On 27/07/2026 10:31, Jeevan Chalke wrote:
Hello ChenhuiMo,
This is a really interesting optimization — doing memcpy() exponentially
is a great idea for larger counts. My main use case for repeat() is
generating large files and padding, where count is usually large, so
this change helps exactly the workload I care about.
That said, I'm not entirely sure how common this usage pattern is in
general production workloads, so I was initially unsure whether the
added complexity is worth it. Setting that aside, the performance gain
for short strings repeated a large number of times is real, so I'm fine
with getting this in.
Yeah, probably the most common case is something like repeat('x',
1000000). It might even be worth special-casing the single-byte case and
turn that into a single memset() call.
A few things worth looking at:
1. The magic number 8 is unexplained. The naive-loop fallback threshold
seems quite low — should it be closer to a few dozen, say 64? I
benchmarked a range of thresholds and counts: the doubling approach
turns out to beat the naive loop even at very small counts (as low as 2–
3), so 8 already looks like a reasonable cutoff; raising it to 64 would
likely make counts in the 8–64 range slower, not faster. Regardless of
the exact value chosen, the constant needs a comment explaining why it
was picked.
2. Because of the exponential growth, CHECK_FOR_INTERRUPTS() granularity
drops off logarithmically; the later doubling steps can copy hundreds of
MB in a single memcpy that can't be interrupted partway through. Would
it help to cap the chunk size, say at 64–100MB? I tested this: capping
adds only a handful of extra memcpy calls even for very large repeats,
and the overhead was within noise; so it looks like a cheap way to bound
worst-case interrupt latency.
Beyond certain size, I'd guess it might even become slower, if the
string no longer fits in the L0 CPU cache for example. Also, if the
fast-path used a constant size, like 64 bytes, maybe the compiler could
optimize the memcpy() into a single SIMD instruction or something.
- Heikki
