[CombToSynth] Use parallel-prefix tree for unsigned comparisons (#9048)

This commit extends the unsigned comparison lowering to support multiple parallel-prefix architectures (Sklanskey, Kogge-Stone, Brent-Kung) in addition to the existing ripple-carry implementation. No functional change in prefix-tree/adder lowering. 

Previously, all comparisons used a ripple-carry style implementation that processed bits sequentially from LSB to MSB, resulting in O(n) depth for n-bit comparisons. This was a significant performance bottleneck for wide comparisons. The comparison lowering is now refactored to use the same parallel-prefix tree algorithms as the adder, reducing depth to O(log n).

The comparison logic is formulated as a prefix computation where equal bits are computed as ~(a_i ^ b_i) and greater bits as ~a_i & b_i, with propagate and generate signals based on equality and greater-than conditions.

Integration tests for all architectures verify logical equivalence via circt-lec.

Author: uenoku

integration_test/circt-synth/comb-lowering-compare.mlir

@@ -0,0 +1,56 @@
+// REQUIRES: libz3
+// REQUIRES: circt-lec-jit
+
+// RUN: circt-opt %s --convert-comb-to-synth --convert-synth-to-comb -o %t.mlir
+
+// RUN: circt-lec %t.mlir %s -c1=icmp_unsigned_ripple_carry -c2=icmp_unsigned_ripple_carry --shared-libs=%libz3 | FileCheck %s --check-prefix=COMB_ICMP_UNSIGNED_RIPPLE_CARRY
+// COMB_ICMP_UNSIGNED_RIPPLE_CARRY: c1 == c2
+hw.module @icmp_unsigned_ripple_carry(in %lhs: i3, in %rhs: i3, out out_ugt: i1, out out_uge: i1, out out_ult: i1, out out_ule: i1) {
+  %ugt = comb.icmp ugt %lhs, %rhs {synth.test.arch = "RIPPLE-CARRY"} : i3
+  %uge = comb.icmp uge %lhs, %rhs {synth.test.arch = "RIPPLE-CARRY"} : i3
+  %ult = comb.icmp ult %lhs, %rhs {synth.test.arch = "RIPPLE-CARRY"} : i3
+  %ule = comb.icmp ule %lhs, %rhs {synth.test.arch = "RIPPLE-CARRY"} : i3
+  hw.output %ugt, %uge, %ult, %ule : i1, i1, i1, i1
+}
+
+// RUN: circt-lec %t.mlir %s -c1=icmp_unsigned_sklanskey -c2=icmp_unsigned_sklanskey --shared-libs=%libz3 | FileCheck %s --check-prefix=COMB_ICMP_UNSIGNED_SKLANSKEY
+// COMB_ICMP_UNSIGNED_SKLANSKEY: c1 == c2
+hw.module @icmp_unsigned_sklanskey(in %lhs: i3, in %rhs: i3, out out_ugt: i1, out out_uge: i1, out out_ult: i1, out out_ule: i1) {
+  %ugt = comb.icmp ugt %lhs, %rhs {synth.test.arch = "SKLANSKEY"} : i3
+  %uge = comb.icmp uge %lhs, %rhs {synth.test.arch = "SKLANSKEY"} : i3
+  %ult = comb.icmp ult %lhs, %rhs {synth.test.arch = "SKLANSKEY"} : i3
+  %ule = comb.icmp ule %lhs, %rhs {synth.test.arch = "SKLANSKEY"} : i3
+  hw.output %ugt, %uge, %ult, %ule : i1, i1, i1, i1
+}
+
+// RUN: circt-lec %t.mlir %s -c1=icmp_unsigned_kogge_stone -c2=icmp_unsigned_kogge_stone --shared-libs=%libz3 | FileCheck %s --check-prefix=COMB_ICMP_UNSIGNED_KOGGE_STONE
+// COMB_ICMP_UNSIGNED_KOGGE_STONE: c1 == c2
+hw.module @icmp_unsigned_kogge_stone(in %lhs: i3, in %rhs: i3, out out_ugt: i1, out out_uge: i1, out out_ult: i1, out out_ule: i1) {
+  %ugt = comb.icmp ugt %lhs, %rhs {synth.test.arch = "KOGGE-STONE"} : i3
+  %uge = comb.icmp uge %lhs, %rhs {synth.test.arch = "KOGGE-STONE"} : i3
+  %ult = comb.icmp ult %lhs, %rhs {synth.test.arch = "KOGGE-STONE"} : i3
+  %ule = comb.icmp ule %lhs, %rhs {synth.test.arch = "KOGGE-STONE"} : i3
+  hw.output %ugt, %uge, %ult, %ule : i1, i1, i1, i1
+}
+
+// RUN: circt-lec %t.mlir %s -c1=icmp_unsigned_brent_kung -c2=icmp_unsigned_brent_kung --shared-libs=%libz3 | FileCheck %s --check-prefix=COMB_ICMP_UNSIGNED_BRENT_KUNG
+// COMB_ICMP_UNSIGNED_BRENT_KUNG: c1 == c2
+hw.module @icmp_unsigned_brent_kung(in %lhs: i4, in %rhs: i4, out out_ugt: i1, out out_uge: i1, out out_ult: i1, out out_ule: i1) {
+  %ugt = comb.icmp ugt %lhs, %rhs {synth.test.arch = "BRENT-KUNG"} : i4
+  %uge = comb.icmp uge %lhs, %rhs {synth.test.arch = "BRENT-KUNG"} : i4
+  %ult = comb.icmp ult %lhs, %rhs {synth.test.arch = "BRENT-KUNG"} : i4
+  %ule = comb.icmp ule %lhs, %rhs {synth.test.arch = "BRENT-KUNG"} : i4
+  hw.output %ugt, %uge, %ult, %ule : i1, i1, i1, i1
+}
+
+// RUN: circt-lec %t.mlir %s -c1=icmp_signed -c2=icmp_signed --shared-libs=%libz3 | FileCheck %s --check-prefix=COMB_ICMP_SIGNED
+// COMB_ICMP_SIGNED: c1 == c2
+hw.module @icmp_signed(in %lhs: i4, in %rhs: i4, out out_ugt: i1, out out_uge: i1, out out_ult: i1, out out_ule: i1) {
+  // Sign comparisons are just unsigned comparisons with inverted inputs and outputs.
+  // No need to test all architectures here.
+  %ugt = comb.icmp sgt %lhs, %rhs {synth.test.arch = "RIPPLE-CARRY"} : i4
+  %uge = comb.icmp sge %lhs, %rhs {synth.test.arch = "SKLANSKEY"} : i4
+  %ult = comb.icmp slt %lhs, %rhs {synth.test.arch = "KOGGE-STONE"} : i4
+  %ule = comb.icmp sle %lhs, %rhs {synth.test.arch = "BRENT-KUNG"} : i4
+  hw.output %ugt, %uge, %ult, %ule : i1, i1, i1, i1
+}