Get rid of bounds check in slice::chunks_exact() and related function…

[sdroege]

…s during construction

LLVM can't figure out in

let rem = self.len() % chunk_size;
let len = self.len() - rem;
let (fst, snd) = self.split_at(len);

and

let rem = self.len() % chunk_size;
let (fst, snd) = self.split_at(rem);

that the index passed to split_at() is smaller than the slice length and
adds a bounds check plus panic for it.

Apart from removing the overhead of the bounds check this also allows
LLVM to optimize code around the ChunksExact iterator better.

[sdroege]

See https://rust.godbolt.org/z/8P74zY for the corresponding assembly of both variants. This is based on the code from #75935, where I noticed this. The variant without bound checks is unrolled by the compiler.

Instead of implementing split_at/split_at_mut manually, it works equally well to simply put a core::intrinsics::assume(len < s.len()) into the code. As I didn't see that used much I didn't do it for this PR but if you prefer that I can switch it over.

Of course even better would be if LLVM would just understand this itself but that seems considerably harder to achieve.

[sdroege]

As @lzutao wrote in the other issue, this might also be #74938

[sdroege]

Seems like #74938 makes this one here obsolete. Might still be worth merging depending on when an update to LLVM 12 is planned.

[pickfire]

Weird, how come there is no reviewers assigned automatically? @Dylan-DPC

[Dylan-DPC-zz]

r? @nagisa

[nagisa]

The only change I would suggest here is to split these inlined algorithms into a private function (split_at_unchecked/split_at_mut_unchecked), while also possibly taking advantage of them to implement the split_at and split_at_mut as well.

Your call really if you want to do this, r=me otherwise.

[pickfire]

@nagisa Would it be good if we have split_at_unchecked and split_at_mut_unchecked to make use of get_unchecked on slice?

[nagisa]

Yeah, to implement the split_at_{,mut}_unckecked you would use get_unchecked much like its done right now in this PR. split_at{,_mut} functions, if rewritten to use the unchecked variants would then also be changed to contain bound-checking code.

Like I said this is not strictly necessary to land this IMO, and even if we don’t make an adjustment here, we can also do the thing in a follow-up.

[sdroege]

I'd be happy to add the split_at_unchecked() here, easy enough to do so. You'd prefer that over using core::intrinsics::assume() here then.

[sdroege]

split_at_unchecked() is added here now and made use of.

[pickfire]

I think we should show an example of invalid use.

[bors]

⌛ Testing commit 8d3cf92 with merge 52d6d94eb07980426e50416d3cf938c5c8335fbb...

[rust-log-analyzer]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

Build completed successfully in 0:02:12
+ /scripts/validate-toolstate.sh
Cloning into 'rust-toolstate'...
<Nothing changed>
HTTPError: HTTP Error 403: Forbidden
b'{"message":"API rate limit exceeded for user ID 7378925.","documentation_url":"https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}'
Traceback (most recent call last):
  File "../../src/tools/publish_toolstate.py", line 288, in <module>
    validate_maintainers(repo, github_token)
  File "../../src/tools/publish_toolstate.py", line 84, in validate_maintainers
    'Accept': 'application/vnd.github.hellcat-preview+json',
  File "/usr/lib/python3.6/urllib/request.py", line 223, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python3.6/urllib/request.py", line 532, in open
    response = meth(req, response)
  File "/usr/lib/python3.6/urllib/request.py", line 642, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.6/urllib/request.py", line 570, in error
    return self._call_chain(*args)
  File "/usr/lib/python3.6/urllib/request.py", line 504, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.6/urllib/request.py", line 650, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 403: Forbidden
  local time: Mon Aug 31 05:18:06 UTC 2020
  network time: Mon, 31 Aug 2020 05:18:06 GMT
== end clock drift check ==
== end clock drift check ==
##[error]Process completed with exit code 1.
Terminate orphan process: pid (5093) (node)
Terminate orphan process: pid (5102) (python)

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @rust-lang/infra. (Feature Requests)

[bors]

💔 Test failed - checks-actions

[sdroege]

The CI failure was some kind of network problem:

b'{"message":"API rate limit exceeded for user ID 7378925.","documentation_url":"https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}'

[Dylan-DPC-zz]

@bors retry

[bors]

⌛ Testing commit 8d3cf92 with merge b4b76c287992b99f73159f22a1482866237c9ebd...

[bors]

💔 Test failed - checks-actions

[sdroege]

Also looks like an unrelated failure

[nagisa]

@bors retry

[bors]

⌛ Testing commit 8d3cf92 with merge 897ef3a...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: nagisa
Pushing 897ef3a to master...