[llvm-bugs] [Bug 34662] New: Performance bug: Clang or LLVM failure to eliminate tail recursion in Linux

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34662

            Bug ID: 34662
           Summary: Performance bug: Clang or LLVM failure to eliminate
                    tail recursion in Linux
           Product: clang
           Version: 5.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: thecppzoo at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

Hello.  For this code:

__int128 factorial(unsigned arg, __int128 soFar) {
    if(arg <= 1) { return soFar; }
    return factorial(arg - 1, arg * soFar);
}

Clang 5.0.0 and others fails to eliminate the tail recursion, as can be seen at
the compiler explorer:
https://godbolt.org/g/iqLwgJ
(optimization level -O3)

factorial(unsigned int, __int128):                         #
@factorial(unsigned int, __int128)
        mov     rcx, rdx
        mov     rax, rsi
        cmp     edi, 2
        jb      .LBB0_2
        push    rax
        mov     edx, edi
        dec     edi
        imul    rcx, rdx
        mul     rdx
        add     rdx, rcx
        mov     rsi, rax
        call    factorial(unsigned int, __int128)
        mov     rcx, rdx
        add     rsp, 8
.LBB0_2:
        mov     rdx, rcx
        ret

Curiously, -target aarch64-linux-gnu also with optimization -O3 produces
non-tail-recursive code.

GCC eliminates the tail recursion

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