opt_lut: always prefer to eliminate 1-LUTs.

These are always either buffers or inverters, and keeping the larger
LUT preserves more source-level information about the design.

diff --git a/passes/opt/opt_lut.cpp b/passes/opt/opt_lut.cpp
index b043f6c10139166536ddd513e7a1d2b078f95904..0eb6b13e54182b71bbd7076c88e5db4c1b457ccb 100644 (file)
--- a/passes/opt/opt_lut.cpp
+++ b/passes/opt/opt_lut.cpp
@@ -154,35 +154,57 @@ struct OptLutWorker
                                        int lutM_arity = lutA_arity + lutB_arity - 1 - common_inputs.size();
                                        log("  Cell A is a %d-LUT. Cell B is a %d-LUT. Cells share %zu input(s) and can be merged into one %d-LUT.\n", lutA_arity, lutB_arity, common_inputs.size(), lutM_arity);
 
-                                       int combine = -1;
-                                       if (combine == -1)
+                                       const int COMBINE_A = 1, COMBINE_B = 2, COMBINE_EITHER = COMBINE_A | COMBINE_B;
+                                       int combine_mask = 0;
+                                       if (lutM_arity > lutA_width)
                                        {
-                                               if (lutM_arity > lutA_width)
-                                               {
-                                                       log("  Not combining LUTs into cell A (combined LUT too wide).\n");
-                                               }
-                                               else if (lutB->get_bool_attribute("\\lut_keep"))
-                                               {
-                                                       log("  Not combining LUTs into cell A (cell B has attribute \\lut_keep).\n");
-                                               }
-                                               else combine = 0;
+                                               log("  Not combining LUTs into cell A (combined LUT wider than cell A).\n");
+                                       }
+                                       else if (lutB->get_bool_attribute("\\lut_keep"))
+                                       {
+                                               log("  Not combining LUTs into cell A (cell B has attribute \\lut_keep).\n");
+                                       }
+                                       else
+                                       {
+                                               combine_mask |= COMBINE_A;
+                                       }
+                                       if (lutM_arity > lutB_width)
+                                       {
+                                               log("  Not combining LUTs into cell B (combined LUT wider than cell B).\n");
+                                       }
+                                       else if (lutA->get_bool_attribute("\\lut_keep"))
+                                       {
+                                               log("  Not combining LUTs into cell B (cell A has attribute \\lut_keep).\n");
+                                       }
+                                       else
+                                       {
+                                               combine_mask |= COMBINE_B;
                                        }
-                                       if (combine == -1)
+
+                                       int combine = combine_mask;
+                                       if (combine == COMBINE_EITHER)
                                        {
-                                               if (lutM_arity > lutB_width)
+                                               log("  Can combine into either cell.\n");
+                                               if (lutA_arity == 1)
                                                {
-                                                       log("  Not combining LUTs into cell B (combined LUT too wide).\n");
+                                                       log("    Cell A is a buffer or inverter, combining into cell B.\n");
+                                                       combine = COMBINE_B;
                                                }
-                                               else if (lutA->get_bool_attribute("\\lut_keep"))
+                                               else if (lutB_arity == 1)
+                                               {
+                                                       log("    Cell B is a buffer or inverter, combining into cell A.\n");
+                                                       combine = COMBINE_A;
+                                               }
+                                               else
                                                {
-                                                       log("  Not combining LUTs into cell B (cell A has attribute \\lut_keep).\n");
+                                                       log("    Arbitrarily combining into cell A.\n");
+                                                       combine = COMBINE_A;
                                                }
-                                               else combine = 1;
                                        }
 
                                        RTLIL::Cell *lutM, *lutR;
                                        pool<SigBit> lutM_inputs, lutR_inputs;
-                                       if (combine == 0)
+                                       if (combine == COMBINE_A)
                                        {
                                                log("  Combining LUTs into cell A.\n");
                                                lutM = lutA;
@@ -190,7 +212,7 @@ struct OptLutWorker
                                                lutR = lutB;
                                                lutR_inputs = lutB_inputs;
                                        }
-                                       else if (combine == 1)
+                                       else if (combine == COMBINE_B)
                                        {
                                                log("  Combining LUTs into cell B.\n");
                                                lutM = lutB;