Fix const-bit-op-tree with single-bit masks (#5993) (#5998)

src/V3Const.cpp

@@ -125,6 +125,7 @@ class ConstBitOpTreeVisitor final : public VNVisitorConst {
         void updateBitRange(const AstShiftR* shiftp) {
             m_lsb += VN_AS(shiftp->rhsp(), Const)->toUInt();
         }
+        void limitBitRangeToLsb() { m_msb = std::min(m_msb, m_lsb); }
         int wordIdx() const { return m_wordIdx; }
         void wordIdx(int i) { m_wordIdx = i; }
         bool polarity() const { return m_polarity; }
@@ -538,6 +539,7 @@ class ConstBitOpTreeVisitor final : public VNVisitorConst {
             Restorer restorer{*this};
             incrOps(nodep, __LINE__);
             iterateConst(nodep->rhsp());
+            if (m_leafp) m_leafp->limitBitRangeToLsb();
             CONST_BITOP_RETURN_IF(m_failed, nodep->rhsp());
             restorer.disableRestore();  // Now all checks passed
         } else if (nodep->type() == m_rootp->type()) {  // And, Or, Xor

test_regress/t/t_opt_const.v

@@ -153,6 +153,7 @@ module Test(/*AUTOARG*/
    bug4857 i_bug4857(.clk(clk), .in(d), .out(bug4857_out));
    bug4864 i_bug4864(.clk(clk), .in(d), .out(bug4864_out));
    bug5186 i_bug5186(.clk(clk), .in(d), .out(bug5186_out));
+   bug5993 i_bug5993(.clk(clk), .in(d[10]));
 
 endmodule
 
@@ -566,3 +567,30 @@ module bug5186(input wire clk, input wire [31:0] in, output out);
       result <= bad;
    assign out = result;
 endmodule
+
+
+// See issue #5993
+// "in4[18]" is just one bit width, so " >> 8'd1" shifts out the bit.
+// BitOpTree ignored implicit "& 1". It caused the bug"
+module bug5993(input wire clk, input wire in);
+
+   reg in3;
+   reg [23:16] in4;
+
+   task automatic checkd(logic gotv, logic expv);
+      if ((gotv) !== (expv)) begin
+         $write("%%Error:  got=%0d exp=%0d\n", gotv, expv);
+         $stop;
+      end
+   endtask
+
+   // verilator lint_off WIDTH
+   wire wire_2 = in3 ? {4{14'b010111101}} : (in4[18] >> 8'b1);
+   // verilator lint_on WIDTH
+
+   always @(posedge clk) begin
+      in3 <= '0;
+      in4 <= in ?  8'b00111__0__10 : 8'b00111__1__10;
+      checkd(wire_2, 1'b0);
+   end
+endmodule

test_regress/t/t_opt_const_dfg.py

@@ -18,7 +18,7 @@ test.compile(verilator_flags2=["-Wno-UNOPTTHREADS", "--stats", test.pli_filename
 test.execute()
 
 if test.vlt:
-    test.file_grep(test.stats, r'Optimizations, Const bit op reduction\s+(\d+)', 39)
+    test.file_grep(test.stats, r'Optimizations, Const bit op reduction\s+(\d+)', 43)
     test.file_grep(test.stats, r'SplitVar, packed variables split automatically\s+(\d+)', 1)
 
 test.passes()