organise

1 files changed, 28 insertions, 0 deletions

diff --git a/3/6_control/23_reverse_engineer_loop.c b/3/6_control/23_reverse_engineer_loop.c
new file mode 100644
index 0000000..922b317
--- /dev/null
+++ b/3/6_control/23_reverse_engineer_loop.c
@@ -0,0 +1,28 @@
+short dw_loop(short x)
+{
+    short y = x/9;
+    short *p = &x;
+    short n = 4*x;
+    do {
+        x += y;
+        (*p) += 5;
+        n += 2;
+    } while (n > 0);
+    return x;
+}
+
+// x in %rdi
+// dw_loop:
+//    movq %rdi, %rdx           # %rbx <- x
+//    movq %rdi, %rcx           # %rcx <- x
+//    idivq $9, %rcx            # %rcx (y) <- x / 9
+//    leaq (,%rdi,4), %rdx      # %rdx (n) <- x * 4
+// .L2:
+//    leaq 5(%rbx,%rcx), %rcx   # %rbx (x) <- 5 + %rbx (x) + %rcx (y)
+//    subq $1, %rdx             # %rdx (n) <- n - 1
+//    testq %rdx, %rdx          # n > 0 ?
+//    jg .L2                    # if n > 0, goto .L2
+//    rep; ret
+//
+//    A. x in rbx, y in rcx, n in rdx
+//    B. The pointer is never used besides incrementing the value at the address. So it is replaced by a incrementing instruction in the loop.