Optimized recursive

    .file   "rec.c"
    .text
    .p2align 4,,15
    .globl  recs
    .type   recs, @function
recs:
.LFB0:
    .cfi_startproc
    xorl    %eax, %eax
    testl   %edi, %edi
    je  .L2
    .p2align 4,,10
    .p2align 3
.L3:
    addl    %edi, %eax
    subl    $1, %edi
    jne .L3
.L2:
    rep
    ret
    .cfi_endproc
.LFE0:
    .size   recs, .-recs
    .section    .text.startup,"ax",@progbits
    .p2align 4,,15
    .globl  main
    .type   main, @function
main:
.LFB1:
    .cfi_startproc
    rep
    ret
    .cfi_endproc
.LFE1:
    .size   main, .-main
    .ident  "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
    .section    .note.GNU-stack,"",@progbits