Add MIR pass to lower call to core::slice::len into Len operand

[shamatar]

During some larger experiment with range analysis I've found that code like let l = slice.len() produces different MIR then one found in bound checks. This optimization pass replaces terminators that are calls to core::slice::len with just a MIR operand and Goto terminator.

It uses some heuristics to remove the outer borrow that is made to call core::slice::len, but I assume it can be eliminated, just didn't find how.

Would like to express my gratitude to @oli-obk who helped me a lot on Zullip

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) soon.

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[shamatar]

Removed the heuristic part that did borrow elimination, it may be resolved (already?) in other optimizations I believe

[shamatar]

That is still surprising that no pass was able to eliminate

 StorageLive(_6);
_6 = _2; _5 = Len((*_6)); StorageDead(_6); 

Most likely LLVM will handle it, but it should be simple to do on MIR level

[scottmcm]

I tried to do some of those cases in #46440 , but IIRC the bigger plan was to do things with full NRVO.

[shamatar]

NRVO is most likely too hard for me, but I think it should not be too hard (well, except of liveness extension) to be able to make 1-2 MIR passes that will eliminate bound checks completely based on the facts from MIR statements. This pass was tested on a trivial code

pub fn bound(index: usize, slice: &[u8]) -> u8 {
if index < slice.len() {
slice[index]
} else {
42
}
}

Where bound checks can be obviously eliminated, but they are not. What I'd like to do is to accumulate the facts like

 _4 = _1;
_5 = Len((*_6));
_3 = Lt(move _4, move _5);
switchInt(move _3) -> [false: bb3, otherwise: bb2];

and be able to make judgements about them in different blocks. In my example the bb3 is just a bound check:

 StorageLive(_7); _7 = _1;
_8 = Len((*_2));
_9 = Lt(_7, _8);
assert(move _9, "index out of bounds: the length is {} but the index is {}", move _8, _7) -> bb4;

So I can for sure say that _9 is true

This will be a good practice before full scale range analysis

[shamatar]

Restructured to follow lowering of intrinsics more closely, but couldn't yet understand how to make it language item. Instead interned len statically in sym

[oli-obk]

@bors try @rust-timer queue

let's see what perf says, too

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit bc11877830550fb9bdbf9c678d3e61b575261edd with merge 18c5138fd6998d04c4fbaeec7188078f2e8a9c7e...

[shamatar]

Hm, what would be the best way to structure it? There is a dependency: I need to first add new LanguageItem to be able to then modify core to add such LanguageItem there, to then use it in the lowering pass.

[bjorn3]

Use #[cfg_attr(not(bootstrap), lang = "slice_len_fn")] in libcore. This disables the lang item when compiling libcore with the bootstrap compiler and enables it when compiling with a rustc compiled from the current checkout source.

[bors]

☀️ Try build successful - checks-actions
Build commit: 18c5138fd6998d04c4fbaeec7188078f2e8a9c7e (18c5138fd6998d04c4fbaeec7188078f2e8a9c7e)

[rust-timer]

Queued 18c5138fd6998d04c4fbaeec7188078f2e8a9c7e with parent b17d9c1, future comparison URL.

[shamatar]

A little unrelated, but at the moment &[T; N].len() coerces into &[T].len(). Is there anything that prevents some kind of specialization of this case to just constant evaluation? May be it should be modified and optimized out at some higher level?

Extra info: made a dirty hack for arrays

 #[inline]
pub const fn len(&self) -> usize {
N
}

There are fancy quircks:

• it's also not lowered to Len
• but later on constant propagation properly substitutes constant N in there

[shamatar]

So far I didn't set any threshold for MIR optimization level that enables this optimization other than > 0 (from mod.rs). What is a standard way to decide it?

[oli-obk]

So far I didn't set any threshold for MIR optimization level that enables this optimization other than > 0 (from mod.rs). What is a standard way to decide it?

Since this is "just" adding another intrinsic-like lowering, we can add it directly without gating on the opt level I believe.

[rust-timer]

Finished benchmarking try commit (18c5138fd6998d04c4fbaeec7188078f2e8a9c7e): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

[bors]

📌 Commit aa53928 has been approved by bjorn3

[bors]

⌛ Testing commit aa53928 with merge 7c7b6938c21b26ad34c9664813ff85689957180d...

[rust-log-analyzer]

The job dist-x86_64-apple failed! Check out the build log: (web)(plain)

Click to see the possible cause of the failure (guessed by this bot)

 Memory: 14 GB
Boot ROM Version: VMW71.00V.13989454.B64.1906190538
Apple ROM Info: [MS_VM_CERT/SHA1/27d66596a61c48dd3dc7216fd715126e33f59ae7]Welcome to the Virtual Machine
SMC Version (system): 2.8f0
Serial Number (system): VMpogIYGdR1H
hw.ncpu: 3
hw.byteorder: 1234
hw.memsize: 15032385536
---
[ 8%] Building CXX object Common/CMakeFiles/lldCommon.dir/Args.cpp.o
error: Connection to server timed out
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make[2]: *** [Common/CMakeFiles/lldCommon.dir/Args.cpp.o] Error 2
make[2]: *** [lib/Core/CMakeFiles/lldCore.dir/Error.cpp.o] Error 2
make[2]: *** Waiting for unfinished jobs....
make[2]: *** [lib/Core/CMakeFiles/lldCore.dir/DefinedAtom.cpp.o] Error 2
make[1]: *** [Common/CMakeFiles/lldCommon.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
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make: *** [all] Error 2
command did not execute successfully, got: exit status: 2
build script failed, must exit now', /Users/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/cmake-0.1.44/src/lib.rs:885:5
finished in 24.179 seconds
failed to run: /Users/runner/work/rust/rust/build/bootstrap/debug/bootstrap dist
Build completed unsuccessfully in 0:44:52

[bors]

💔 Test failed - checks-actions

[shamatar]

@bjorn3 Looks like random error in CI, error: Connection to server timed out during build, can you make bors retry?

[bjorn3]

@bors retry

[bors]

⌛ Testing commit aa53928 with merge 4573a4a...

[bors]

☀️ Test successful - checks-actions
Approved by: bjorn3
Pushing 4573a4a to master...

[pnkfelix]

Just a quick heads up: I noticed this PR injected a few regressions to max-rss, up to 8.3% on incr-patched: println on inflate-debug.

From skimming the results, I am not sure its worth taking any kind of corrective action to address the regressions. The max-rss results across the board are all over the place (e.g. some losses, and some wins on the order of -5%).

I'm leaving the note more to remind people that when you do a perf run, its a good idea to try to double-check the max-rss as well as the instructions:u. (And if you notice something like this, and explicitly decide that the PR is worth the cost, then just make a comment saying that you've made that decision.)

(Arguably we should change the perf.rlo site to enable both instruction:u and max-rss to be reported simultaneously on the graph, and then make that the default presentation... then people won't have to remember to select max-rss.)