[Synth] Implement operation reuse for LowerVariadic

[joaovam]

This patch implements a subset-sharing heuristic in the LowerVariadic pass, as suggested in Issue #9712.
Currently, the LowerVariadic pass lowers variadic operations into binary trees independently, leading to redundant AIG gates.

I added a greedy subset-matching heuristic. Before a variadic aig.and_inv is lowered to a binary tree, the pass scans the current block for existing operations of the same type. Checks if the operation is a strict subset of the current one. Substitute the operands with the result of the other operation.

Example

For the testcase:

module Test(
  input  [5:0] in,	
  output out1, out2
  );

  assign out1 = in[0] & in[1] & in[2] & in[3] & in[4];
  assign out2 =         in[1] & in[2] & in[3] & in[4];
endmodule

The output is:

module {
  hw.module @Test(in %in : i6, out out1 : i1, out out2 : i1) {
    %0 = comb.extract %in from 0 : (i6) -> i1
    %1 = comb.extract %in from 1 : (i6) -> i1
    %2 = comb.extract %in from 2 : (i6) -> i1
    %3 = comb.extract %in from 3 : (i6) -> i1
    %4 = comb.extract %in from 4 : (i6) -> i1
    %5 = synth.aig.and_inv %1, %2 : i1
    %6 = synth.aig.and_inv %3, %4 : i1
    %7 = synth.aig.and_inv %5, %6 : i1
    %8 = synth.aig.and_inv %7, %0 : i1
    hw.output %8, %7 : i1, i1
  }
}

[uenoku]

Cool, thank you for sending for PR! Can we make this an option of the pass for area/timing trade off? Note that there are failures in LEC but this change itself is not at fault. This patch exposed a bug in other pass, let me investigate :) Ah it seems there is a correctness issue with the current implementation. Let me extract a counterexample.

[joaovam]

Hi @uenoku, I believe I'm almost done with making it O(N). I have a question, you asked me to make it part of an option. Did you mean I should create a new option that enables this specific optimization, or should I use an existing one?

[uenoku]

Yes please add a new option to

circt/include/circt/Dialect/Synth/Transforms/SynthPasses.td

Line 92 in 5bd66cd

"Lower operators with timing information">

.

[joaovam]

@uenoku I have updated the pass to use a map of the available andInverterOp operations in the block. I have also added a new option, reuse-subsets as a guard to the new code. But I have a question about it, how to enable the option in circt-synth? I could only test it via a unit test.

[uenoku]

Could you conditionally pass the option here when options.synthesisStrategy == OptimizationStrategyArea for now?

circt/lib/Dialect/Synth/Transforms/SynthesisPipeline.cpp

Lines 84 to 88 in ba22599

	pm.addPass(createLowerVariadicPass<comb::XorOp>(options.timingAware));
	} else if (options.targetIR.getValue() == TargetIR::MIG) {
	// For MIG, lower variadic And, Or, and Xor since MIG cannot keep variadic
	// representation.
	pm.addPass(createLowerVariadicPass<comb::AndOp, comb::OrOp, comb::XorOp>(

[joaovam]

Done. I see that createLowerVariadicPass is called in other places in that same file. Should I also update those calls?