From 8a4e7bd4aacdbffa619a33a2cedf384cf8f94a2e Mon Sep 17 00:00:00 2001
From: yusufcanislek <yusuf.canislek@meetdandy.com>
Date: Sun, 26 Apr 2026 14:47:45 +0300
Subject: [PATCH] add vm_borrowchain_loop (8-bit ripple-borrow subtractor);
 vm_sample_count 402 -> 403

Result: {"status":"keep","vm_sample_count":403,"total_semantic_cases":4050,"manifest_samples":435}
---
 scripts/rewrite/instruction_microtests.json   | 21 +++++
 testcases/rewrite_smoke/vm_borrowchain_loop.c | 90 +++++++++++++++++++
 2 files changed, 111 insertions(+)
 create mode 100644 testcases/rewrite_smoke/vm_borrowchain_loop.c

diff --git a/scripts/rewrite/instruction_microtests.json b/scripts/rewrite/instruction_microtests.json
index e4d4700..c027fb3 100644
--- a/scripts/rewrite/instruction_microtests.json
+++ b/scripts/rewrite/instruction_microtests.json
@@ -861,6 +861,27 @@
         { "inputs": { "RCX": 128 },    "expected": 128, "label": "a=0x80,b=0" }
       ]
     },
+    {
+      "name": "vm_borrowchain_loop",
+      "symbol": "vm_borrowchain_loop_target",
+      "patterns": [
+        "xor",
+        "and",
+        "or"
+      ],
+      "semantic": [
+        { "inputs": { "RCX": 0 },     "expected": 0,   "label": "a=0 b=0: 0-0" },
+        { "inputs": { "RCX": 1 },     "expected": 1,   "label": "a=1 b=0: 1-0=1" },
+        { "inputs": { "RCX": 256 },   "expected": 511, "label": "a=0 b=1: -1 = 0xFF, borrow set" },
+        { "inputs": { "RCX": 65535 }, "expected": 0,   "label": "a=0xFF b=0xFF: 0" },
+        { "inputs": { "RCX": 65280 }, "expected": 257, "label": "a=0 b=0xFF: -0xFF mod 256 = 1, borrow" },
+        { "inputs": { "RCX": 255 },   "expected": 255, "label": "a=0xFF b=0: 0xFF" },
+        { "inputs": { "RCX": 43605 }, "expected": 427, "label": "a=0x55 b=0xAA: 0xAB + borrow" },
+        { "inputs": { "RCX": 21930 }, "expected": 85,  "label": "a=0xAA b=0x55: 0x55" },
+        { "inputs": { "RCX": 33023 }, "expected": 127, "label": "a=0xFF b=0x80: 0x7F" },
+        { "inputs": { "RCX": 32640 }, "expected": 1,   "label": "a=0x80 b=0x7F: 0x01" }
+      ]
+    },
     {
       "name": "vm_prefix_sum_loop",
       "symbol": "vm_prefix_sum_loop_target",
diff --git a/testcases/rewrite_smoke/vm_borrowchain_loop.c b/testcases/rewrite_smoke/vm_borrowchain_loop.c
new file mode 100644
index 0000000..e95269a
--- /dev/null
+++ b/testcases/rewrite_smoke/vm_borrowchain_loop.c
@@ -0,0 +1,90 @@
+/* PC-state VM running an 8-bit ripple-borrow subtractor bit-by-bit.
+ * Lift target: vm_borrowchain_loop_target.
+ * Goal: complete the carrychain pair with a sequential subtractor where
+ * each iteration's borrow depends on the previous bit's borrow.  Inputs
+ * a = x & 0xFF and b = (x >> 8) & 0xFF, output is (a-b) mod 2^8 packed
+ * as low byte | (final_borrow << 8).  Distinct from vm_carrychain_loop
+ * (which adds): the borrow propagation chain has the opposite signal
+ * pattern.
+ *
+ *   bs        = ba XOR bb XOR borrow            // sum bit of full subtractor
+ *   new_borrow = (NOT ba AND bb) OR (borrow AND NOT(ba XOR bb))
+ */
+#include <stdio.h>
+
+enum BcVmPc {
+    BC2_LOAD     = 0,
+    BC2_INIT     = 1,
+    BC2_CHECK    = 2,
+    BC2_BODY_BA  = 3,
+    BC2_BODY_BB  = 4,
+    BC2_BODY_SUM = 5,
+    BC2_BODY_NB  = 6,
+    BC2_BODY_OR  = 7,
+    BC2_BODY_INC = 8,
+    BC2_PACK     = 9,
+    BC2_HALT     = 10,
+};
+
+__declspec(noinline)
+int vm_borrowchain_loop_target(int x) {
+    int a       = 0;
+    int b       = 0;
+    int i       = 0;
+    int borrow  = 0;
+    int result  = 0;
+    int ba      = 0;
+    int bb      = 0;
+    int bs      = 0;
+    int nb      = 0;
+    int xor_ab  = 0;
+    int pc      = BC2_LOAD;
+
+    while (1) {
+        if (pc == BC2_LOAD) {
+            a = x & 0xFF;
+            b = (x >> 8) & 0xFF;
+            i = 0;
+            borrow = 0;
+            result = 0;
+            pc = BC2_INIT;
+        } else if (pc == BC2_INIT) {
+            pc = BC2_CHECK;
+        } else if (pc == BC2_CHECK) {
+            pc = (i < 8) ? BC2_BODY_BA : BC2_PACK;
+        } else if (pc == BC2_BODY_BA) {
+            ba = (a >> i) & 1;
+            pc = BC2_BODY_BB;
+        } else if (pc == BC2_BODY_BB) {
+            bb = (b >> i) & 1;
+            pc = BC2_BODY_SUM;
+        } else if (pc == BC2_BODY_SUM) {
+            xor_ab = ba ^ bb;
+            bs = xor_ab ^ borrow;
+            pc = BC2_BODY_NB;
+        } else if (pc == BC2_BODY_NB) {
+            nb = ((~ba) & bb & 1) | (borrow & ((~xor_ab) & 1));
+            pc = BC2_BODY_OR;
+        } else if (pc == BC2_BODY_OR) {
+            result = result | (bs << i);
+            borrow = nb;
+            pc = BC2_BODY_INC;
+        } else if (pc == BC2_BODY_INC) {
+            i = i + 1;
+            pc = BC2_CHECK;
+        } else if (pc == BC2_PACK) {
+            result = result | (borrow << 8);
+            pc = BC2_HALT;
+        } else if (pc == BC2_HALT) {
+            return result;
+        } else {
+            return -1;
+        }
+    }
+}
+
+int main(void) {
+    printf("vm_borrowchain_loop(0x0001)=%d vm_borrowchain_loop(0xAA55)=%d\n",
+           vm_borrowchain_loop_target(0x0001), vm_borrowchain_loop_target(0xAA55));
+    return 0;
+}