Use subslice patterns in slice methods

[cuviper]

For all of the methods that pick off the first or last element, we can
use subslice patterns to implement them directly, rather than relying on
deeper indexing function calls. At a minimum, this means the generated
code will rely less on inlining for performance, but in some cases it
also optimizes better.

[rust-highfive]

r? @kennytm

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[cuviper]

At a minimum, this means the generated code will rely less on inlining for performance, but in some cases it also optimizes better.

To back this up, here is the compiler explorer comparing the immutable methods. Note the missing functions in the output, indicating that LLVM merged them with their counterpart. Then last demonstrates improved optimization:

example::last:
        xor     ecx, ecx
        mov     rdx, rsi
        sub     rdx, 1
        lea     rax, [rdi + 4*rdx]
        cmovb   rax, rcx
        cmp     rdx, rsi
        cmovae  rax, rcx
        ret

example::last_pattern:
        xor     eax, eax
        sub     rsi, 1
        lea     rcx, [rdi + 4*rsi]
        cmovae  rax, rcx
        ret

Here is the same exploration for the mutable methods. It looks like LLVM's merge-functions pass was less successful here, outputing split methods that are identical AFAICS. Then last_mut shows the exact same optimization difference as last did.

[cuviper]

Note the missing functions in the output, indicating that LLVM merged them with their counterpart.

This can be confirmed by unchecking the .text box, then at the end you'll see the aliases:

        .globl  example::split_last_pattern
        .type   example::split_last_pattern,@function
.set example::split_last_pattern, example::split_last
        .globl  example::first
        .type   example::first,@function
.set example::first, example::first_pattern
        .globl  example::split_first_pattern
        .type   example::split_first_pattern,@function
.set example::split_first_pattern, example::split_first

        .globl  example::first_mut
        .type   example::first_mut,@function
.set example::first_mut, example::first_mut_pattern

[Centril]

r? @Centril

This looks beautiful, @bors r+

[bors]

📌 Commit 53be0cc has been approved by Centril

[tspiteri]

It looks like LLVM's merge-functions pass was less successful here, outputing split methods that are identical AFAICS.

I think LLVM merges functions which have equivalent IR, in fact the alias is already specified in the IR when functions are merged.