Hello internals,
I’d like to share an early proposal idea that introduces the ability to
express loop unrolling directly from userland using a simple attribute
syntax: #[Unroll(N)].
Motivation
Loop unrolling is a well-known optimization strategy used by compilers to
reduce the overhead of conditional jumps and improve CPU pipelining by
expanding the loop body multiple times.
In PHP, we often hand-unroll loops for performance gains in hot paths, like
so:
for ($i = 0; $i < $n; $i += 4) {
$sum += $array[$i] + $array[$i + 1] + $array[$i + 2] + $array[$i + 3];
}
This is tedious and error-prone, especially when the logic is more complex.
Idea: #[Unroll(N)] Syntax
What if PHP allowed users to annotate loops like this?
#[Unroll(4)]for ($i = 0; $i < $n; $i++) {
$sum += $array[$i];
}
And the compiler could then transform this into an equivalent unrolled
version at compile time, removing the need for manual duplication and
keeping the source readable.
Proof-of-Concept
In a patched version of the PHP compiler, we implemented a special-case
optimization for exactly this loop pattern:
<?php
// Proposal idea of enabling the unrolling in the userland.
function measure(string $label, callable $fn): void {
$start = hrtime(true);
$result = $fn();
$end = hrtime(true);
$elapsed = ($end - $start) / 1e9;
echo "Sum: {$result}, {$label}: " . number_format($elapsed, 6) . "
seconds\n";
}
$array = range(1, 1e6);
$n = count($array);
measure("Normal for loop", function () use ($array, $n) {
$sum = 0;
for ($i = 0; $i < $n; $i++) {
$sum += $array[$i];
}
return $sum;
}); // Sum: 500000500000, Normal for loop: 0.047172 seconds
measure("Manual Unrolled x4", function () use ($array, $n) {
$sum = 0;
for ($i = 0; $i < $n; $i += 4) {
$sum += $array[$i] + $array[$i + 1] + $array[$i + 2] + $array[$i + 3];
}
return $sum;
}); // Sum: 500000500000, Manual Unrolled x4: 0.042200 seconds
measure("Manual Unrolled x8", function () use ($array, $n) {
$sum = 0;
for ($i = 0; $i < $n; $i += 8) {
$sum += $array[$i] + $array[$i + 1] + $array[$i + 2] + $array[$i + 3]
+ $array[$i + 4] + $array[$i + 5] + $array[$i + 6] +
$array[$i + 7];
}
return $sum;
}); // Sum: 500000500000, Manual Unrolled x8: 0.036472 seconds
// Attributed Unroll - this is handled by your modified compiler
measure("Attributed Unrolled #[Unroll(4)]", function () use ($array, $n) {
$sum = 0;
// @TODO: KhaledAlam: Unrolling logic of zend_compile_for( )
// #[Unroll(4)]
for ($i = 0; $i < $n; $i++) {
$sum += $array[$i];
}
return $sum;
}); // Sum: 0, Attributed Unrolled #[Unroll(4)] expected to be less
than normal loop
um: 500000500000, Normal for loop: 0.006646 seconds
Sum: 500000500000, Manual Unrolled x4: 0.005646 seconds
Sum: 500000500000, Manual Unrolled x8: 0.004885 seconds
Sum: 500000500000, Attributed Unrolled #[Unroll(4)]: expected to be
less than normal loop
Limitations & Scope
This is not a full RFC yet, just an idea seeking feedback. Current
limitations:
No generic AST replacement yet, just a special-case demo.
Doesn't handle complex control flow, non-trivial statements, or
multi-variable iteration.
Attribute is ignored in production PHP, requires patched compiler.
Open Questions
Is there interest in allowing user-directed optimizations like this?
Would PHP be willing to expand attribute-driven compiler behavior in
this direction?
What are the risks in exposing low-level optimizations to users?
Could this be generalized in the JIT instead?
Source & Demo
Happy to share an initial POC underdevelopment patch[0] if there's interest.
Looking forward to your thoughts.
Best regards,
Khaled Alam
[0] https://gist.github.com/khaledalam/2f2237733e1cc3ec74d597b1b20d94df