x86-64 Assembly test - jle

Question

This:

testl   %esi, %esi
jle .L3
movl    %esi, %eax

If testl do a logical AND on esi the result can't never be less but only equals, either if esi is 0. In this way movl can't be reached. It's that true, or I'm missing somethings.

Step two:

f1:
    pushq   %rbp
    movq    %rsp, %rbp
    testl   %esi, %esi
    jle .L3
    movl    %esi, %eax
.L2:
    incb    (%rdi)
    incq    %rdi
    decq    %rax
    jne .L2
.L3:
    popq    %rbp
    ret

In a hypothetical C translation if .L3 consists of pop then ret and the branch take place it's possible to determine the value returned by the function?

Answer 1

"Less than or equal" is defined as: ZF=1 or SF != OF

The TEST instruction sets ZF and SF based on a logical AND between the operands, and clears OF.

So in effect you end up with the condition ZF or SF, meaning "Less than or equal to zero" (i.e. the jump would be taken if (signed int)esi <= 0 in this case).

Edit: For the second part of your question, it looks like it's doing something along these lines:

void f1(char *c, int len)
{
    if (len > 0) {
        for (i = len; i != 0; i--) {
            (*c)++;
            c++;   
        }
    }
}