In the C programming language, Duff's device is a technique of manually implementing loop unrolling by interleaving `do-while loop` and a `switch` statement.
Questions tagged [duffs-device]
22 questions
180
votes
11 answers
How does Duff's device work?
I've read the article on Wikipedia on the Duff's device, and I don't get it. I am really interested, but I've read the explanation there a couple of times and I still don't get it how the Duff's device works.
What would a more detailed explanation…
hhafez
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22
votes
5 answers
The behavior of the for loop split between switch cases
While playing with the code I've noticed a strange behavior that I do not know to explain the logic behind
void foo(int n)
{
int m = n;
while (--n > 0)
{
switch (n)
{
case -1:
case 0:
…
dEmigOd
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16
votes
2 answers
Switch case weird scoping
Reviewing some 3rd party C code I came across something like:
switch (state) {
case 0:
if (c=='A') { // open brace
// code...
break; // brace not closed!
case 1:
// code...
break;
} // close brace!
case 2:
//…
Ricibob
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11
votes
2 answers
Does Duff's Device Speed up Java Code?
Using the stock Sun 1.6 compiler and JRE/JIT, is it a good idea to use the sort of extensive unroll exemplified by Duff's Device to unroll a loop? Or does it end up as code obfuscation with no performance benefit?
The Java profiling tools I've used…
bmargulies
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9
votes
2 answers
Duff's device in Swift
We know that a Duff's device makes use of interlacing the structures of a fallthrough switch and a loop like:
send(to, from, count)
register short *to, *from;
register count;
{
register n = (count + 7) / 8;
switch (count % 8) {
case 0:…
loretoparisi
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8
votes
1 answer
Mixed 'switch' and 'while' in C
I've recently read this page about strange C snippet codes. Most of them was understandable. But I can't understand this one:
switch(c & 3) while((c -= 4) >= 0){
foo(); case 3:
foo(); case 2:
foo(); case 1:
foo(); case 0:
}
Can…
frogatto
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8
votes
3 answers
Revising the syntax of Duff's device - Is this legal C/C++?
Only last night I encountered the curious Duff's device for the first time. I've been doing some reading on it, and I don't think it's that daunting to understand. What I am curious about is the strange syntax (from Wikipedia):
register short *to,…
monkey0506
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6
votes
5 answers
How can Duff's device code be compiled?
I understood why Duff's device is faster than normal loop code which can be unrolled but is not optimized. But I can't understand how the code can be compiled yet.
I guess it's a trick about the switch syntax. But not anymore.
How can do while…
Benjamin
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6
votes
3 answers
Is Duff's device still useful?
I see that Duff's device is just to do loop unrolling in C.
https://en.wikipedia.org/wiki/Duff%27s_device
I am not sure why it is still useful nowadays. Isn't that the compiler should be smart enough to do loop-unrolling?
user1424739
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5
votes
1 answer
Is there any problem in jumping into if(false) block?
I've read already a couple times (e.g. here Compiler: What if condition is always true / false) that any decent c++ compiler will opt-out something like
if(false)
{
...
}
But what if there is an intentional jump into this if(false) block. I'm…
ezegoing
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5
votes
2 answers
Porting duff's device from C to JavaScript
I have this kind of Duff's device in C and it works fine (format text as money):
#include
#include
char *money(const char *src, char *dst)
{
const char *p = src;
char *q = dst;
size_t len;
len = strlen(src);
…
David Ranieri
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5
votes
13 answers
Can I use Duff's Device on an array in C?
I have a loop here and I want to make it run faster. I am passing in a large array. I recently heard of Duff's Device can it be applied to this for loop? any ideas?
for (i = 0; i < dim; i++) {
for (j = 0; j < dim; j++) {
…
Ben Fossen
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4
votes
1 answer
Writing a macro for Duff's Device
From Zed Shaw's, Learn C the Hard Way,in exercise 23, he talks about Duff's Device. Here is Duff's Device for reference:
int duffs_device(char *from, char *to, int count)
{
{
int n = (count + 7) / 8;
switch(count % 8) {
…
Fumbster
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- 7
3
votes
3 answers
C switch statement with do-while interleaved
Possible Duplicate:
How does Duff's device work?
I am trying to understand how this is working. Any help would be appreciated.
#include
void duff(int count)
{
int n=(count+7)/8;
printf("n=%d count =%d\n",n,count%8);
…
Pkp
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1
vote
2 answers
Duff device in PHP not possible?
I've been told that a duff device doesn't work with PHP, because the switch and case construct working different. I've found this duff devive on php.net, my question is what is wrong with this device? Or didn't I understand a duff device? In my…
Micromega
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