Description
What problem does this solve or what need does it fill?
When writing the core Fetch related code, there's a large amount of unwrap_or_else(|| debug_unreachable_unchecked() to suggest to the compiler that, in release builds, the Option is never None and unwrapping is always safe. This is supposed to hint to the compiler that this branch is dead.
However, when attempting to compare this pattern against an actual get_unchecked implementation for SparseSets seems to still generates two jump instructions (at least on x86 platforms). You can see this here: https://godbolt.org/z/e3EPE1vxW, the full assembly output has been copied below.
example::get_checked:
mov rax, qword ptr [rdi + 24]
shl rsi, 4
mov rax, qword ptr [rax + rsi + 8]
shl rax, 3
add rax, qword ptr [rdi]
ret
example::get_unwrap_checked_unreachable:
mov rcx, rsi
shl rcx, 4
add rcx, qword ptr [rdi + 24]
xor eax, eax
cmp qword ptr [rdi + 40], rsi
cmovbe rcx, rax
jbe .LBB1_4
cmp qword ptr [rcx], 0
je .LBB1_2
mov rax, qword ptr [rcx + 8]
shl rax, 3
add rax, qword ptr [rdi]
.LBB1_4:
ret
.LBB1_2:
xor eax, eax
ret
What solution would you like?
Internal, pub(crate) get_unchecked(_mut) variants on SparseSet and derivative metadata stores and storages that is only used for release builds.
What alternative(s) have you considered?
Leaving this untouched. Eat the performance penalty.