refactoring

bitsandbytes/autograd/_functions.py

@@ -245,10 +245,11 @@ class MatMul8bitLt(torch.autograd.Function):
                 subA = A[:, idx]
                 state.subB = B[:, idx].t().contiguous()
                 state.idx = idx
-            elif state.CxB is None:
-                # B in in 8-bit row-major, we can transform it back to 16-bit to extract outlier dimensions
-                # we also need to convert it to the turing/ampere format
-                state.CxB, state.SB = F.transform(state.CB, to_order=formatB)
+            else:
+                if state.CxB is None:
+                    # B in in 8-bit row-major, we can transform it back to 16-bit to extract outlier dimensions
+                    # we also need to convert it to the turing/ampere format
+                    state.CxB, state.SB = F.transform(state.CB, to_order=formatB)
         else:
             if not state.has_fp16_weights and state.CxB is None:
                 state.CxB, state.SB = F.transform(state.CB, to_order=formatB)
@@ -355,19 +356,24 @@ class MatMul8bitLt(torch.autograd.Function):
 
         if req_gradA:
             C32grad, Sgrad = F.transform(Cgrad, "col32")
-            if state.CxBt is None and state.has_fp16_weights:
-                CBt = state.CBt
-            elif state.CxBt is None:
-                assert state.CBt is None
-                CB = state.CB.half()
-                SCB = state.SCB.unsquezee(1).half()
-                SCBt = state.SCBt.unsquezee(1).half()
-                Bt = (CB * SCB).t().contiguous()
-                CBt = (Bt / SCBt).t().to(torch.int8)
-
-            CxBt, SBt = F.transform(
-                CBt, to_order=formatB, transpose=True
-            )
+            if state.CxBt is None:
+                if state.has_fp16_weights:
+                    CBt = state.CBt
+                else:
+                    # Restore CBt from CB
+                    assert state.CBt is None, "CBt should not be stored in state"
+                    CB = state.CB.half()
+                    SCB = state.SCB.unsquezee(1).half()
+                    SCBt = state.SCBt.unsquezee(1).half()
+                    Bt = (CB * SCB).t().contiguous()
+                    CBt = (Bt / SCBt).t().to(torch.int8)
+
+                # intentionally, do not store CxBt into state  
+                CxBt, SBt = F.transform(
+                    CBt, to_order=formatB, transpose=True
+                )
+            else:
+                CxBt = state.CxBt
             gradA32, SgradA32 = F.igemmlt(C32grad, CxBt, Sgrad, SBt)
             grad_A = F.mm_dequant(gradA32, SgradA32, SCgrad, state.SCBt).view(ctx.grad_shape)