Add --fno-slice to disable array assignment slicing (#5644).

Changes

@@ -24,6 +24,8 @@ Verilator 5.031 devel
 * Add `--no-std-package` as subset-alias of `--no-std` (#5607).
 * Add `lint_off --contents` in configuration files (#5606).
 * Add `--waiver-multiline` for context-sensitive `--waiver-output` (#5608).
+* Add `--fno-inline-funcs` to disable function inlining.
+* Add `--fno-slice` to disable array assignment slicing (#5644).
 * Add error on illegal enum base type (#3010). [Iztok Jeras]
 * Add error on `wait` with missing `.triggered` (#4457).
 * Add error when improperly storing to parameter (#5147). [Gökçe Aydos]
@@ -31,7 +33,6 @@ Verilator 5.031 devel
 * Add coverage point hierarchy to coverage reports (#5575) (#5576). [Andrew Nolte]
 * Add warning on global constraints (#5625). [Ryszard Rozak, Antmicro Ltd.]
 * Add error on `solve before` or soft constraints of `randc` variable.
-* Add `--fno-inline-funcs` to disable function inlining.
 * Improve concatenation performance (#5598) (#5599) (#5602). [Geza Lore]
 * Fix dotted reference in delay value (#2410).
 * Fix `function fork...join_none` regression with unknown type (#4449).

docs/guide/exe_verilator.rst

@@ -617,6 +617,8 @@ Summary:
 
 .. option:: -fno-reorder
 
+.. option:: -fno-slice
+
 .. option:: -fno-split
 
 .. option:: -fno-subst

src/V3Options.cpp

@@ -1332,6 +1332,7 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc,
     DECL_OPTION("-fmerge-const-pool", FOnOff, &m_fMergeConstPool);
     DECL_OPTION("-freloop", FOnOff, &m_fReloop);
     DECL_OPTION("-freorder", FOnOff, &m_fReorder);
+    DECL_OPTION("-fslice", FOnOff, &m_fSlice);
     DECL_OPTION("-fsplit", FOnOff, &m_fSplit);
     DECL_OPTION("-fsubst", FOnOff, &m_fSubst);
     DECL_OPTION("-fsubst-const", FOnOff, &m_fSubstConst);

src/V3Options.h

@@ -399,6 +399,7 @@ private:
     bool m_fMergeConstPool = true;  // main switch: -fno-merge-const-pool
     bool m_fReloop;      // main switch: -fno-reloop: reform loops
     bool m_fReorder;     // main switch: -fno-reorder: reorder assignments in blocks
+    bool m_fSlice = true;  // main switch: -fno-slice: array assignment slicing
     bool m_fSplit;       // main switch: -fno-split: always assignment splitting
     bool m_fSubst;       // main switch: -fno-subst: substitute expression temp values
     bool m_fSubstConst;  // main switch: -fno-subst-const: final constant substitution
@@ -696,6 +697,7 @@ public:
     bool fMergeConstPool() const { return m_fMergeConstPool; }
     bool fReloop() const { return m_fReloop; }
     bool fReorder() const { return m_fReorder; }
+    bool fSlice() const { return m_fSlice; }
     bool fSplit() const { return m_fSplit; }
     bool fSubst() const { return m_fSubst; }
     bool fSubstConst() const { return m_fSubstConst; }

src/V3Slice.cpp

@@ -39,6 +39,8 @@
 
 #include "V3Slice.h"
 
+#include "V3Stats.h"
+
 VL_DEFINE_DEBUG_FUNCTIONS;
 
 //*************************************************************************
@@ -54,6 +56,9 @@ class SliceVisitor final : public VNVisitor {
     //  AstInitItem::user2()        -> Corresponding first elemIdx
     const VNUser2InUse m_inuser2;
 
+    // STATE - across all visitors
+    VDouble0 m_statAssigns;  // Statistic tracking
+
     // STATE - for current visit position (use VL_RESTORER)
     AstNode* m_assignp = nullptr;  // Assignment we are under
     bool m_assignError = false;  // True if the current assign already has an error
@@ -218,6 +223,46 @@ class SliceVisitor final : public VNVisitor {
         return newp;
     }
 
+    bool assignOptimize(AstNodeAssign* nodep) {
+        // Return true if did optimization
+        AstNodeDType* const dtp = nodep->lhsp()->dtypep()->skipRefp();
+        AstNode* stp = nodep->rhsp();
+        const AstUnpackArrayDType* const arrayp = VN_CAST(dtp, UnpackArrayDType);
+        if (!arrayp) return false;
+        if (VN_IS(stp, CvtPackedToArray)) return false;
+
+        // Any isSc variables must be expanded regardless of --fno-slice
+        const bool hasSc
+            = nodep->exists([&](const AstVarRef* refp) -> bool { return refp->varp()->isSc(); });
+        if (!hasSc && !v3Global.opt.fSlice()) {
+            m_okInitArray = true;  // VL_RESTORER in visit(AstNodeAssign)
+            return false;
+        }
+
+        UINFO(4, "Slice optimizing " << nodep << endl);
+        ++m_statAssigns;
+
+        // Left and right could have different ascending/descending range,
+        // but #elements is common and all variables are realigned to start at zero
+        // Assign of an ascending range slice to a descending range one must reverse
+        // the elements
+        AstNodeAssign* newlistp = nullptr;
+        const int elements = arrayp->rangep()->elementsConst();
+        for (int elemIdx = 0; elemIdx < elements; ++elemIdx) {
+            AstNodeAssign* const newp
+                = nodep->cloneType(cloneAndSel(nodep->lhsp(), elements, elemIdx),
+                                   cloneAndSel(nodep->rhsp(), elements, elemIdx));
+            if (debug() >= 9) newp->dumpTree("-  new: ");
+            newlistp = AstNode::addNext(newlistp, newp);
+        }
+        if (debug() >= 9) nodep->dumpTree("-  Deslice-Dn: ");
+        nodep->replaceWith(newlistp);
+        VL_DO_DANGLING(nodep->deleteTree(), nodep);
+        // Normal edit iterator will now iterate on all of the expansion assignments
+        // This will potentially call this function again to resolve next level of slicing
+        return true;
+    }
+
     void visit(AstNodeAssign* nodep) override {
         // Called recursively on newly created assignments
         if (nodep->user1SetOnce()) return;  // Process once
@@ -225,34 +270,10 @@ class SliceVisitor final : public VNVisitor {
         if (debug() >= 9) nodep->dumpTree("-  Deslice-In: ");
         VL_RESTORER(m_assignError);
         VL_RESTORER(m_assignp);
+        VL_RESTORER(m_okInitArray);  // Set in assignOptimize
         m_assignError = false;
         m_assignp = nodep;
-        AstNodeDType* const dtp = nodep->lhsp()->dtypep()->skipRefp();
+        if (assignOptimize(nodep)) return;
-        AstNode* stp = nodep->rhsp();
-        if (const AstUnpackArrayDType* const arrayp = VN_CAST(dtp, UnpackArrayDType)) {
-            if (!VN_IS(stp, CvtPackedToArray)) {
-                // Left and right could have different ascending/descending range,
-                // but #elements is common and all variables are realigned to start at zero
-                // Assign of an ascending range slice to a descending range one must reverse
-                // the elements
-                AstNodeAssign* newlistp = nullptr;
-                const int elements = arrayp->rangep()->elementsConst();
-                for (int elemIdx = 0; elemIdx < elements; ++elemIdx) {
-                    AstNodeAssign* const newp
-                        = nodep->cloneType(cloneAndSel(nodep->lhsp(), elements, elemIdx),
-                                           cloneAndSel(nodep->rhsp(), elements, elemIdx));
-                    if (debug() >= 9) newp->dumpTree("-  new: ");
-                    newlistp = AstNode::addNext(newlistp, newp);
-                }
-                if (debug() >= 9) nodep->dumpTree("-  Deslice-Dn: ");
-                nodep->replaceWith(newlistp);
-                VL_DO_DANGLING(nodep->deleteTree(), nodep);
-                // Normal edit iterator will now iterate on all of the expansion assignments
-                // This will potentially call this function again to resolve next level of
-                // slicing
-                return;
-            }
-        }
         iterateChildren(nodep);
     }
 
@@ -337,7 +358,9 @@ class SliceVisitor final : public VNVisitor {
 public:
     // CONSTRUCTORS
     explicit SliceVisitor(AstNetlist* nodep) { iterate(nodep); }
-    ~SliceVisitor() override = default;
+    ~SliceVisitor() override {
+        V3Stats::addStat("Optimizations, Slice array assignments", m_statAssigns);
+    }
 };
 
 //######################################################################

test_regress/t/t_opt_slice.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2024 by Wilson Snyder. 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-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile(verilator_flags2=['--sc', '--stats'])
+
+test.file_grep(test.stats, r'Optimizations, Slice array assignments\s+(\d+)', 3)
+
+test.passes()

test_regress/t/t_opt_slice.v

@@ -0,0 +1,23 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2024 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t (/*AUTOARG*/
+   // Outputs
+   o1a2,
+   // Inputs
+   i1a2
+   );
+
+   input i1a2 [1:0];
+   output logic o1a2 [1:0];
+
+   always o1a2 = i1a2;
+
+   initial begin
+      $write("*-* All Finished *-*\n");
+      $finish;
+   end
+endmodule

test_regress/t/t_opt_slice_no.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2024 by Wilson Snyder. 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-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+test.top_filename = 't/t_opt_slice.v'
+
+test.compile(verilator_flags2=['--sc', '--stats', '-fno-slice'])
+
+test.file_grep(test.stats, r'Optimizations, Slice array assignments\s+(\d+)', 2)
+
+test.passes()