leaf’ functions where possible.
I am not sure how much good it is to enforce leaf functions with
a @leaf annotation.
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Slide 32:
Experiment with prefetching?
Are D intrinsics offering instructions to perform prefetching?
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LDC2 is supports SIMD on Windows32 too.
So for this code:
void main() {
alias double2 = __vector(double[2]);
auto a = new double[200];
auto b = cast(double2[])a;
double2 tens = [10.0, 10.0];
b[] += tens;
}
LDC2 compiles it to:
movl $200, 4(%esp)
movl $__D11TypeInfo_Ad6__initZ, (%esp)
calll __d_newarrayiT
movl %edx, %esi
movl %eax, (%esp)
movl $16, 8(%esp)
movl $8, 4(%esp)
calll __d_array_cast_len
testl %eax, %eax
je LBB0_3
movapd LCPI0_0, %xmm0
.align 16, 0x90
LBB0_2:
movapd (%esi), %xmm1
addpd %xmm0, %xmm1
movapd %xmm1, (%esi)
addl $16, %esi
decl %eax
jne LBB0_2
LBB0_3:
xorl %eax, %eax
addl $12, %esp
popl %esi
ret
It uses addpd that works with two doubles at the same time.
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The Reddit thread contains a link to this page, a compiler for a
C variant from Intel that's optimized for SIMD:
http://ispc.github.io/
Some of the syntax of ispc:
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The first of these statements is cif, indicating an if statement
that is expected to be coherent. The usage of cif in code is just
the same as if:
cif (x < y) {
...
} else {
...
}
cif provides a hint to the compiler that you expect that most of
the executing SPMD programs will all have the same result for the
if condition.
Along similar lines, cfor, cdo, and cwhile check to see if all
program instances are running at the start of each loop
iteration; if so, they can run a specialized code path that has
been optimized for the "all on" execution mask case.
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foreach_tiled(y = y0 ... y1, x = 0 ... w,
u = 0 ... nsubsamples, v = 0 ... nsubsamples) {
float du = (float)u * invSamples, dv = (float)v * invSamples;
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I'll take a better look at ispc.
Bye,
bearophile
