[llvm-bugs] [Bug 47247] New: Unroll small loops implied by llvm.assume

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47247

            Bug ID: 47247
           Summary: Unroll small loops implied by llvm.assume
           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

#include <cstddef>


unsigned sum_small(unsigned data[], size_t count) {
    if ( count > 4)   {
    __builtin_unreachable();
    }
    unsigned sum = 0;
    for (size_t i = 0; i < count; ++i) {
        sum += data[i];
    }
    return sum;
}

LLVM generates loop:
sum_small(unsigned int*, unsigned long):                        #
@sum_small(unsigned int*, unsigned long)
        test    rsi, rsi
        je      .LBB0_1
        xor     ecx, ecx
        xor     eax, eax
.LBB0_4:                                # =>This Inner Loop Header: Depth=1
        add     eax, dword ptr [rdi + 4*rcx]
        add     rcx, 1
        cmp     rsi, rcx
        jne     .LBB0_4
        ret
.LBB0_1:
        xor     eax, eax
        ret

GCC unrolls loop:
sum_small(unsigned int*, unsigned long):
        test    rsi, rsi
        je      .L4
        mov     eax, DWORD PTR [rdi]
        cmp     rsi, 1
        je      .L1
        add     eax, DWORD PTR [rdi+4]
        cmp     rsi, 2
        je      .L1
        add     eax, DWORD PTR [rdi+8]
        cmp     rsi, 3
        je      .L1
        add     eax, DWORD PTR [rdi+12]
        ret
.L4:
        xor     eax, eax
.L1:
        ret

https://godbolt.org/z/Przjfo

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