add some practice problems from 4HEADmaster

1 files changed, 50 insertions, 0 deletions

diff --git a/4/practice_problem_4_4_rsum.c b/4/practice_problem_4_4_rsum.c
new file mode 100644
index 0000000..44960e6
--- /dev/null
+++ b/4/practice_problem_4_4_rsum.c
@@ -0,0 +1,50 @@
+#include <stdio.h>
+
+long rsum(long *start, long count) {
+	if (count <= 0)
+		return 0;
+	return *start + rsum(start + 1, count - 1);
+}
+
+int main() {
+	long nums[] = {1, 2, 3};
+	printf("%d\n", rsum(nums, 3));
+}
+
+/*
+long rsum(long *start, long count)
+start in %rdi, count in %rsi
+test:
+	xorq 			%rax, %rax 	return = 0
+	andq 			%rsi, %rsi,  Set CC again?
+	jle return 		Count <= 0, or jump
+	push %rbx		keep original rbx around
+	mrmovq (%rdi), %rbx 	Get *start
+	irmovq $8,%r8 		Constant 8
+	irmovq $1,%r9 		Constant 1
+	addq %r8,%rdi 		start++
+	subq %r9,%rsi 		count--. Set CC
+	call rsum		Recurse
+	addq %rbx, %rax		Accumulate
+	popq %rbx		Restore original rbx
+return:
+	ret
+*/
+
+/*
+0000000000001149 <rsum>:
+    1149:	48 85 f6             	test   %rsi,%rsi
+    114c:	7e 16                	jle    1164 <rsum+0x1b>
+
+    114e:	53                   	push   %rbx
+    114f:	48 8b 1f             	mov    (%rdi),%rbx
+    1152:	48 83 ee 01          	sub    $0x1,%rsi
+    1156:	48 83 c7 08          	add    $0x8,%rdi
+    115a:	e8 ea ff ff ff       	call   1149 <rsum>
+    115f:	48 01 d8             	add    %rbx,%rax
+    1162:	5b                   	pop    %rbx			Return accumulated %rbx
+    1163:	c3                   	ret
+
+    1164:	b8 00 00 00 00       	mov    $0x0,%eax 		Return 0 after first test
+    1169:	c3                   	ret
+*/