Support soft unions (#5912) (#5932)

docs/CONTRIBUTORS

@@ -196,6 +196,7 @@ Ricardo Barbedo
 Richard Myers
 Risto Pejašinović
 Robert Balas
+Robin Heinemann
 Rupert Swarbrick
 Ryan Ziegler
 Ryszard Rozak

src/V3AstNodeDType.h

@@ -1388,13 +1388,20 @@ public:
     string verilogKwd() const override { return "struct"; }
 };
 class AstUnionDType final : public AstNodeUOrStructDType {
+    bool m_isSoft;  // Is a "union soft"
+
 public:
     // UNSUP: bool isTagged;
     // VSigning below is mispurposed to indicate if packed or not
-    AstUnionDType(FileLine* fl, VSigning numericUnpack)
-        : ASTGEN_SUPER_UnionDType(fl, numericUnpack) {}
+    // isSoft implies packed
+    AstUnionDType(FileLine* fl, bool isSoft, VSigning numericUnpack)
+        : ASTGEN_SUPER_UnionDType(fl, numericUnpack)
+        , m_isSoft(isSoft) {
+        packed(packed() | m_isSoft);
+    }
     ASTGEN_MEMBERS_AstUnionDType;
     string verilogKwd() const override { return "union"; }
+    bool isSoft() const { return m_isSoft; }
 };
 
 #endif  // Guard

src/V3Width.cpp

@@ -2882,10 +2882,14 @@ class WidthVisitor final : public VNVisitor {
                 pushDeletep(itemp->valuep()->unlinkFrBack());
             }
         }
+        const bool isHardPackedUnion
+            = nodep->packed() && VN_IS(nodep, UnionDType) && !VN_CAST(nodep, UnionDType)->isSoft();
+
         // Determine bit assignments and width
         if (VN_IS(nodep, UnionDType) || nodep->packed()) {
             int lsb = 0;
             int width = 0;
+            bool first = true;
             // Report errors on first member first
             AstMemberDType* itemp;
             // MSB is first, so loop backwards
@@ -2895,11 +2899,17 @@ class WidthVisitor final : public VNVisitor {
                 if (itemp->isFourstate()) nodep->isFourstate(true);
                 itemp->lsb(lsb);
                 if (VN_IS(nodep, UnionDType)) {
-                    width = std::max(width, itemp->width());
+                    const int itemWidth = itemp->width();
+                    if (!first && isHardPackedUnion && itemWidth != width) {
+                        itemp->v3error("Hard packed union members must have equal size "
+                                       "(IEEE 1800-2023 7.3.1)");
+                    }
+                    width = std::max(width, itemWidth);
                 } else {
                     lsb += itemp->width();
                     width += itemp->width();
                 }
+                first = false;
             }
             nodep->widthForce(width, width);  // Signing stays as-is, as parsed from declaration
         } else {

src/verilog.y

@@ -2304,7 +2304,7 @@ struct_unionDecl<nodeUOrStructDTypep>:  // IEEE: part of data_type
         /*cont*/    struct_union_memberListEnd
                         { $$ = $<nodeUOrStructDTypep>4; $$->addMembersp($5); SYMP->popScope($$); }
         |       yUNION taggedSoftE packedSigningE '{'
-        /*mid*/         { $<nodeUOrStructDTypep>$ = new AstUnionDType{$1, $3}; SYMP->pushNew($<nodeUOrStructDTypep>$); }
+        /*mid*/         { $<nodeUOrStructDTypep>$ = new AstUnionDType{$1, $2, $3}; SYMP->pushNew($<nodeUOrStructDTypep>$); }
         /*cont*/    struct_union_memberListEnd
                         { $$ = $<nodeUOrStructDTypep>5; $$->addMembersp($6); SYMP->popScope($$); }
         ;
@@ -2440,9 +2440,9 @@ random_qualifier<qualifiers>:   // ==IEEE: random_qualifier
         |       yRANDC                                  { $$ = VMemberQualifiers::none(); $$.m_randc = true; }
         ;
 
-taggedSoftE:
-                /*empty*/                               { }
-        |       ySOFT                                   { BBUNSUP($<fl>1, "Unsupported: 'union soft'"); }
+taggedSoftE<cbool>:
+                /*empty*/                               { $$ = false; }
+        |       ySOFT                                   { $$ = true; }
         //UNSUP yTAGGED                                 { UNSUP }
         ;
 

test_regress/t/t_dpi_result_type_bad.v

@@ -27,10 +27,10 @@ module t_dpi_result_type_bad;
    typedef struct      packed { bit [63:0] x; bit [63:0] y; } struct_2_state_128;
 
    // 2-state packed unions of width > 32
-   typedef union       packed { bit [ 32:0] x; bit y; } union_2_state_33;
-   typedef union       packed { bit [ 63:0] x; bit y; } union_2_state_64;
-   typedef union       packed { bit [ 64:0] x; bit y; } union_2_state_65;
-   typedef union       packed { bit [127:0] x; bit y; } union_2_state_128;
+   typedef union       packed { bit [ 32:0] x; bit [ 32:0] y; } union_2_state_33;
+   typedef union       packed { bit [ 63:0] x; bit [ 63:0] y; } union_2_state_64;
+   typedef union       packed { bit [ 64:0] x; bit [ 64:0] y; } union_2_state_65;
+   typedef union       packed { bit [127:0] x; bit [127:0] y; } union_2_state_128;
 
    // 4-state packed arrays of any size
    typedef logic [  0:0] array_4_state_1_t;
@@ -59,17 +59,17 @@ module t_dpi_result_type_bad;
    typedef struct        packed { logic [63:0] x; bit [63:0] y; } struct_4_state_128;
 
    // 4-state packed unions of any size
-   typedef union         packed { logic [  0:0] x; bit y; } union_4_state_1;
-   typedef union         packed { logic [  1:0] x; bit y; } union_4_state_2;
-   typedef union         packed { logic [  7:0] x; bit y; } union_4_state_8;
-   typedef union         packed { logic [  8:0] x; bit y; } union_4_state_9;
-   typedef union         packed { logic [ 15:0] x; bit y; } union_4_state_16;
-   typedef union         packed { logic [ 16:0] x; bit y; } union_4_state_17;
-   typedef union         packed { logic [ 31:0] x; bit y; } union_4_state_32;
-   typedef union         packed { logic [ 32:0] x; bit y; } union_4_state_33;
-   typedef union         packed { logic [ 63:0] x; bit y; } union_4_state_64;
-   typedef union         packed { logic [ 64:0] x; bit y; } union_4_state_65;
-   typedef union         packed { logic [127:0] x; bit y; } union_4_state_128;
+   typedef union         packed { logic [  0:0] x; bit [  0:0] y; } union_4_state_1;
+   typedef union         packed { logic [  1:0] x; bit [  1:0] y; } union_4_state_2;
+   typedef union         packed { logic [  7:0] x; bit [  7:0] y; } union_4_state_8;
+   typedef union         packed { logic [  8:0] x; bit [  8:0] y; } union_4_state_9;
+   typedef union         packed { logic [ 15:0] x; bit [ 15:0] y; } union_4_state_16;
+   typedef union         packed { logic [ 16:0] x; bit [ 16:0] y; } union_4_state_17;
+   typedef union         packed { logic [ 31:0] x; bit [ 31:0] y; } union_4_state_32;
+   typedef union         packed { logic [ 32:0] x; bit [ 32:0] y; } union_4_state_33;
+   typedef union         packed { logic [ 63:0] x; bit [ 63:0] y; } union_4_state_64;
+   typedef union         packed { logic [ 64:0] x; bit [ 64:0] y; } union_4_state_65;
+   typedef union         packed { logic [127:0] x; bit [127:0] y; } union_4_state_128;
 
    //======================================================================
    // Imports

test_regress/t/t_export_packed_struct.v

@@ -6,9 +6,9 @@
 
 typedef logic [5:0] udata6_t;
 
-typedef union packed {
-  udata6_t    a;
-  logic [2:0] b;
+typedef union soft packed {
+    udata6_t      a;
+    logic [2 : 0] b;
 } sub_t;
 
 typedef struct packed {

test_regress/t/t_export_packed_struct2.v

@@ -6,10 +6,10 @@
 
 // Packed struct in package
 package TEST_TYPES;
-  typedef union packed {
-    logic [64:0] a;
-    logic [2:0]  b;
-  } sub_t;
+    typedef union soft packed {
+        logic [64 : 0] a;
+        logic [2 : 0]  b;
+    } sub_t;
   typedef struct packed {
     struct packed {  // Anonymous packed struct
       logic a;

test_regress/t/t_union_hard_bad.out

@@ -0,0 +1,6 @@
+%Error: t/t_union_hard_bad.v:11:21: Hard packed union members must have equal size (IEEE 1800-2023 7.3.1)
+                                  : ... note: In instance 't'
+   11 |         bit [7 : 0] val1;
+      |                     ^~~~
+        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+%Error: Exiting due to

test_regress/t/t_union_hard_bad.py

@@ -0,0 +1,16 @@
+#!/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('vlt')
+
+test.lint(fails=True, expect_filename=test.golden_filename)
+
+test.passes()

test_regress/t/t_union_hard_bad.v

@@ -0,0 +1,25 @@
+// 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*/);
+
+    union packed {
+        bit [7 : 0] val1;
+        bit [3 : 0] val2;
+    } u;
+
+    initial begin
+        u.val1 = 8'h7c;
+        if(u.val1 != 8'h7c) $stop;
+        u.val2 = 4'h6;
+        if(u.val2 != 4'h6) $stop;
+        $display("%p", u);
+        if(u.val1 != 8'h76) $stop;
+        $write("*-* All Finished *-*\n");
+        $finish;
+    end
+endmodule

test_regress/t/t_union_soft.py

@@ -11,6 +11,8 @@ import vltest_bootstrap
 
 test.scenarios('vlt')
 
-test.lint(fails=True, expect_filename=test.golden_filename)
+test.compile()
+
+test.execute()
 
 test.passes()

test_regress/t/t_union_soft.v

@@ -11,13 +11,24 @@ module t(/*AUTOARG*/);
       bit [3:0] val2;
    } u;
 
+   union soft packed {
+       bit [7 : 0] val1;
+       bit [3 : 0] val2;
+   } u2;
+
    initial begin
       u.val1 = 8'h7c;
       if (u.val1 != 8'h7c) $stop;
       u.val2 = 4'h6;
       if (u.val2 != 4'h6) $stop;
       $display("%p", u);
-      if (u.ual1 != 8'h76) $stop;
+      if(u.val1 != 8'h76) $stop;
+      u2.val1 = 8'h7c;
+      if(u2.val1 != 8'h7c) $stop;
+      u2.val2 = 4'h6;
+      if(u2.val2 != 4'h6) $stop;
+      $display("%p", u2);
+      if(u2.val1 != 8'h76) $stop;
       $write("*-* All Finished *-*\n");
       $finish;
    end