From 0ea8201c8677a9eb93173d72f67cd8cdbb002fac Mon Sep 17 00:00:00 2001
From: xaedes <xaedes@gmail.com>
Date: Wed, 26 Apr 2023 20:14:33 +0200
Subject: [PATCH] bug fix for cpy backward pass

---
 ggml.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/ggml.c b/ggml.c
index 723e55d99..cde938eae 100644
--- a/ggml.c
+++ b/ggml.c
@@ -12809,11 +12809,15 @@ static void ggml_compute_backward(struct ggml_context * ctx, struct ggml_tensor
         case GGML_OP_CPY:
             {
                 // necessary for llama
+                // cpy overwrites value of src1 by src0 and returns view(src1)
+                // the overwriting is mathematically equivalent to:
+                // tensor = src0 * 1 + src1 * 0
                 if (src0->grad) {
+                    // dsrc0 = dtensor * 1
                     src0->grad = ggml_add_impl(ctx, src0->grad, tensor->grad, inplace);
                 }
                 if (src1->grad) {
-                    src1->grad = ggml_add_impl(ctx, src1->grad, tensor->grad, inplace);
+                    // dsrc1 = dtensor * 0 -> noop
                 }
             } break;
         case GGML_OP_CONT: