[llvm-bugs] [Bug 42119] New: Missed after-loop range optimizations

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42119

            Bug ID: 42119
           Summary: Missed after-loop range optimizations
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

Clang trunk -g0 -O3 -march=haswell

void g(unsigned x);
unsigned p(unsigned x, unsigned n) {

    while (x < n) {
            g(x);
            x++;
    }

    // x == n

    return x;
}

unsigned p2(unsigned x, unsigned n) {

    while (x < n) {
            g(x);
            x++;
    }

    return n;
}


Clang somehow did not realize that x == n after this while loop, it is a missed
LLVM IR optimization.. 

p(unsigned int, unsigned int):
        push    rbp
        push    rbx
        push    rax
        mov     ebx, edi
        cmp     edi, esi
        jae     .LBB0_4
        mov     ebp, esi // move this instruction above cmp
.LBB0_2:                          
        mov     edi, ebx
        call    g(unsigned int)
        inc     ebx
        cmp     ebp, ebx
        jne     .LBB0_2
        mov     ebx, ebp // useless
.LBB0_4:
        mov     eax, ebx // move from ebp
        add     rsp, 8
        pop     rbx
        pop     rbp
        ret


for p2, Clang generates better code:
p2(unsigned int, unsigned int):
        push    rbp
        push    rbx
        push    rax
        mov     ebx, esi
        cmp     edi, esi
        jae     .LBB0_3
        mov     ebp, edi
.LBB0_2:                             
        mov     edi, ebp
        call    g(unsigned int)
        inc     ebp
        cmp     ebx, ebp
        jne     .LBB0_2
.LBB0_3:
        mov     eax, ebx
        add     rsp, 8
        pop     rbx
        pop     rbp
        ret

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