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rust source #1
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mrustc (master)
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pub unsafe fn is_equal_raw( mut x: *const u8, mut y: *const u8, mut n: usize, ) -> bool { while n >= 4 { let vx = x.cast::<u32>().read_unaligned(); let vy = y.cast::<u32>().read_unaligned(); if vx != vy { return false; } x = x.add(4); y = y.add(4); n -= 4; } if n >= 2 { if x.cast::<u16>().read_unaligned() != y.cast::<u16>().read_unaligned() { return false; } x = x.add(2); y = y.add(2); n -= 2; } if n > 0 { if x.read() != y.read() { return false; } } true }
rust source #2
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mrustc (master)
rustc 1.0.0
rustc 1.1.0
rustc 1.10.0
rustc 1.11.0
rustc 1.12.0
rustc 1.13.0
rustc 1.14.0
rustc 1.15.1
rustc 1.16.0
rustc 1.17.0
rustc 1.18.0
rustc 1.19.0
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rustc 1.23.0
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rustc 1.35.0
rustc 1.36.0
rustc 1.37.0
rustc 1.38.0
rustc 1.39.0
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rustc 1.43.0
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Source code
pub unsafe fn is_equal_raw( mut x: *const u8, mut y: *const u8, n: usize, ) -> bool { // If we don't have enough bytes to do 4-byte at a time loads, then // handle each possible length specially. Note that I used to have a // byte-at-a-time loop here and that turned out to be quite a bit slower // for the memmem/pathological/defeat-simple-vector-alphabet benchmark. if n < 4 { return match n { 0 => true, 1 => x.read() == y.read(), 2 => { x.cast::<u16>().read_unaligned() == y.cast::<u16>().read_unaligned() } // I also tried copy_nonoverlapping here and it looks like the // codegen is the same. 3 => x.cast::<[u8; 3]>().read() == y.cast::<[u8; 3]>().read(), _ => unreachable!() //std::hint::unreachable_unchecked(), }; } // When we have 4 or more bytes to compare, then proceed in chunks of 4 at // a time using unaligned loads. // // Also, why do 4 byte loads instead of, say, 8 byte loads? The reason is // that this particular version of memcmp is likely to be called with tiny // needles. That means that if we do 8 byte loads, then a higher proportion // of memcmp calls will use the slower variant above. With that said, this // is a hypothesis and is only loosely supported by benchmarks. There's // likely some improvement that could be made here. The main thing here // though is to optimize for latency, not throughput. // SAFETY: The caller is responsible for ensuring the pointers we get are // valid and readable for at least `n` bytes. We also do unaligned loads, // so there's no need to ensure we're aligned. (This is justified by this // routine being specifically for short strings.) let xend = x.add(n.wrapping_sub(4)); let yend = y.add(n.wrapping_sub(4)); while x < xend { let vx = x.cast::<u32>().read_unaligned(); let vy = y.cast::<u32>().read_unaligned(); if vx != vy { return false; } x = x.add(4); y = y.add(4); } let vx = xend.cast::<u32>().read_unaligned(); let vy = yend.cast::<u32>().read_unaligned(); vx == vy }
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