Unexpected behaviour of for loop and if statement

[ymherklotz]

Steps to reproduce the issue

Consider the following piece of code:

module top  (y, clk, sel);
   output wire y  ;
   input       clk; 
   input       sel;
   reg         reg_assign = (1'h0) ;
   reg [1:0]   reg_count = (1'h0) ;
   assign y = reg_assign ;
   always @(posedge clk) 
     if (sel)
       for (reg_count = 0; reg_count < 2; reg_count = reg_count + 1'h1)
         if (0);
         else reg_assign <= 1;
     else reg_assign <=  0;
endmodule

First of all, the for loop in the code does seem a bit dodgy, however, I would still expect reg_assign to be set to 1 when sel is high. When sel is low, reg_assign should then be reset to 0.

However, when synthesised with

Yosys 0.8+618 (git sha1 acd8bc0a, clang  -fPIC -Os)

using

yosys -p "read_verilog rtl.v; synth; write_verilog -noattr synth.v"

reg_assign is set to a constant 0 instead of to what the value of sel is. Removing the dead if statement in the for loop results in the correct behaviour.

I have also attached a folder containing a test bench and SymbiYosys script to compare the design to the synthesised net list.

Expected behaviour

I would expect this to be implemented by assigning sel to y. This is actually also the output of a previous version of Yosys (Yosys 0.8+508 (git sha1 c2ea3746, clang 8.0.0 -fPIC -Os))

module top(y, clk, sel);
  input clk;
  wire reg_assign;
  wire [1:0] reg_count;
  input sel;
  output y;
  assign reg_assign = 1'h0;
  assign reg_count = 2'h0;
  assign y = sel;
endmodule

Actual behaviour

However, with Yosys, y is set to a constant 0.

module top(y, clk, sel);
  input clk;
  wire reg_assign;
  wire [1:0] reg_count;
  input sel;
  output y;
  assign reg_assign = 1'h0;
  assign reg_count = 2'h0;
  assign y = 1'h0;
endmodule

test.zip

[daveshah1]

Reverting #1169 seems to give the expected result (I don't think assigning sel to y is correct, but the register looks good):

module top(y, clk, sel);
  input clk;
  reg reg_assign = 1'h0;
  wire [1:0] reg_count;
  input sel;
  output y;
  always @(posedge clk)
      reg_assign <= sel;
  assign reg_count = 2'h0;
  assign y = reg_assign;
endmodule

cc @whitequark

[ymherklotz]

Ah yes you are right, assigning sel directly does not seem correct, clocking it is definitely right.

[whitequark]

@daveshah1 Good catch, I'll fix the pass in a moment.

[whitequark]

Fixed in #1267.