Use a faster allocation size check in slice::from_raw_parts

[saethlin]

I've been perusing through the codegen changes that result from turning on the standard library debug assertions. The previous check in here uses saturating arithmetic, which in my experience sometimes makes LLVM just fail to optimize things around the saturating operation.

Here is a demo of the codegen difference: https://godbolt.org/z/WMEqrjajW
Before:

example::len_check_old:
        mov     rax, rdi
        mov     ecx, 3
        mul     rcx
        setno   cl
        test    rax, rax
        setns   al
        and     al, cl
        ret

example::len_check_old:
        mov     rax, rdi
        mov     ecx, 8
        mul     rcx
        setno   cl
        test    rax, rax
        setns   al
        and     al, cl
        ret

After:

example::len_check_new:
        movabs  rax, 3074457345618258603
        cmp     rdi, rax
        setb    al
        ret

example::len_check_new:
        shr     rdi, 60
        sete    al
        ret

Running rustc-perf locally, this looks like up to a 4.5% improvement when debug-assertions-std = true.

Thanks @LegionMammal978 (I think that's you?) for turning my idea into a much cleaner implementation.

r? @thomcc

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[thomcc]

Thanks! Looks good.

This is only used in debug builds (of the stdlib) so no need for a perf run or restricting rollup (but no need to force it either, since maybe it could impact compile times for other unknown reasons).

@bors r+

[bors]

📌 Commit cfcb0a2 has been approved by thomcc

It is now in the queue for this repository.