From 300c64773b20f5b48896dcb87cb0b33dccd53d2d Mon Sep 17 00:00:00 2001
From: Akash Levy <akash@silimate.com>
Date: Thu, 9 Apr 2026 16:20:26 -0700
Subject: [PATCH] Fix offset in splitcells

---
 passes/cmds/splitcells.cc   | 24 ++++++++--
 tests/various/splitcells.ys | 90 +++++++++++++++++++++++++++++++++++++
 2 files changed, 110 insertions(+), 4 deletions(-)
 create mode 100644 tests/various/splitcells.ys

diff --git a/passes/cmds/splitcells.cc b/passes/cmds/splitcells.cc
index 759e9775b..4f86e28d5 100644
--- a/passes/cmds/splitcells.cc
+++ b/passes/cmds/splitcells.cc
@@ -147,6 +147,19 @@ struct SplitcellsWorker
 			if (GetSize(outsig) <= 1) return 0;
 			int width = GetSize(outsig);
 
+			// Compute user-facing bit index from the wire connected to Q
+			SigSpec raw_q = cell->getPort(ID::Q);
+			auto user_index = [&](int idx) -> int {
+				if (idx < GetSize(raw_q) && raw_q[idx].is_wire()) {
+					Wire *w = raw_q[idx].wire;
+					if (w->upto)
+						return w->start_offset + w->width - 1 - raw_q[idx].offset;
+					else
+						return w->start_offset + raw_q[idx].offset;
+				}
+				return idx;
+			};
+
 			std::vector<int> slices;
 			slices.push_back(0);
 
@@ -168,6 +181,9 @@ struct SplitcellsWorker
 			{
 				int slice_msb = slices[i]-1;
 				int slice_lsb = slices[i-1];
+				int name_lsb = user_index(slice_lsb);
+				int name_msb = user_index(slice_msb);
+				if (name_lsb > name_msb) std::swap(name_lsb, name_msb);
 
 				std::string base_name = cell->name.str();
 				IdString slice_name;
@@ -178,11 +194,11 @@ struct SplitcellsWorker
 						base_name = base_name.substr(0, bracket_pos);
 					}
 					slice_name = module->uniquify(base_name + stringf(
-						"%c%d%c", format[0], slice_lsb, format[1]));
+						"%c%d%c", format[0], name_lsb, format[1]));
 				} else {
-					slice_name = module->uniquify(base_name + (slice_msb == slice_lsb ?
-						stringf("%c%d%c", format[0], slice_lsb, format[1]) :
-						stringf("%c%d%c%d%c", format[0], slice_msb, format[2], slice_lsb, format[1])));
+					slice_name = module->uniquify(base_name + (name_msb == name_lsb ?
+						stringf("%c%d%c", format[0], name_lsb, format[1]) :
+						stringf("%c%d%c%d%c", format[0], name_msb, format[2], name_lsb, format[1])));
 				}
 
 				Cell *slice = module->addCell(slice_name, cell);
diff --git a/tests/various/splitcells.ys b/tests/various/splitcells.ys
new file mode 100644
index 000000000..121629ba6
--- /dev/null
+++ b/tests/various/splitcells.ys
@@ -0,0 +1,90 @@
+# Test 1: Basic splitcells blast on a zero-base register [5:0]
+# Cell names should use indices 0..5
+
+read_verilog <<EOT
+module test_zero_base (
+  input clk,
+  input [5:0] d,
+  output reg [5:0] q
+);
+  always @(posedge clk) q <= d;
+endmodule
+EOT
+proc
+splitcells -blast -format []:
+
+select -assert-count 6 t:$dff
+select -assert-count 1 c:*[0]
+select -assert-count 1 c:*[5]
+select -assert-none c:*[6]
+
+design -reset
+
+# Test 2: splitcells blast on a non-zero base register [22:17]
+# Cell names must use user-facing indices 17..22, not 0..5
+
+read_verilog <<EOT
+module test_nonzero_base (
+  input clk,
+  input [22:17] d,
+  output reg [22:17] q
+);
+  always @(posedge clk) q <= d;
+endmodule
+EOT
+proc
+splitcells -blast -format []:
+
+select -assert-count 6 t:$dff
+select -assert-count 1 c:*[17]
+select -assert-count 1 c:*[18]
+select -assert-count 1 c:*[19]
+select -assert-count 1 c:*[20]
+select -assert-count 1 c:*[21]
+select -assert-count 1 c:*[22]
+
+design -reset
+
+# Test 3: splitcells blast with offset=1 register [8:1]
+# Cell names should use indices 1..8, not 0..7
+
+read_verilog <<EOT
+module test_offset_one (
+  input clk,
+  input [8:1] d,
+  output reg [8:1] q
+);
+  always @(posedge clk) q <= d;
+endmodule
+EOT
+proc
+splitcells -blast -format []:
+
+select -assert-count 8 t:$dff
+select -assert-count 1 c:*[1]
+select -assert-count 1 c:*[8]
+
+design -reset
+
+# Test 4: splitcells non-blast with non-zero base and divergent fanout
+# Slice range names should use user-facing indices (e.g. [22:20], [19:17])
+
+read_verilog <<EOT
+module test_nonblast_nonzero (
+  input clk,
+  input [22:17] d,
+  output reg [22:17] q,
+  output [2:0] lo,
+  output [2:0] hi
+);
+  always @(posedge clk) q <= d;
+  assign lo = q[19:17];
+  assign hi = q[22:20];
+endmodule
+EOT
+proc
+splitcells -format []:
+
+select -assert-count 2 t:$dff
+select -assert-count 1 c:*[19:17]
+select -assert-count 1 c:*[22:20]