From 368f59d5f26d6349b95e4db9ba8ca9fdac0c387c Mon Sep 17 00:00:00 2001
From: Nick Brereton <85175726+nbstrike@users.noreply.github.com>
Date: Wed, 30 Sep 2026 20:17:00 -0400
Subject: [PATCH] Optimize exhaustive case if-chains with an unconditional else
 (#8554)

---
 src/V3Case.cpp                             |  17 ++-
 test_regress/t/t_case_exhaustive_nocase.py |  21 +++
 test_regress/t/t_case_exhaustive_nocase.v  | 143 +++++++++++++++++++++
 3 files changed, 178 insertions(+), 3 deletions(-)
 create mode 100755 test_regress/t/t_case_exhaustive_nocase.py
 create mode 100644 test_regress/t/t_case_exhaustive_nocase.v

diff --git a/src/V3Case.cpp b/src/V3Case.cpp
index ef24d8a56..26a40cd58 100644
--- a/src/V3Case.cpp
+++ b/src/V3Case.cpp
@@ -1001,11 +1001,22 @@ class CaseVisitor final : public VNVisitor {
             itemp->addCondsp(newCondp);
         }
 
-        // If there was no default, add a empty one, this greatly simplifies below code
+        // If there was no default, make one up, this greatly simplifies below code
         // and constant propagation will just eliminate it for us later.
         if (!hasDefault) {
-            nodep->addItemsp(new AstCaseItem{
-                nodep->fileline(), new AstConst{nodep->fileline(), AstConst::BitTrue{}}, nullptr});
+            if (m_caseDetailsValid && m_caseDetails.exhaustive
+                && !m_caseDetails.exhaustiveOverEnumOnly) {
+                // Case is proven exhaustive, so the last is reached only when its test holds,
+                // make it unconditional
+                AstCaseItem* const lastp = VN_AS(nodep->itemsp()->lastp(), CaseItem);
+                pushDeletep(lastp->condsp()->unlinkFrBackWithNext());
+                lastp->addCondsp(new AstConst{lastp->fileline(), AstConst::BitTrue{}});
+            } else {
+                // Otherwise needs an explicit empty default
+                nodep->addItemsp(new AstCaseItem{
+                    nodep->fileline(), new AstConst{nodep->fileline(), AstConst::BitTrue{}},
+                    nullptr});
+            }
         }
 
         // Now build the IF statement tree
diff --git a/test_regress/t/t_case_exhaustive_nocase.py b/test_regress/t/t_case_exhaustive_nocase.py
new file mode 100755
index 000000000..2a3286a5d
--- /dev/null
+++ b/test_regress/t/t_case_exhaustive_nocase.py
@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of either the GNU Lesser General Public License Version 3
+# or the Perl Artistic License Version 2.0.
+# SPDX-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile(verilator_flags2=["--stats -fno-case"])
+
+test.execute()
+
+test.file_grep(test.stats, r'Optimizations, Cases complex\s+(\d+)', 9)
+test.file_grep(test.stats, r'Optimizations, Tables created\s+(\d+)', 1)
+
+test.passes()
diff --git a/test_regress/t/t_case_exhaustive_nocase.v b/test_regress/t/t_case_exhaustive_nocase.v
new file mode 100644
index 000000000..0f52dc978
--- /dev/null
+++ b/test_regress/t/t_case_exhaustive_nocase.v
@@ -0,0 +1,143 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+// verilog_format: on
+
+// Exhaustive cases lowered as if/else chains, checked against reference expressions.
+
+// verilator lint_off CASEINCOMPLETE
+// verilator lint_off CASEOVERLAP
+// verilator lint_off CASEWITHX
+// verilator lint_off CASEX
+
+module t (
+    input clk
+);
+
+  typedef enum logic [1:0] {
+    E0,
+    E1,
+    E2
+  } e_t;
+
+  integer cyc = 0;
+
+  wire [1:0] sel_plain = cyc[1:0];
+  wire [2:0] s3 = cyc[2:0];
+  e_t e;
+  assign e = e_t'(cyc[1:0]);
+
+  logic [3:0] y_plain, y_casez, y_casex, y_never, y_inside, y_overlap, y_unique, y_enum, y_incompl;
+
+  always_comb begin
+    case (sel_plain)
+      2'd0: y_plain = 4'h1;
+      2'd1: y_plain = 4'h2;
+      2'd2: y_plain = 4'h4;
+      2'd3: y_plain = 4'h8;
+    endcase
+  end
+
+  always_comb begin
+    casez (s3)
+      3'b1??: y_casez = 4'h1;
+      3'b01?: y_casez = 4'h2;
+      3'b001: y_casez = 4'h3;
+      3'b000: y_casez = 4'h4;
+    endcase
+  end
+
+  always_comb begin
+    casex (s3)
+      3'bxx1: y_casex = 4'h1;
+      3'bx10: y_casex = 4'h2;
+      3'b100: y_casex = 4'h3;
+      3'b000: y_casex = 4'h4;
+    endcase
+  end
+
+  // Items with X never match in a plain case: the last item is unreachable
+  always_comb begin
+    case (s3[1:0])
+      2'b0x: y_never = 4'hf;
+      2'd0: y_never = 4'h1;
+      2'd1: y_never = 4'h2;
+      2'd2: y_never = 4'h3;
+      2'd3: y_never = 4'h4;
+      2'b1x: y_never = 4'he;
+    endcase
+  end
+
+  always_comb begin
+    case (s3) inside
+      3'b1??: y_inside = 4'h1;
+      3'b0?1: y_inside = 4'h2;
+      3'b0?0: y_inside = 4'h3;
+    endcase
+  end
+
+  // First match wins; the last item is reached only for values it covers
+  always_comb begin
+    casez (s3)
+      3'b??1: y_overlap = 4'h1;
+      3'b1??: y_overlap = 4'h2;
+      3'b0?0: y_overlap = 4'h3;
+      3'b1?0: y_overlap = 4'h4;
+    endcase
+  end
+
+  always_comb begin
+    unique case (sel_plain)
+      2'd3: y_unique = 4'h1;
+      2'd2: y_unique = 4'h2;
+      2'd1: y_unique = 4'h3;
+      2'd0: y_unique = 4'h4;
+    endcase
+  end
+
+  // Covers every enum value but not every bit pattern: value 3 matches no item
+  always_comb begin
+    y_enum = 4'h0;
+    unique0 case (e)
+      E0: y_enum = 4'h1;
+      E1: y_enum = 4'h2;
+      E2: y_enum = 4'h3;
+    endcase
+  end
+
+  always_comb begin
+    y_incompl = 4'h0;
+    case (sel_plain)
+      2'd0: y_incompl = 4'h1;
+      2'd1: y_incompl = 4'h2;
+      2'd2: y_incompl = 4'h3;
+    endcase
+  end
+
+  always @(posedge clk) begin
+`ifdef TEST_VERBOSE
+    $write("[%0t] cyc=%0d s3=%b %h %h %h %h %h %h %h %h %h\n", $time, cyc, s3, y_plain, y_casez,
+           y_casex, y_never, y_inside, y_overlap, y_unique, y_enum, y_incompl);
+`endif
+    `checkd(y_plain, 4'h1 << sel_plain);
+    `checkd(y_casez, (s3[2] ? 4'h1 : s3[1] ? 4'h2 : s3[0] ? 4'h3 : 4'h4));
+    `checkd(y_casex, (s3[0] ? 4'h1 : s3[1] ? 4'h2 : s3[2] ? 4'h3 : 4'h4));
+    `checkd(y_never, 4'(s3[1:0]) + 4'h1);
+    `checkd(y_inside, (s3[2] ? 4'h1 : s3[0] ? 4'h2 : 4'h3));
+    `checkd(y_overlap, (s3[0] ? 4'h1 : s3[2] ? 4'h2 : 4'h3));
+    `checkd(y_unique, 4'h4 - 4'(sel_plain));
+    `checkd(y_enum, (sel_plain == 2'd3 ? 4'h0 : 4'(sel_plain) + 4'h1));
+    `checkd(y_incompl, (sel_plain == 2'd3 ? 4'h0 : 4'(sel_plain) + 4'h1));
+    cyc <= cyc + 1;
+    if (cyc == 20) begin
+      $write("*-* All Finished *-*\n");
+      $finish;
+    end
+  end
+endmodule