Rust is unable to figure out that `if` inside the nested loop is executed at most once per outer loop iteration

[95th]

Rust unable to figure out that if inside the nested loop is executed at most once per outer loop iteration.

https://users.rust-lang.org/t/how-to-make-compiler-automatically-elide-bound-checks/34148/6

pub fn foo() -> usize {
    let mut index = 0;
    for i in 0..100 {
        assert!(index <= i);  // This generates a panic jmp
        for j in 0..100 {
            if i == j {
                index += 1;
                break;
            }
        }
    }
    index
}

Generated asm:

foo::foo:
 push    rax
 mov     rcx, -1
 mov     dl, 1
 xor     r8d, r8d
 xor     esi, esi
 test    r8b, r8b
 je      .LBB7_2
 jmp     .LBB7_9       // here
.LBB7_7:
 add     rcx, -1
 add     dl, 1
 mov     rsi, rax
 cmp     rax, 100
 je      .LBB7_8
 test    r8b, r8b
 jne     .LBB7_9         // here
.LBB7_2:
 lea     rax, [rsi, +, 1]
 mov     edi, eax
 and     edi, 7
 cmp     rsi, 7
 jb      .LBB7_5
 movzx   esi, dl
 and     esi, 7
 add     rsi, rcx
.LBB7_4:
 add     rsi, 8
 jne     .LBB7_4
.LBB7_5:
 test    rdi, rdi
 je      .LBB7_7
.LBB7_6:
 add     rdi, -1
 jne     .LBB7_6
 jmp     .LBB7_7
.LBB7_8:
 pop     rcx
 ret
.LBB7_9:
 call    std::panicking::begin_panic
 ud2

Rust version: 1.38 (stable)
platform: Ubuntu 18.04

[95th]

Sample C++ which handles this case correctly (no exception call generated): https://godbolt.org/z/dPJSxZ

#include <vector>

int foo(std::vector<int> const& a, std::vector<int> const& b) {
    int index = 0;
    int x = 0;
    for (int i = 0; i < a.size(); i++) {
        for (int j = 0; j < b.size(); i++) {
            if (i == j) {
                x = a.at(index);  // no exception generate by this
                index += 1;
                break;
            }
        }
    }
    return x;
}

clang (https://godbolt.org/z/gj8Xoe) still produces the exception call. (I guess because the LLVM doesn't handle this properly).

[mati865]

@rustbot modify labels: -O-linux