From e2ab4646567a32cdc8ae47cd1f7db8abe0e381dd Mon Sep 17 00:00:00 2001
From: Ethan Sifferman <ethan@sifferman.dev>
Date: Thu, 2 Jul 2026 16:20:20 -0700
Subject: [PATCH] Fix unequal-length string comparison

---
 elab_expr.cc                           | 11 ++++++++
 ivtest/ivltests/param_string_compare.v | 36 ++++++++++++++++++++++++++
 ivtest/regress-sv.list                 |  1 +
 3 files changed, 48 insertions(+)
 create mode 100644 ivtest/ivltests/param_string_compare.v

diff --git a/elab_expr.cc b/elab_expr.cc
index 6c9d7b17a..291db1a08 100644
--- a/elab_expr.cc
+++ b/elab_expr.cc
@@ -916,6 +916,17 @@ NetExpr* PEBComp::elaborate_expr(Design*des, NetScope*scope,
       eval_expr(lp, l_width_);
       eval_expr(rp, r_width_);
 
+	// test_width() does not equalize operand widths for string
+	// operands, but constant evaluation expects equal widths.
+      if (left_->expr_type() == IVL_VT_STRING ||
+	  right_->expr_type() == IVL_VT_STRING) {
+	    if (dynamic_cast<NetEConst*>(lp) && dynamic_cast<NetEConst*>(rp)) {
+		  unsigned use_wid = max(lp->expr_width(), rp->expr_width());
+		  lp = pad_to_width(lp, use_wid, false, *this);
+		  rp = pad_to_width(rp, use_wid, false, *this);
+	    }
+      }
+
 	// Handle some operand-specific special cases...
       switch (op_) {
 	  case 'E': /* === */
diff --git a/ivtest/ivltests/param_string_compare.v b/ivtest/ivltests/param_string_compare.v
new file mode 100644
index 000000000..61e1f81e6
--- /dev/null
+++ b/ivtest/ivltests/param_string_compare.v
@@ -0,0 +1,36 @@
+// Regression test: comparing a `string` parameter to a string constant of
+// a different length must constant-fold cleanly instead of tripping the
+// "lv.len() == rv.len()" assertion in the NetEBComp evaluation functions.
+// Covers ==, !=, ===, !==, ==? and !=? with the string operand on either
+// side and both shorter and longer than the other operand.
+
+module main #(parameter string Target = "A",
+	      parameter string Long = "ABC");
+
+   integer errors = 0;
+
+   // Equality / inequality, parameter shorter than the literal.
+   if (Target == "AA") initial begin errors += 1; $display("FAILED: Target == \"AA\""); end
+   if (Target != "A")  initial begin errors += 1; $display("FAILED: Target != \"A\""); end
+
+   // Literal on the left.
+   if ("AA" == Target) initial begin errors += 1; $display("FAILED: \"AA\" == Target"); end
+
+   // Parameter longer than the literal.
+   if (Long == "AB")   initial begin errors += 1; $display("FAILED: Long == \"AB\""); end
+   if (Long != "ABC")  initial begin errors += 1; $display("FAILED: Long != \"ABC\""); end
+
+   // Two string parameters of different lengths.
+   if (Target == Long) initial begin errors += 1; $display("FAILED: Target == Long"); end
+
+   // Case equality.
+   if (Target === "AA") initial begin errors += 1; $display("FAILED: Target === \"AA\""); end
+   if (Target !== "A")  initial begin errors += 1; $display("FAILED: Target !== \"A\""); end
+
+   // Wildcard equality.
+   if (Target ==? "AA") initial begin errors += 1; $display("FAILED: Target ==? \"AA\""); end
+   if (Target !=? "A")  initial begin errors += 1; $display("FAILED: Target !=? \"A\""); end
+
+   initial #1 if (errors == 0) $display("PASSED");
+
+endmodule
diff --git a/ivtest/regress-sv.list b/ivtest/regress-sv.list
index dade32337..ee2dc7757 100644
--- a/ivtest/regress-sv.list
+++ b/ivtest/regress-sv.list
@@ -416,6 +416,7 @@ net_string_fail		CE,-g2005-sv		ivltests
 package_vec_part_select	normal,-g2005-sv	ivltests
 packeda			normal,-g2009		ivltests
 packeda2		normal,-g2009		ivltests
+param_string_compare	normal,-g2009		ivltests
 parameter_in_generate2	CE,-g2005-sv		ivltests
 parameter_no_default	normal,-g2005-sv	ivltests
 parameter_no_default_fail1 CE			ivltests