Porting some code from C to D I found the inline assembler very
convenient. This is the C code (using an external NASM file):
// dot_product returns dot product t*w of n elements. n is rounded
// up to a multiple of 8. Result is scaled down by 8 bits.
#ifdef NOASM // no assembly language
int dot_product(short *t, short *w, int n) {
int sum=0;
n=(n+7)&-8;
for (int i=0; i<n; i+=2) {
if (lol >= 21567) printf("dp %d %d %d %d %d %d\n", n, i, t[i], w[i],
t[i+1], w[i+1]);
sum+=(t[i]*w[i]+t[i+1]*w[i+1]) >> 8;
}
return sum;
}
#else // The NASM version uses MMX and is about 8 times faster.
extern "C" int dot_product(short *t, short *w, int n); // in NASM
#endif
In D, I can move the ASM inside the function, so there is no need for two
declarations:
extern (C) int dot_product(short *t, short *w, const int n) {
version (D_InlineAsm_X86_64) asm {
naked;
mov RCX, RDX; // n
mov RAX, RDI; // a
mov RDX, RSI; // b
cmp RCX, 0;
jz done;
sub RAX, 16;
sub RDX, 16;
pxor XMM0, XMM0; // sum = 0
loop: // each loop sums 4 products
movdqa XMM1, [RAX+RCX*2];// put parital sums of vector product in
xmm1
pmaddwd XMM1, [RDX+RCX*2];
psrad XMM1, 8;
paddd XMM0, XMM1;
sub RCX, 8;
ja loop;
movdqa XMM1, XMM0; // add 4 parts of xmm0 and return in
eax
psrldq XMM1, 8;
paddd XMM0, XMM1;
movdqa XMM1, XMM0;
psrldq XMM1, 4;
paddd XMM0, XMM1;
movq RAX, XMM0;
done:
ret;
} else {
int sum = 0;
for (int i = 0; i < n; i += 4) {
sum += (t[i ]*w[i ] + t[i+1]*w[i+1]) >> 8;
sum += (t[i+2]*w[i+2] + t[i+3]*w[i+3]) >> 8;
}
return sum;
}
}
This example also shows, how 'naked' should probably not be applied to the
function declaration, because it contains non-asm code as well. (It could
be "naked asm" though.) For compatibility with GDC (and in fact the
original NASM code), I used extern(C) here as the parameter passing
strategy.
This may also serve as a practical use case for vector operations.
