Fix false LATCH warning on 'unique if' (#3088).

Changes

@@ -25,7 +25,7 @@ Verilator 5.001 devel
 * Introduce a new combinational logic optimizer (DFG), that can yield
   significant performance improvements on some designs. [Geza Lore, Shunyao CAD]
 * Add --binary option as alias of --main --exe --build --timing (#3625).
-  For designs where C++ was only used to make a simple testbench we
+  For designs where C++ was only used to make a simple no-I/O testbench, we
   recommend abandoning that C++, and instead letting Verilator build it
   with --binary (or --main).
 
@@ -38,6 +38,7 @@ Verilator 5.001 devel
 * Add --dump-tree-dot to enable dumping Ast Tree .dot files (#3636). [Marcel Chang]
 * Add --get-supported to determine what features are in Verilator.
 * Add error on real edge event control.
+* Fix false LATCH warning on 'unique if' (#3088). [Rachit Nigam]
 * Fix cell assigning integer array parameters (#3299). [Michael Platzer]
 * Fix LSB error on --hierarchical submodules (#3539). [danbone]
 * Fix $display of fixed-width numbers (#3565). [Iztok Jeras]

src/V3Assert.cpp

@@ -98,7 +98,7 @@ private:
         // Add a internal if to check assertions are on.
         // Don't make this a AND term, as it's unlikely to need to test this.
         FileLine* const fl = nodep->fileline();
-        AstNode* const newp = new AstIf{
+        AstNodeIf* const newp = new AstIf{
             fl,
             (force ? new AstConst{fl, AstConst::BitTrue{}}
                    :  // If assertions are off, have constant propagation rip them out later
@@ -108,6 +108,7 @@ private:
                           new AstCMath{fl, "vlSymsp->_vm_contextp__->assertOn()", 1})
                       : static_cast<AstNode*>(new AstConst{fl, AstConst::BitFalse{}}))),
             nodep};
+        newp->isBoundsCheck(true);  // To avoid LATCH warning
         newp->user1(true);  // Don't assert/cover this if
         return newp;
     }
@@ -164,6 +165,7 @@ private:
 
             if (bodysp && passsp) bodysp = bodysp->addNext(passsp);
             ifp = new AstIf{nodep->fileline(), propp, bodysp};
+            ifp->isBoundsCheck(true);  // To avoid LATCH warning
             bodysp = ifp;
         } else if (VN_IS(nodep, Assert) || VN_IS(nodep, AssertIntrinsic)) {
             if (nodep->immediate()) {
@@ -176,6 +178,7 @@ private:
             if (failsp) failsp = newIfAssertOn(failsp, force);
             if (!failsp) failsp = newFireAssertUnchecked(nodep, "'assert' failed.");
             ifp = new AstIf{nodep->fileline(), propp, passsp, failsp};
+            ifp->isBoundsCheck(true);  // To avoid LATCH warning
             // It's more LIKELY that we'll take the nullptr if clause
             // than the sim-killing else clause:
             ifp->branchPred(VBranchPred::BP_LIKELY);
@@ -253,6 +256,7 @@ private:
             AstIf* const checkifp
                 = new AstIf{nodep->fileline(), new AstLogNot{nodep->fileline(), ohot},
                             newFireAssert(nodep, "'unique if' statement violated"), newifp};
+            checkifp->isBoundsCheck(true);  // To avoid LATCH warning
             checkifp->branchPred(VBranchPred::BP_UNLIKELY);
             nodep->replaceWith(checkifp);
             pushDeletep(nodep);
@@ -323,6 +327,7 @@ private:
                         nodep->fileline(), new AstLogNot{nodep->fileline(), ohot},
                         newFireAssert(nodep,
                                       "synthesis parallel_case, but multiple matches found")};
+                    ifp->isBoundsCheck(true);  // To avoid LATCH warning
                     ifp->branchPred(VBranchPred::BP_UNLIKELY);
                     nodep->addNotParallelp(ifp);
                 }
@@ -425,6 +430,7 @@ private:
                               new AstEq{fl, new AstConst{fl, monNum},
                                         newMonitorNumVarRefp(nodep, VAccess::READ)}},
                 stmtsp};
+            ifp->isBoundsCheck(true);  // To avoid LATCH warning
             ifp->branchPred(VBranchPred::BP_UNLIKELY);
             AstNode* const newp = new AstAlways{fl, VAlwaysKwd::ALWAYS, nullptr, ifp};
             m_modp->addStmtsp(newp);
@@ -443,6 +449,7 @@ private:
             // Add "always_comb if (__Vstrobe) begin $display(...); __Vstrobe = '0; end"
             AstNode* const stmtsp = nodep;
             AstIf* const ifp = new AstIf{fl, new AstVarRef{fl, varp, VAccess::READ}, stmtsp};
+            ifp->isBoundsCheck(true);  // To avoid LATCH warning
             ifp->branchPred(VBranchPred::BP_UNLIKELY);
             AstNode* const newp = new AstAlwaysPostponed{fl, ifp};
             stmtsp->addNext(new AstAssign{fl, new AstVarRef{fl, varp, VAccess::WRITE},

src/V3AstNodeOther.h

@@ -504,7 +504,7 @@ class AstNodeIf VL_NOT_FINAL : public AstNodeStmt {
     // @astgen op3 := elsesp : List[AstNode]
 private:
     VBranchPred m_branchPred;  // Branch prediction as taken/untaken?
-    bool m_isBoundsCheck;  // True if this if node was inserted for array bounds checking
+    bool m_isBoundsCheck;  // True if this if node is for assertion/bounds checking
 protected:
     AstNodeIf(VNType t, FileLine* fl, AstNode* condp, AstNode* thensp, AstNode* elsesp)
         : AstNodeStmt{t, fl} {

test_regress/t/t_lint_latch_4.v

@@ -7,21 +7,27 @@ module test (
   input      [2:0] a,
   input      [3:0] c,
 
-  output reg [7:0] b
+  output reg [7:0] o1,
+  output reg [7:0] o2
 );
 
-  integer i;
+   integer         i;
 
-  always @ (*)
-  begin
-    case(a)
-      {3'b000}: b = 8'd1;
-      {3'b001}:
-        for(i=0;i<4;i=i+1) b[i*2+:2] = 2'(c[i]);
-      {3'b010}: b = 8'd3;
-      {3'b011}: b = 8'd4;
-      default : b = 0;
-    endcase
-  end
+   always @ (*) begin
+      case(a)
+        {3'b000}: o1 = 8'd1;
+        {3'b001}:
+          for(i=0;i<4;i=i+1) o1[i*2+:2] = 2'(c[i]);
+        {3'b010}: o1 = 8'd3;
+        {3'b011}: o1 = 8'd4;
+        default : o1 = 0;
+      endcase
+   end
+
+   always_comb begin
+      unique if (a[0]) o2 = 1;
+      else if (a[1]) o2 = 2;
+      else o2 = 3;
+   end
 
 endmodule