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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteDuff’s Device is a C loop-unrolling technique that uses a switch with fall-through to handle a partial group of operations before processing full groups. JavaScript can adapt the remainder-handling idea, but it cannot use the original C construction literally. Neither form guarantees faster execution: measure the real workload on its target runtime.
What is Duff’s Device?
Tom Duff devised the technique for a real-time animation program, not as a general-purpose memory-copy optimization. His reproduced note, dated 10 November 1983, describes copying shorts to the programmed I/O data register of an Evans & Sutherland Picture System II. The animation program was running about 50% slower than it needed to, by Duff’s historical estimate—not a modern benchmark result. In a later message dated 29 August 1988, he said he invented the device while at Lucasfilm.
The destination in the original example is deliberately a fixed register address: each write sends the next value to the same programmed I/O location. It does not increment as a destination pointer would in an ordinary memory-to-memory copy. Duff cautioned that comparing the device with memcpy can miss this difference in workload. Tom Duff’s reproduced note and 1988 message
As Duff put it, “The point of the device is to express general loop unrolling directly in C.” He also wrote, “I feel a combination of pride and revulsion at this discovery.” Russ Cox’s historical account says Duff first described the device in a November 1983 email, posted a revised note in May 1984, and gave it its name in that message; Cox also notes that Bjarne Stroustrup used a variant in The C++ Programming Language. Russ Cox’s account of Duff’s Device
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How does the switch handle the remainder?
The original pattern unrolls eight operations into one loop body. For a positive integer count, count % 8 gives the number of operations in the first partial group, while (count + 7) / 8 gives the number of loop groups.
The switch selects a case label positioned within the unrolled loop body. Since the cases have no intervening break statements, execution falls through the remaining operations. Once it reaches the loop’s end, later passes execute all eight operations in each full group.
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For example, with a count of 11, the remainder is 3 and the group count is 2. Execution begins at case 3, runs three operations, then continues through a second loop pass with eight operations. The total is 11. This is not a conventional switch in which each case handles one independent branch: the fall-through is the mechanism that performs the work.
/* Schematic C pattern: write to a fixed I/O register. */
void copy_to_register(volatile short *to, short *from, int count)
{
int groups = (count + 7) / 8;
switch (count % 8) {
case 0: do { *to = *from++;
case 7: *to = *from++;
case 6: *to = *from++;
case 5: *to = *from++;
case 4: *to = *from++;
case 3: *to = *from++;
case 2: *to = *from++;
case 1: *to = *from++;
} while (--groups > 0);
}
}This schematic follows the original control-flow idea; actual device access, types, qualifiers, and input validation depend on the program. C permits case labels within the switch body even when they occur inside a nested loop statement, which is what makes this unusual arrangement possible. Duff’s original example Explanation of the grouping arithmetic
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What inputs and language rules matter?
Guard nonpositive counts and validate the range
The classic do-while form assumes a positive count. Do not call it with zero or a negative value: the first operation runs before the loop condition is checked, so the code can write when it should do no work. Check that count is positive and that the input contains at least that many values. A production implementation should also account for the limits and types of its count arithmetic.
C and JavaScript do not permit the same construction
The interleaving relies on C’s case-label placement rules. In JavaScript, a case clause must be directly inside its switch block; it cannot label an assignment nested inside a loop as the C version does. A JavaScript adaptation can arrange a switch to fall through the tail of an unrolled sequence, but that is an adaptation of the remainder-handling idea—not a literal port of Duff’s Device. JavaScript adaptation and benchmark discussion
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For other interpreted languages, do not infer that the C or JavaScript pattern ports directly. Check that language’s case-label placement, fall-through behavior, and execution model before using or describing an equivalent.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does loop unrolling make interpreted code faster?
Not reliably. Fewer loop-control operations can help in some circumstances, but a hand-unrolled pattern can also increase code size, make control flow harder to follow, and interact differently with each compiler or runtime. Apple’s archived performance guidance recommends establishing a baseline and reevaluating unrolled code; it warns that unrolling usually increases code size and memory footprint and can raise paging risk. Apple’s archived performance guidance
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Duff’s advice was to measure generated code: “Transformations like this can only be justified by measuring the resulting code.” For compiled C, inspect the output and benchmark on the target hardware with the production compiler and settings. For JavaScript, benchmark the real workload with the target engine and version; a result from one engine, CPU, or benchmark does not establish a general speedup.
In a 2026 article, Vladimir Lazutkin reported a 19.5% win for a Duff-style JavaScript variant in a Node 22 benchmark on an i9-11900K. He described results ranging up to 40% across the JavaScript configurations he tested, with outcomes also near parity or worse in some configurations. Those are that author’s environment-specific results, not independent tests or expected gains for other programs. Lazutkin’s JavaScript benchmark article
Which loop approach should you choose?
| Approach | Best fit | Trade-off to check |
|---|---|---|
| Plain loop | Clear default for ordinary repeated work | Measure whether loop overhead matters in the actual workload. |
| Manual unrolling with a tail loop | Code that needs explicit groups but can keep remainder handling separate | More code and boundary logic; verify count cases and benchmark the target. |
| Duff-style switch and loop | A case where fall-through remainder handling is valid and demonstrated to help | Unfamiliar control flow, language-specific rules, and possible code-size cost. |
For ordinary memory copying, use an appropriate built-in or library copy operation rather than assuming the original fixed-register I/O pattern is a better substitute. For any unrolling strategy, test zero, small counts, exact multiples of eight, and counts with each possible remainder; confirm input bounds and compare runtime as well as code size.
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