PHP Loop Unrolling: An #[Unroll(N)] Attribute RFC Idea

Hello,

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:#

https://3v4l.org/T23MH


<?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

[PHP-DEV] [RFC Idea] Proposal: Loop Unrolling in Userland via #[Unroll(N)] Attribute