Interesting integer signed overflow testcase

foo.c

#include <stdio.h>
#include <string.h>

int main(void)
{
    char t[] = "abcdefghijklmnopqrstuvwxyz";
    for (int i = 0, n = strlen(t); i < n; i++)
    {
        if (t[i] >= 'a' && t[i] <= 'z')
        {
            while ((t[i] - 13) < 'a')
            {
                t[i] += 26;
            }                
            printf("%c", (t[i] - 13));
        }
    }
    printf("\n");
}

// output with clang -std=c99 at any optimization level:
// nopqrabcdabcdabcdefghijklm

foo.s

# clang -Os output:
        .file   "20130902.c"
        .text
        .globl  main
        .type   main,@function
main:                                   # @main
# BB#0:
        push    EDI
        push    ESI
        sub     ESP, 52
        lea     ESI, DWORD PTR [ESP + 25]
        mov     DWORD PTR [ESP], ESI
        mov     DWORD PTR [ESP + 8], 27
        mov     DWORD PTR [ESP + 4], .Lmain.t
        call    memcpy
        mov     DWORD PTR [ESP], ESI
        call    strlen
        mov     ESI, EAX
        test    ESI, ESI
        jle     .LBB0_9
# BB#1:
        xor     EDI, EDI
        mov     AL, 97
        jmp     .LBB0_2
.LBB0_8:                                # %._crit_edge9
                                        #   in Loop: Header=BB0_2 Depth=1
        mov     AL, BYTE PTR [ESP + EDI + 25]
.LBB0_2:                                # %.lr.ph5
                                        # =>This Loop Header: Depth=1
                                        #     Child Loop BB0_4 Depth 2
        mov     CL, AL
        add     CL, -97
        cmp     CL, 25
        ja      .LBB0_7
# BB#3:                                 # %.preheader
                                        #   in Loop: Header=BB0_2 Depth=1
        movsx   ECX, AL
        add     ECX, -13
        cmp     ECX, 96
        jg      .LBB0_6
.LBB0_4:                                # %.lr.ph
                                        #   Parent Loop BB0_2 Depth=1
                                        # =>  This Inner Loop Header: Depth=2
        add     AL, 26
        movsx   ECX, AL
        add     ECX, -13
        cmp     ECX, 97
        jl      .LBB0_4
# BB#5:                                 # %._crit_edge
                                        #   in Loop: Header=BB0_2 Depth=1
        mov     BYTE PTR [ESP + EDI + 25], AL
.LBB0_6:                                #   in Loop: Header=BB0_2 Depth=1
        mov     DWORD PTR [ESP], ECX
        call    putchar
.LBB0_7:                                #   in Loop: Header=BB0_2 Depth=1
        inc     EDI
        cmp     EDI, ESI
        jne     .LBB0_8
.LBB0_9:                                # %._crit_edge6
        mov     DWORD PTR [ESP], 10
        call    putchar
        xor     EAX, EAX
        add     ESP, 52
        pop     ESI
        pop     EDI
        ret
.Ltmp0:
        .size   main, .Ltmp0-main

        .type   .Lmain.t,@object        # @main.t
        .section        .rodata.str1.1,"aMS",@progbits,1
.Lmain.t:
        .asciz   "abcdefghijklmnopqrstuvwxyz"
        .size   .Lmain.t, 27


        .section        ".note.GNU-stack","",@progbits