[Seq] Remove incorrect canonicalization and reject registers with pre…

…sets(#7289)

This PR removes a canonicalization that incorrectly replaces self-connect registers with their non-constant reset values.
This PR also adds additional conditions for presets. We have to revisit to handle preset values. 

Fix #7266.

lib/Dialect/Seq/SeqOps.cpp

@@ -547,6 +547,11 @@ void FirRegOp::getAsmResultNames(OpAsmSetValueNameFn setNameFn) {
 std::optional<size_t> FirRegOp::getTargetResultIndex() { return 0; }
 
 LogicalResult FirRegOp::canonicalize(FirRegOp op, PatternRewriter &rewriter) {
+  // Don't canonicalize if there is a preset value for now.
+  // TODO: Handle a preset value.
+  if (op.getPresetAttr())
+    return failure();
+
   // If the register has a constant zero reset, drop the reset and reset value
   // altogether.
   if (auto reset = op.getReset()) {
@@ -576,20 +581,14 @@ LogicalResult FirRegOp::canonicalize(FirRegOp op, PatternRewriter &rewriter) {
     return false;
   };
 
-  if (isConstant()) {
-    if (auto resetValue = op.getResetValue()) {
-      // If the register has a reset value, we can replace it with that.
-      rewriter.replaceOp(op, resetValue);
+  if (isConstant() && !op.getResetValue()) {
+    if (isa<seq::ClockType>(op.getType())) {
+      rewriter.replaceOpWithNewOp<seq::ConstClockOp>(
+          op, seq::ClockConstAttr::get(rewriter.getContext(), ClockConst::Low));
     } else {
-      if (isa<seq::ClockType>(op.getType())) {
-        rewriter.replaceOpWithNewOp<seq::ConstClockOp>(
-            op,
-            seq::ClockConstAttr::get(rewriter.getContext(), ClockConst::Low));
-      } else {
-        auto constant = rewriter.create<hw::ConstantOp>(
-            op.getLoc(), APInt::getZero(hw::getBitWidth(op.getType())));
-        rewriter.replaceOpWithNewOp<hw::BitcastOp>(op, op.getType(), constant);
-      }
+      auto constant = rewriter.create<hw::ConstantOp>(
+          op.getLoc(), APInt::getZero(hw::getBitWidth(op.getType())));
+      rewriter.replaceOpWithNewOp<hw::BitcastOp>(op, op.getType(), constant);
     }
     return success();
   }
@@ -652,8 +651,9 @@ LogicalResult FirRegOp::canonicalize(FirRegOp op, PatternRewriter &rewriter) {
 }
 
 OpFoldResult FirRegOp::fold(FoldAdaptor adaptor) {
-  // If the register has a symbol, we can't optimize it away.
-  if (getInnerSymAttr())
+  // If the register has a symbol or preset value, we can't optimize it away.
+  // TODO: Handle a preset value.
+  if (getInnerSymAttr() || getPresetAttr())
     return {};
 
   // If the register is held in permanent reset, replace it with its reset

test/Dialect/Seq/canonicalization.mlir

@@ -39,10 +39,13 @@ hw.module @FirRegReset(in %clk : !seq.clock, in %in : i32, in %r : i1, in %v : i
   %true = hw.constant true
 
   // Registers that update to themselves should be replaced with their reset
-  // value.
-  %reg0 = seq.firreg %reg0 clock %clk reset sync %r, %v : i32
-  hw.instance "reg0" @Observe(x: %reg0: i32) -> ()
-  // CHECK: hw.instance "reg0" @Observe(x: %v: i32) -> ()
+  // value only when a reset value is constant.
+  %reg0a = seq.firreg %reg0a clock %clk reset sync %r, %v : i32
+  %reg0b = seq.firreg %reg0b clock %clk reset sync %r, %c0_i32 : i32
+  hw.instance "reg0a" @Observe(x: %reg0a: i32) -> ()
+  hw.instance "reg0b" @Observe(x: %reg0b: i32) -> ()
+  // CHECK: hw.instance "reg0a" @Observe(x: %reg0a: i32) -> ()
+  // CHECK-NEXT: hw.instance "reg0b" @Observe(x: %c0_i32: i32) -> ()
 
   // Registers that never reset should drop their reset value.
   %reg1 = seq.firreg %in clock %clk reset sync %false, %v : i32
@@ -72,6 +75,10 @@ hw.module @FirRegReset(in %clk : !seq.clock, in %in : i32, in %r : i1, in %v : i
   // CHECK: hw.instance "reg3a" @Observe(x: %reg3a: i32) -> ()
   // CHECK: hw.instance "reg3b" @Observe(x: %reg3b: i32) -> ()
   // CHECK: hw.instance "reg3c" @Observe(x: %c0_i32: i32) -> ()
+
+  // A register with preset value is not folded right now
+  // CHECK: %reg_preset = seq.firreg
+  %reg_preset = seq.firreg %reg_preset clock %clk reset sync %r, %c0_i32 preset 3: i32
 }
 
 // CHECK-LABEL: @FirRegAggregate