How to left-align a node in Graphviz?

I am trying to display digital circuit netlist with graphviz.

I am resorting to :

• record shapes to represent input/output ports.
• rankdir=LR to suggest a left to right alignment

The following code works fine. However, during layout, some inputs (here "i3") can be misaligned : I would expect i3 to be left-aligned, as for i1 and i2.

How can I do that ?

digraph G {
  graph [rankdir = LR];
  node[shape=record];
  c1[  label="{ {<i1>i1|<i2>i2}| c1 | {<f>f} }"];
  c2[  label="{ {<i0>i0}| c2 | {<out_0>out_0} }"];
  c3[  label="{ {<i0>i0}| c3 | {<out_0>out_0} }"];
  c4[  label="{ {<i0>i0|<i1>i1}| c4 | {<out_0>out_0} }"];
  i1;   i2;   i3;   f;   i1 -> c2:i0[ label="w(1)"];
  i2 -> c3:i0[ label="w(2)"];
  i3 -> c4:i1[ label="w(4)"];
  c1:f -> c4:i0[label="w(3)"];
  c2:out_0 -> c1:i1[label="w(5)"];
  c3:out_0 -> c1:i2[label="w(6)"];
  c4:out_0 -> f[label="w(7)"];
}

Just specify that i1, i2 and i3 share the same rank:

digraph G {
  graph [rankdir = LR];
  node[shape=record];
  c1[  label="{ {<i1>i1|<i2>i2}| c1 | {<f>f} }"];
  c2[  label="{ {<i0>i0}| c2 | {<out_0>out_0} }"];
  c3[  label="{ {<i0>i0}| c3 | {<out_0>out_0} }"];
  c4[  label="{ {<i0>i0|<i1>i1}| c4 | {<out_0>out_0} }"];
  i1;   i2;   i3;   f;   i1 -> c2:i0[ label="w(1)"];
  i2 -> c3:i0[ label="w(2)"];
  i3 -> c4:i1[ label="w(4)"];
  c1:f -> c4:i0[label="w(3)"];
  c2:out_0 -> c1:i1[label="w(5)"];
  c3:out_0 -> c1:i2[label="w(6)"];
  c4:out_0 -> f[label="w(7)"];

  {rank=same; i1; i2; i3}
}