Synthesis warning for out of bounds signal

[scorbetta]

Description

I keep having the following warnings from the synthesis phase:

/openlane/designs/LAYER/src/HL_NEURON.v:95: Warning: Range select [15:8] out of bounds on signal `\weights': Setting all 8 result bits to undef.
//...
/openlane/designs/LAYER/src/HL_NEURON.v:109: Warning: Range select [127:120] out of bounds on signal `\weights': Setting all 8 result bits to undef.

This points to a signal named weights defined as follows in the referenced file:

wire [NUM_INPUTS*WIDTH-1:0] weights;

NUM_INPUTS and WIDTH parameters get values from the top-level. The top-level has the following default values:

parameter NUM_INPUTS = 16
parameter NUM_OUTPUTS = 8
parameter WIDTH = 8
parameter FRAC_BITS = 5

So that weights has actual dimension of 128 bits. Meaning the warning above is wrong. Strangely, the synthesis output results/synthesis/HIDDEN_LAYER.v contains the signal with the correct width.

The only way I found to bypass this is to add the following to the config.tcl:

set ::env(QUIT_ON_SYNTH_CHECKS) 0

But this is not the way to go. So what is the problem with the above case? Am I missing something?

Thanks S

Expected Behavior

Synthesis pass

Environment report

open_pdks cd1748bb197f9b7af62a54507de6624e30363943
WARNING: issue-survey appears to be running inside the OpenLane
container.

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environment.

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---

Kernel: Linux v6.5.0-18-generic
Distribution: centos 7
Python: v3.6.8 (OK)
OpenLane Git Version: 9dbd8b5ea2bd891bed4dcc97df5c7439083f0368
python-venv: INSTALLED
---
PDK Version Verification Status: OK
---
Git Log (Last 3 Commits)

9dbd8b5e 2024-01-11T15:26:56+02:00 Fix `-synth_explore` crash (#2085) - Kareem Farid -  (HEAD, tag: 2024.01.12, origin/master, origin/HEAD, master)
a005df1f 2024-01-08T10:51:06+02:00 Updated link to newer version of spanish docs (#2082) - Laboratorio de Investigación en Microelectrónica y Arquitectura de Computadoras, EIE -- UCR -  (tag: 2024.01.09)
8bea8c51 2023-12-26T13:17:21+02:00 Update `OpenROAD` (#2062) - Kareem Farid -  (tag: 2023.12.27)
---
Git Remotes

origin  git@github.com:The-OpenROAD-Project/OpenLane.git (fetch)
origin  git@github.com:The-OpenROAD-Project/OpenLane.git (push)

Reproduction material

n/a

Relevant log output

/openlane/designs/LAYER/src/HL_NEURON.v:95: Warning: Range select [15:8] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:96: Warning: Range select [23:16] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:97: Warning: Range select [31:24] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:98: Warning: Range select [39:32] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:99: Warning: Range select [47:40] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:100: Warning: Range select [55:48] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:101: Warning: Range select [63:56] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:102: Warning: Range select [71:64] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:103: Warning: Range select [79:72] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:104: Warning: Range select [87:80] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:105: Warning: Range select [95:88] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:106: Warning: Range select [103:96] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:107: Warning: Range select [111:104] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:108: Warning: Range select [119:112] out of bounds on signal `\weights': Setting all 8 result bits to undef.
/openlane/designs/LAYER/src/HL_NEURON.v:109: Warning: Range select [127:120] out of bounds on signal `\weights': Setting all 8 result bits to undef.

[kareefardi]

@scorbetta Can you provide the RTL of your design?

[scorbetta]

Sure thing. I'm attaching the OpenLANE config file as well. I had to remove the runs folder due to space. LAYER.tar.gz

[scorbetta]

@kareefardi Any idea?

[kareefardi]

@scorbetta Sorry I have been a bit busy this week. I am checking it out now.

[kareefardi]

@scorbetta These warnings are generated when each file is read before analyzing the hierarchy. If you consider HL_NEURON by itself, it's default parameter value doesn't account for the out of bounds signal.

The whole system wouldn't be affect by this warning however I think it's best if you fix it inside the module itself to avoid future problems when reusing the module.

[scorbetta]

@kareefardi Got it. I can live with them then, however is this really how it should properly work? I mean, when I synthesize multiple Verilog files that belong to a given hierarchy, I would expect the top-level to forward values to the lower levels within the hierarchy. Otherwise there is no point to get a top with fixed parameters. And indeed that's how commercial tools do.

[kareefardi]

@scorbetta I think we may be able to do that with -defer in Yosys when reading the Verilog files but this requires discussion internally on whether we want to do that. I will keep this issue open and update it.

[scorbetta]

@kareefardi Indeed that's what I did. I am now using a patched synth.tcl script for synthesis. I added the -defer option to the read_verilog commands, and placed a hierarchy -check -top $vtop command later. Warnings go away.