Change:
results in the following assembly When using the check is correctly optimized away: This happens both on all stable versions I have tested and on the most recent nightly.
In fact, this has nothing to do with slices: change from 3 to 4 (or other powers of 2) to see everything optimized away. Edit: reduced it some more:
There are no optimization in GCC and Clang either: Should be optimized to only return true?
So this is more as LLVM part: modify labels: +A-LLVM +A-mir-opt Of course this might be improved with MIR-opt. But how hard it would be? I don't know.
What's interesting is that the power of 2 optimization is done by rustc somewhere - the same MIR is translated to radically different LLVM-IR when N is 2^n. Trying to find where this happens without knowledge of the compiler is rather difficult, though :)
Clang could do that, change in my snippet about to see. But still GCC doesn't do that.
needs to set a nuw flag. Then it will probably fold.
Cool. So if I understand correctly, the following is happening: The bounds check boils down to basically . This can be transformed to (fair enough) This is further transformed to (as the llvm ir shows) () And for some reason, LLVM can't seem to deduce that this comparison will always be false. Adding this as an assumption will make the check be optimized:
Ignore me, I missed that the assume caused the IR to have However, it does optimize if a is inserted instead of a , has the following opt: I don't know why the same doesn't exist for ? maybe no one considered it until now? LLVM doesn't know about this optimization but it is apparently valid (thanks John Regehr) so I'll submit a patch to LLVM.
I've submitted a patch to LLVM adding this optimization:
Does anyone want to file a bug against GCC? I registered an account two days ago but they hasn't accepted my account yet.
^ as you can see above this optimization has landed in LLVM master, so whenever LLVM is bumped we should get this for free.
Does it land with LLVM 11?
it's not in RC-1 which was released a week ago, I don't know if they will include it in a future release candidate or not.
This will be solved in LLVM 12 as my commit missed the cutoff (it was before the issue was even filed)
I filed the gcc bug:
I confirmed that with the bounds check is removed.
Can we close this issue?
could you test again and close this issue?
this has been fixed, would be nice to add a codegen test for this, asserting that can't panic

Discussion:
<ins> //@ assembly-output: emit-asm //@ compile-flags:-Copt-level=3 //@ only-x86_64 #![crate_type = "lib"] #[no_mangle] type T = u8; type T1 = (T, T, T, T, T, T, T, T); type T2 = [T; 8]; #[no_mangle] // CHECK-LABEL: foo1a // CHECK: cmp // CHECK-NEXT: set // CHECK-NEXT: ret pub fn foo1a(a: T1, b: T1) -> bool { a == b } #[no_mangle] // CHECK-LABEL: foo1b // CHECK: mov // CHECK-NEXT: cmp // CHECK-NEXT: set // CHECK-NEXT: ret pub fn foo1b(a: &T1, b: &T1) -> bool { a == b } </ins>

<ins> //@ compile-flags: -O //@ min-llvm-version: 17 #![crate_type = "lib"] #[no_mangle] // CHECK-LABEL: @foo // CHECK: getelementptr inbounds // CHECK-NEXT: load i64 // CHECK-NEXT: icmp eq i64 // CHECK-NEXT: br i1 #[no_mangle] pub fn foo(input: &mut &[u64]) -> Option<u64> { let (first, rest) = input.split_first()?; *input = rest; Some(*first) } </ins>

<ins> //@ compile-flags: -O // XXX: The x86-64 assembly get optimized correclty. But llvm-ir output is not until llvm 18? //@ min-llvm-version: 18 #![crate_type = "lib"] pub enum K{ A(Box<[i32]>), B(Box<[u8]>), C(Box<[String]>), D(Box<[u16]>), } #[no_mangle] // CHECK-LABEL: @get_len // CHECK: getelementptr inbounds // CHECK-NEXT: load // CHECK-NEXT: ret i64 // CHECK-NOT: switch pub fn get_len(arg: &K)->usize{ match arg { K::A(ref lst)=>lst.len(), K::B(ref lst)=>lst.len(), K::C(ref lst)=>lst.len(), K::D(ref lst)=>lst.len(), } } </ins>

<ins> //@ compile-flags: -O // This regress since Rust version 1.72. //@ min-llvm-version: 18.1.4 #![crate_type = "lib"] use std::convert::TryInto; const N: usize = 24; #[no_mangle] // CHECK-LABEL: @example // CHECK-NOT: unwrap_failed pub fn example(a: Vec<u8>) -> u8 { if a.len() != 32 { return 0; } let a: [u8; 32] = a.try_into().unwrap(); a[15] + a[N] } </ins>

<ins> //@ compile-flags: -O //@ min-llvm-version: 17 #![crate_type = "lib"] // CHECK-LABEL: @write_u8_variant_a // CHECK: getelementptr // CHECK-NEXT: icmp ugt #[no_mangle] pub fn write_u8_variant_a( bytes: &mut [u8], buf: u8, offset: usize, ) -> Option<&mut [u8]> { let buf = buf.to_le_bytes(); bytes .get_mut(offset..).and_then(|bytes| bytes.get_mut(..buf.len())) } </ins>

<ins> // in Rust 1.73, -O and opt-level=3 optimizes differently //@ compile-flags: -C opt-level=3 //@ min-llvm-version: 17 #![crate_type = "lib"] use std::cmp::max; #[no_mangle] // CHECK-LABEL: @foo // CHECK-NOT: slice_start_index_len_fail // CHECK-NOT: unreachable pub fn foo(v: &mut Vec<u8>, size: usize)-> Option<&mut [u8]> { if v.len() > max(1, size) { let start = v.len() - size; Some(&mut v[start..]) } else { None } } </ins>

<ins> //@ compile-flags: -O //@ min-llvm-version: 18 #![crate_type = "lib"] // CHECK-LABEL: @div2 // CHECK: ashr i32 %a, 1 // CHECK-NEXT: ret i32 #[no_mangle] pub fn div2(a: i32) -> i32 { a.div_euclid(2) } </ins>

<ins> //@ compile-flags: -O #![crate_type = "lib"] const N: usize = 3; pub type T = u8; #[no_mangle] // CHECK-LABEL: @split_mutiple // CHECK-NOT: unreachable pub fn split_mutiple(slice: &[T]) -> (&[T], &[T]) { let len = slice.len() / N; slice.split_at(len * N) } </ins>

<ins> //@ compile-flags: -O //@ min-llvm-version: 17 #![crate_type = "lib"] #[no_mangle] // CHECK-LABEL: @foo // CHECK: {{.*}}: // CHECK: ret // CHECK-NOT: unreachable pub fn foo(arr: &mut [u32]) { for i in 0..arr.len() { for j in 0..i { assert!(j < arr.len()); } } } </ins>

<ins> //@ compile-flags: -O //@ min-llvm-version: 18 #![crate_type = "lib"] use std::ptr::NonNull; // CHECK-LABEL: @slice_ptr_len_1 // CHECK: {{.*}}: // CHECK-NEXT: ret i64 %ptr.1 #[no_mangle] pub fn slice_ptr_len_1(ptr: *const [u8]) -> usize { let ptr = ptr.cast_mut(); if let Some(ptr) = NonNull::new(ptr) { ptr.len() } else { // We know ptr is null, so we know ptr.wrapping_byte_add(1) is not null. NonNull::new(ptr.wrapping_byte_add(1)).unwrap().len() } } </ins>