Change zero replication width error to ZEROREPL warning (#4753) (#4762)

docs/CONTRIBUTORS

@@ -134,6 +134,7 @@ Oleh Maksymenko
 Patrick Stewart
 Paul Wright
 Pawel Sagan
+Pengcheng Xu
 Peter Debacker
 Peter Horvath
 Peter Monsson

docs/guide/warnings.rst

@@ -2131,3 +2131,46 @@ List Of Warnings
    Inactive region. Such processes do get resumed in the same time slot
    somewhere in the Active region. Issued only if Verilator is run with the
    :vlopt:`--timing` option.
+
+.. option:: ZEROREPL
+
+   Warns that zero is used as the replication value in the replication
+   operator. This is specified as an error by IEEE 1800-2017 11.4.12.1.
+
+   Faulty example:
+
+   .. code-block:: sv
+      :linenos:
+      :emphasize-lines: 5
+
+         module dut
+            #(parameter int MY_PARAM = 0);
+           reg [7:0] data;
+           always @* begin
+             data = {MY_PARAM{1'b1}}; //<--- WARNING
+           end
+         endmodule
+
+   Results in the following error:
+
+   .. code-block::
+
+       %Error-ZEROREPL: test.v:5:22: Replication value of 0 is only legal under a concatenation (IEEE 1800-2017 11.4.12.1)
+
+   Note that in some cases, this warning may be false, when a condition
+   upstream or downstream of the access means the zero replication will
+   never execute or be used.
+
+   Repaired example:
+
+   .. code-block:: sv
+      :linenos:
+      :emphasize-lines: 2
+
+         module dut
+            #(parameter int MY_PARAM = 1); //<--- REPAIRED
+           reg [7:0] data;
+           always @* begin
+             data = {MY_PARAM{1'b1}};
+           end
+         endmodule

src/V3Error.h

@@ -165,6 +165,7 @@ public:
         WIDTHTRUNC,     // Width mismatch- lhs < rhs
         WIDTHXZEXPAND,  // Width mismatch- lhs > rhs xz filled
         ZERODLY,        // #0 delay
+        ZEROREPL,       // Replication width of zero
         _ENUM_MAX
         // ***Add new elements below also***
     };
@@ -210,7 +211,7 @@ public:
             "UNDRIVEN", "UNOPT", "UNOPTFLAT", "UNOPTTHREADS",
             "UNPACKED", "UNSIGNED", "UNUSEDGENVAR", "UNUSEDPARAM", "UNUSEDSIGNAL",
             "USERERROR", "USERFATAL", "USERINFO", "USERWARN",
-            "VARHIDDEN", "WAITCONST", "WIDTH", "WIDTHCONCAT",  "WIDTHEXPAND", "WIDTHTRUNC", "WIDTHXZEXPAND", "ZERODLY",
+            "VARHIDDEN", "WAITCONST", "WIDTH", "WIDTHCONCAT",  "WIDTHEXPAND", "WIDTHTRUNC", "WIDTHXZEXPAND", "ZERODLY", "ZEROREPL",
             " MAX"
         };
         // clang-format on
@@ -228,7 +229,7 @@ public:
         return (m_e == ASSIGNIN || m_e == BADSTDPRAGMA || m_e == BLKANDNBLK || m_e == BLKLOOPINIT
                 || m_e == CONTASSREG || m_e == ENCAPSULATED || m_e == ENDLABEL || m_e == ENUMVALUE
                 || m_e == IMPURE || m_e == PINNOTFOUND || m_e == PKGNODECL
-                || m_e == PROCASSWIRE  // Says IEEE
+                || m_e == PROCASSWIRE || m_e == ZEROREPL  // Says IEEE
         );
     }
     // Warnings to mention manual

src/V3Width.cpp

@@ -781,7 +781,7 @@ class WidthVisitor final : public VNVisitor {
                 if (!constp) nodep->v3error("Replication value isn't a constant.");
                 if (times == 0
                     && !VN_IS(nodep->backp(), Concat)) {  // Concat Visitor will clean it up.
-                    nodep->v3error("Replication value of 0 is only legal under a concatenation"
+                    nodep->v3warn(ZEROREPL, "Replication value of 0 is only legal under a concatenation"
                                    " (IEEE 1800-2017 11.4.12.1)");
                     times = 1;  // Set to 1, so we can continue looking for errors
                 }

test_regress/t/t_math_repl_bad.out

@@ -1,12 +1,12 @@
-%Error: t/t_math_repl_bad.v:12:14: Replication value of 0 is only legal under a concatenation (IEEE 1800-2017 11.4.12.1)
+%Error-ZEROREPL: t/t_math_repl_bad.v:12:14: Replication value of 0 is only legal under a concatenation (IEEE 1800-2017 11.4.12.1)
                                           : ... note: In instance 't'
    12 |       o = {0 {1'b1}};   
       |              ^
+                 ... For error description see https://verilator.org/warn/ZEROREPL?v=latest
 %Warning-WIDTHEXPAND: t/t_math_repl_bad.v:12:9: Operator ASSIGN expects 32 bits on the Assign RHS, but Assign RHS's REPLICATE generates 1 bits.
                                               : ... note: In instance 't'
    12 |       o = {0 {1'b1}};   
       |         ^
-                      ... For warning description see https://verilator.org/warn/WIDTHEXPAND?v=latest
                       ... Use "/* verilator lint_off WIDTHEXPAND */" and lint_on around source to disable this message.
 %Error: t/t_math_repl_bad.v:13:43: Replication value isn't a constant.
                                  : ... note: In instance 't'