fwdlist - A simply linked list in Rust.

A simple forward linked list, see the API documentation. The crate is also available on crates.io.

It's a linked list. Its not cache friendly, its relatively slow when you think about it, but it allows for O(1) insertion... after the current cursor location, maybe you care about that.

Avoiding unsafe

One of the goal here is to play with Rust and see how much unsafe is needed. It turns out that you can implement the basics of a simply linked list without using unsafe.

The mutable iterator and cursor both need mutable acces to the list with a different lifetime than the mutable reference on the list itself. The compiler cannot infer that auto-magically and needs a bit of our help.

penultimate_link() performances

Sometimes the code is more convoluted than necessary to please the borrow checker. Some unsafe code would make the code not only easier to read, but also as we might believe naively, more efficient for the machine.

The best example here is penultimate_link(), which returns a mutable reference to last but one link of the list.

To illustrate what this function returns, let's assume the following list:

head_link -> node1.next -> node2.next -> node3.next -> nil

In this case, penultimate_link() will return a mutable reference to node2.next. It is then trivial to implement pop_back() with a simple Option.take().

See penultimate_link() and penultimate_link_with_unsafe() implementations further below.

Assembly output

Take a look at the assembly outputs (cargo build --release) below:

• penultimate_link():

0000000000016200 <::only_safe::>:
   16200:	48 8b 4f 08          	mov    0x8(%rdi),%rcx
   16204:	31 c0                	xor    %eax,%eax
   16206:	48 85 c9             	test   %rcx,%rcx
   16209:	74 1f                	je     1622a <::only_safe::+0x2a>
   1620b:	31 c0                	xor    %eax,%eax
   1620d:	0f 1f 00             	nopl   (%rax)
   16210:	48 89 ca             	mov    %rcx,%rdx
   16213:	48 8b 4a 08          	mov    0x8(%rdx),%rcx
   16217:	48 85 c9             	test   %rcx,%rcx
   1621a:	74 0e                	je     1622a <::only_safe::+0x2a>
   1621c:	48 83 79 08 00       	cmpq   $0x0,0x8(%rcx)
   16221:	75 ed                	jne    16210 <::only_safe::+0x10>
   16223:	48 83 c2 08          	add    $0x8,%rdx
   16227:	48 89 d0             	mov    %rdx,%rax
   1622a:	c3                   	retq

• penultimate_link_with_unsafe():

00000000000168a0 <::with_unsafe::>:
   168a0:	31 c0                	xor    %eax,%eax
   168a2:	48 83 7f 08 00       	cmpq   $0x0,0x8(%rdi)
   168a7:	74 18                	je     168c1 <::with_unsafe::+0x21>
   168a9:	48 83 c7 08          	add    $0x8,%rdi
   168ad:	0f 1f 00             	nopl   (%rax)
   168b0:	48 8b 0f             	mov    (%rdi),%rcx
   168b3:	48 83 79 08 00       	cmpq   $0x0,0x8(%rcx)
   168b8:	48 89 f8             	mov    %rdi,%rax
   168bb:	48 8d 79 08          	lea    0x8(%rcx),%rdi
   168bf:	75 ef                	jne    168b0 <::with_unsafe::+0x10>
   168c1:	c3                   	retq

Assembly quick analysis

The first thing to note, is how well the original code is translated from high level Option and Box to simple null-able pointers.

• penultimate_link() is a loop with two conditional branches inside, and it tests twice every nodes of the list (exactly like in the Rust code). One test on every next_link, before testing it again when it become the new link to work on new every new iteration.
• penultimate_with_unsafe() is a loop with only one condition, but it keeps a “prev_link” pointer handy, again like in the Rust code.

Looking at the assembly with my ridiculously weak knowledge of modern CPU architecture, I infer that penultimate_link() requires twice the amount of branches predictions and both functions perform two data read per iteration.

Considering how modern CPUs seems to pipeline/pre-fetch like crazy, the two branchs predictions should pretty much cost like only one.

Callgrind/Cachegrind (valgrind) analysis

After adding #[inline(never)] on both penultimate_link* functions, I ran valgrind like so:

$ valgrind --tool=callgrind --dump-instr=yes --trace-jump=yes --cache-sim=yes --branch-sim=yes --collect-atstart=no --toggle-collect=*penultimate_link* target/release/fwdlist... --test one_penultimate

We basically get the following report:

• Ir: instruction read, penultimate_link() has more instructions and so more instruction read.
• Dr: data read. penultimate_with_unsafe() performs one more loop iteration, reading 2 more data.
• D1mr: data read misses on L1 cache. Similar between the two.
• DLmr: data read misses on Last Level cache. Interestingly, penultimate_with_unsafe() has more misses.
• Bc: Conditional branches. Confirms that penultimate_link() has two vs one conditions.
• Bcm: Conditional branches misses. penultimate_with_unsafe() gets one more, maybe the extra iteration?

Benchmark

penultimate_link() is faster than penultimate_with_unsafe() on real hardware.

Benchmarks with List<i64> and BIGLIST_SIZE=2²⁰ (list takes ~16Mib):

AMD Phenom(tm) II X4 965 Processor
penultimate_safe        ... bench:   3651099 ns/iter (+/- 35924)
penultimate_with_unsafe ... bench:   3687377 ns/iter (+/- 33386)

Intel(R) Core(TM) i7-2720QM CPU @ 2.20GHz
penultimate_safe        ... bench:   2333951 ns/iter (+/- 27634)
penultimate_with_unsafe ... bench:   2334611 ns/iter (+/- 43642)

Intel(R) Core(TM) i5-3320M CPU @ 2.60GHz
penultimate_safe        ... bench:   1675111 ns/iter (+/- 106477)
penultimate_with_unsafe ... bench:   2127297 ns/iter (+/- 128966)

Benchmarks with List<i64> and BIGLIST_SIZE=2³⁰ (list takes ~16Gib):

Intel(R) Xeon(R) CPU E5-1650 0 @ 3.20GHz
penultimate_safe        ... bench: 2399497518 ns/iter (+/- 357540058)
penultimate_with_unsafe ... bench: 2509462341 ns/iter (+/- 377119880)

Performances conclusion

Convoluted safe code vs simpler unsafe code doesn't necessary mean that unsafe code is going to be faster. In our specific case penultimate_with_unsafe() is indeed slower!

This is great because with safe Rust code, the compiler basically proves for us that there is no possible memory bugs. Any code refactoring cannot possibly introduce new memory bugs, the compiler wouldn't let it pass.

Happy hacking!