On Thursday, 15 May 2014 at 13:10:29 UTC, Dicebot wrote:
If compiler lacks contextual knowledge, than only means that
range is not actually semantically equivalent to a loop.
Not really. Here is a simple example, a sawtooth generator that
goes from 1 to 0 with a cycle length >=2 (nyquist).
import std.algorithm;
import std.math;
struct gen_sawtooth {
enum bool empty = false;
float front;
float delta;
this(immutable float samps_per_cycle){
front = 1.0f;
delta = 1.0f/samps_per_cycle;
}
void popFront() {
front -= delta;
if(front < 0.0f) front += 1.0f;
}
}
void rangefill_sawtooth(float[] a, immutable float
samps_per_cycle){
auto gen = gen_sawtooth(samps_per_cycle);
fill(a,gen);
}
void fill_sawtooth(float[] a,immutable float samps_per_cycle) {
if(a.length==0) return;
immutable float delta = 1.0f/samps_per_cycle;
immutable max_cycle_length =
cast(uint)floor(samps_per_cycle*1.0001+1);
immutable lastcycle = a.length - max_cycle_length;
float v = 1.0f;
uint i = 0;
do {
do{ // main inner loop
a[i] = v;
++i;
v -= delta;
} while(v>=0.0f);
v += 1.0f;
} while( i < lastcycle );
for(;i<a.length;++i){ // can be optimized further
a[i] = v;
v -= delta;
if (v<0.0f) v += 1.0f;
}
}
The generated code for the range based version does not create a
fast innerloop, it lacks enough context and smarts:
fill_sawtooth main inner loop:
loop:
incl %eax
leal -2(%rax), %edx
movss %xmm1, (%rbx,%rdx,4)
subss %xmm0, %xmm1
ucomiss %xmm3, %xmm1
jae loop
rangefill_sawtooth inner loop:
loop:
movss %xmm3, (%rsi)
subss %xmm2, %xmm3
decq %rdi
ucomiss %xmm3, %xmm0
jbe skip
addss %xmm1, %xmm3
skip:
addq $4, %rsi
testq %rdi, %rdi
jne loop
What is wrong is assumption that such kinds of loops are
anything but tiny minority and this warrants generic "ranges
can never be as fast as loops" statement.
Not without significant work…
Floating point math is inaccurate, that means the compiler
will have to guess what kind of accuracy you are happy with…
AFAIK it is actually not true. Floating point standard defines
basic precision guarantees
Minimum yes. That makes drift unpredictable. Sometimes you care
more about deltas than absolute values.
