Few LLVM lint "Unusual"

In Issue #48227 I've seen code compiled with "-C passes=lint", I've used it on some of my code and I've found few problems, reduced below:

fn foo(_: &u32, _: &u32) {}
fn main() {
    let a = 0;
    foo(&a, &a);
}


...>rustc -C passes=lint test1.rs
Unusual: noalias argument aliases another argument
  call void @_ZN4bug13foo17h2a852a9d6b40f8a0E(i32* noalias readonly dereferenceable(4) %1, i32* noalias readonly dereferenceable(4) %1)

fn main() {
    (1 .. 9).take_while(|i| i * i <= 10).any(|_| true);
}


...>rustc -C passes=lint test2.rs
Unusual: noalias argument aliases another argument
  %3 = call i32 @"_ZN86_$LT$$RF$$u27$b$u20$i32$u20$as$u20$core..ops..arith..Mul$LT$$RF$$u27$a$u20$i32$GT$$GT$3mul17hdb3638dfdb52b819E"(i32* noalias readonly dereferenceab
le(4) %1, i32* noalias readonly dereferenceable(4) %1)

fn main() {
    (1 .. 9).filter(|_| true).sum::<u32>();
}


...>rustc -C passes=lint test3.rs
Unusual: Return statement in function with noreturn attribute
  ret void

rustc 1.25.0-nightly (3ec5a99aa 2018-02-14)
binary: rustc
commit-hash: 3ec5a99aaa0084d97a9e845b34fdf03d1462c475
commit-date: 2018-02-14
host: x86_64-pc-windows-gnu
release: 1.25.0-nightly
LLVM version: 6.0

Duplicate of #7463

I guess I’ll reopen, since this contains minimal and actionable code samples.

This seems like the LLVM lint disagrees with the statement somewhere in the compiler sources saying that noalias on read-only data is fine even if thins alias -- because we can still reorder them?

Doesn't noalias also factor into optimizing pointer comparisons?

Well, whoever wrote that code strongly things that is not the case...

Also, depending on what exactly clang does, that would be unsound:

int foo(restrict int *x, restrict int *y) {
  (x == y) ? 1 : 0
}

This can not be optimized to return 0.

Hm, you're right, and indeed I couldn't find any such optimization in a quick skim of instcombine.

Rollup merge of rust-lang#59639 - cuviper:ignore-uninhabited, r=sanxiyn

Rollup merge of rust-lang#59639 - cuviper:ignore-uninhabited, r=eddyb

Unusual: Return statement in function with noreturn attribute

This case is fixed by #59639 as of nightly-2019-04-05.

Removing I-unsound label. The remaining 2 cases ("noalias argument aliases another argument") seem to be caused by the LLVM lint being a bit overzealous. Maybe we should close this altogether?

Those noalias lints should be relaxed by D60239, which is in LLVM 9. I will try to do a new rustc build with lints enabled to see if anything remains.

Can confirm they're gone on rustc 1.41.0-nightly (797fd9262 2019-11-26) with -Cpasses=lint. In some other code, some of these have shown up:

Unusual: unreachable immediately preceded by instruction without side effects
  unreachable, !dbg !112

This shouldn't really be an issue and is just a bit sloppy codegen by rustc.

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Few LLVM lint "Unusual" #48229

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Few LLVM lint "Unusual" #48229

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leonardo-m commented on Feb 15, 2018

nagisa commented on Feb 16, 2018

nagisa commented on Feb 16, 2018

RalfJung commented on Mar 28, 2018

hanna-kruppe commented on Mar 28, 2018

RalfJung commented on Mar 28, 2018

hanna-kruppe commented on Mar 28, 2018

cuviper commented on Apr 5, 2019

jonas-schievink commented on Dec 10, 2019

cuviper commented on Dec 10, 2019

jonas-schievink commented on Dec 10, 2019

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