From b9bf3afbb52aebcc0f93df55b55ec0f0f521b1ab Mon Sep 17 00:00:00 2001
From: Mike Vink <ivi@vinkies.net>
Date: Sun, 16 Feb 2025 18:21:50 +0100
Subject: add some practice problems from 4

---
 4/practice_problem_4_3_iaddq.c | 79 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)
 create mode 100644 4/practice_problem_4_3_iaddq.c

(limited to '4/practice_problem_4_3_iaddq.c')

diff --git a/4/practice_problem_4_3_iaddq.c b/4/practice_problem_4_3_iaddq.c
new file mode 100644
index 0000000..4e8add2
--- /dev/null
+++ b/4/practice_problem_4_3_iaddq.c
@@ -0,0 +1,79 @@
+#include <stdio.h>
+
+long sum(long *start, long count)
+{
+	long sum = 0;
+	while (count) {
+		sum += *start;
+		start++;
+		count--;
+	}
+	return sum;
+}
+
+int main() {
+	long start[] = { 42, 11, 12 };
+	return printf("%d\n", sum(start, (long)1));
+}
+
+/*
+long sum(long *start, long count)
+start in %rdi, count in %rsi
+sum:
+	movl $0, %eax		sum = 0
+	jmp .12			Goto test
+.L3:				loop:
+	addq (%rdi), %rax 		Add *start
+	addq $8, %rdi 			start++
+	subq $1, %rsi 			count--
+.L2: 				test:
+	testq %rsi, %rsi 		Test sum
+	jne .L3 			If !=O, goto loop
+	rep; ret			Return
+*/
+
+/*
+long sum(long *start, long count)
+start in %rdi, count in %rsi
+sum:
+	irmovq $8,%r8 		Constant 8
+	irmovq $1,%r9 		Constant 1
+	xorq %rax,%rax		sum = 0
+	andq %rsi,%rsi		Set cc
+	jmp test		Goto test
+loop:
+	mrmovq (%rdi), %r10 	Get *start
+	addq %r10,%rax		Add to sum
+	addq %r8,%rdi 		start++
+	subq %r9,%rsi 		count--. Set CC
+test:
+	jne loop		Stop when 0
+	ret
+*/
+
+/* Using iaddq
+First byte
+- C, 12
+- 0, first variant of iadd
+
+Second byte
+- F, 15, absence of first argument
+- rB, register number 0-15 to add V to
+
+Other 0-8 bytes V
+
+long sum(long *start, long count)
+start in %rdi, count in %rsi
+sum:
+	xorq %rax,%rax		sum = 0
+	andq %rsi,%rsi		Set cc (nop), ZF=0 if count != 0 else 1, SF=0 if count > 0, OF=0
+	jmp test		Goto test
+loop:
+	mrmovq (%rdi), %r10 	Get *start
+	addq %r10,%rax		Add to sum
+	iaddq $8,%rdi 		start++
+	iaddq $-1,%rsi 		count--. Set CC
+test:
+	jne loop		Stop when 0, (~ZF)
+	ret
+*/
-- 
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