[Synth] Improve the truth table expansion of > 6 inputs

[okekayode]

Concerns #10419

We improve the truth table expansion of > 6 inputs by replacing the bit-by-bit implementation ofexpandTruthTableToInputSpaceGeneric with a word-level one. To do so we generate a mask corresponding to the rows that are expanded and set them all at once. This reduces the time complexity from O(2^numExpandedInputs) to O(2^numOrigInputs).

[uenoku]

Fantastic, thanks! I think it would be worth checking the sparsity of the truthtable and flip if necessary. Also can you add a bit more tests for here?

circt/unittests/Support/TruthTableTest.cpp

Lines 330 to 345 in 9fa974e

	TEST(TruthTableUtilTest, ExpandTruthTableToInputSpaceGenericPath) {
	// 3-input parity embedded into inputs [x0, x3, x6].
	APInt tt(8, 0b10010110);
	SmallVector<unsigned> mapping = {0, 3, 6};
	APInt expanded = circt::detail::expandTruthTableToInputSpace(tt, mapping, 7);
	APInt expected(128, 0);
	for (unsigned expandedIdx = 0; expandedIdx < 128; ++expandedIdx) {
	unsigned origIdx = ((expandedIdx >> 0) & 1U) |
	(((expandedIdx >> 3) & 1U) << 1) |
	(((expandedIdx >> 6) & 1U) << 2);
	if (tt[origIdx])
	expected.setBit(expandedIdx);
	}
	EXPECT_EQ(expanded, expected);
	}

Also could you make the PR description a bit more verbose? A few sentences would be great!

[okekayode]

Fantastic, thanks! I think it would be worth checking the sparsity of the truthtable and flip if necessary. Also can you add a bit more tests for here?

circt/unittests/Support/TruthTableTest.cpp

Lines 330 to 345 in 9fa974e

	TEST(TruthTableUtilTest, ExpandTruthTableToInputSpaceGenericPath) {
	// 3-input parity embedded into inputs [x0, x3, x6].
	APInt tt(8, 0b10010110);
	SmallVector<unsigned> mapping = {0, 3, 6};
	APInt expanded = circt::detail::expandTruthTableToInputSpace(tt, mapping, 7);
	APInt expected(128, 0);
	for (unsigned expandedIdx = 0; expandedIdx < 128; ++expandedIdx) {
	unsigned origIdx = ((expandedIdx >> 0) & 1U) |
	(((expandedIdx >> 3) & 1U) << 1) |
	(((expandedIdx >> 6) & 1U) << 2);
	if (tt[origIdx])
	expected.setBit(expandedIdx);
	}
	EXPECT_EQ(expanded, expected);
	}

Also could you make the PR description a bit more verbose? A few sentences would be great!

🫡

[uenoku]

Thanks!