Weird Assembly Generated by VC++ When CASE Selector is Unsigned Long Long

Question

I compiled the following function in VC++2010 to see what optimized (/O2) 32bit code was generated (note the unsigned long long type for switch statement) :

int proc1(unsigned long long a, char*s) {

    switch (a ) {
    case -1L  :
        printf("-1");
        break;
    case -11  :
        printf("-11");
        break;
    case -3  :
        printf("-3");
        break;
    case 21  :
        printf("21");
        break;
    case 25  :
        printf("25");
        break;
    case 29  :
        printf("29");
        break;
    case 31  :
        printf("31");
        break;
    case 40  :
        printf("40");
        break;
    }
    return strlen(s);
}

Here is part of the generated code:

_TEXT   SEGMENT
_a$ = 8                         ; size = 8
_s$ = 16                        ; size = 4
?proc1@@YAH_KPAD@Z PROC                 ; proc1, COMDAT

; 5    : int proc1(unsigned long long a, char*s) {

    push    ebp
    mov ebp, esp

; 6    :    
; 7    :    switch (a ) {

    mov ecx, DWORD PTR _a$[ebp+4]
    mov eax, DWORD PTR _a$[ebp]
    test    ecx, ecx
    ja  SHORT $LN13@proc1
    jb  SHORT $LN15@proc1
    cmp eax, 40                 ; 00000028H
    ja  SHORT $LN13@proc1
$LN15@proc1:
    cmp eax, 40                 ; 00000028H
    jne SHORT $LN16@proc1
    test    ecx, ecx
    je  SHORT $LN1@proc1
$LN16@proc1:
    test    ecx, ecx
    ja  SHORT $LN14@proc1
    jb  SHORT $LN17@proc1
    cmp eax, 29                 ; 0000001dH
    ja  SHORT $LN14@proc1

But why it uses ja + jb after the test ecx, ecx? i.e.:

test    ecx, ecx
ja  SHORT $LN13@proc1
jb  SHORT $LN15@proc1

From what I understand, the test ecx,ecx is to test if the ecx is zero. Hence, I expect a je/jz/jnz after the instruction. ja should test zf=0/cf=1 while jb should test zf=0/cf=0. So it is not appropriate to use them after the test instruction. And it even makes less sense to me to have a jb following the ja since both expects zf=0 and cf is not set properly before them.

Weird Assembly Generated by VC++ When CASE Selector is Unsigned Long Long

Answers (1)

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