One missed array bound test elision

[leonardo-m]

Perhaps I'm missing something, but I think in this code rustc should not add a bound test:

#![feature(core_intrinsics)]
const N: usize = 1000;

pub fn foo(arr: &[usize; N]) -> usize {
    let mut total = 0;
    for i in 0 .. N {
        let k = i % 10;
        let index = i >> k;
        // unsafe { std::intrinsics::assume(index < N) };
        total += arr[index];
    }
    total
}

Currently it generates:

example::foo:
	push    rax
	xor     r8d, r8d
	movabs  r9, -3689348814741910323
	xor     esi, esi
.LBB0_1:
	mov     rax, rsi
	mul     r9
	shr     rdx, 2
	and     rdx, -2
	lea     rax, [rdx + 4*rdx]
	mov     rcx, rsi
	sub     rcx, rax
	mov     rax, rsi
	shr     rax, cl
	cmp     rax, 999
	ja      .LBB0_4
	add     rsi, 1
	add     cl, 1
	mov     rdx, rsi
	shr     rdx, cl
	cmp     rdx, 1000
	jae     .LBB0_3
	add     r8, qword ptr [rdi + 8*rax]
	add     r8, qword ptr [rdi + 8*rdx]
	add     rsi, 1
	cmp     rsi, 1000
	jne     .LBB0_1
	mov     rax, r8
	pop     rcx
	ret
.LBB0_3:
	mov     rax, rdx
.LBB0_4:
	lea     rdx, [rip + .L__unnamed_1]
	mov     esi, 1000
	mov     rdi, rax
	call    qword ptr [rip + core::panicking::panic_bounds_check@GOTPCREL]
	ud2

Using rustc 1.45.0-nightly 9310e3bd4 2020-05-21.

[istankovic]

I think this has been solved. At least with 1.75 nightly in release mode I get no bound checks:

playground::foo: # @playground::foo
# %bb.0:
	xorl	%esi, %esi
	movabsq	$-3689348814741910323, %r9      # imm = 0xCCCCCCCCCCCCCCCD
	xorl	%r8d, %r8d

.LBB0_1:                                # =>This Inner Loop Header: Depth=1
	movq	%r8, %rax
	mulq	%r9
	shrl	$2, %edx
	andl	$-2, %edx
	leal	(%rdx,%rdx,4), %eax
	movl	%r8d, %ecx
	subl	%eax, %ecx
	movq	%r8, %rax
	shrq	%cl, %rax
	addq	(%rdi,%rax,8), %rsi
	leaq	1(%r8), %rax
	incb	%cl
                                        # kill: def $cl killed $cl killed $ecx
	shrq	%cl, %rax
	addq	(%rdi,%rax,8), %rsi
	addq	$2, %r8
	cmpq	$1000, %r8                      # imm = 0x3E8
	jne	.LBB0_1
# %bb.2:
	movq	%rsi, %rax
	retq
                                        # -- End function

[leonardo-m]

Verified, and good.