Add --unused-regexp

bin/verilator

@@ -296,6 +296,7 @@ descriptions in the next sections for more information.
      -U<var>                    Undefine preprocessor define
     --unroll-count <loops>      Tune maximum loop iterations
     --unroll-stmts <stmts>      Tune maximum loop body size
+    --unused-regexp <regexp>    Tune UNUSED lint signals
      -V                         Verbose version and config
      -v <filename>              Verilog library
      -Werror-<message>          Convert warning to error
@@ -834,6 +835,12 @@ unrolled.  See also BLKLOOPINIT warning.
 Rarely needed.  Specifies the maximum number of statements in a loop for
 that loop to be unrolled. See also BLKLOOPINIT warning.
 
+=item --unused-regexp I<regexp>
+
+Rarely needed.  Specifies a simple regexp with * and ? that if a signal
+name matches will suppress the UNUSED warning.  Defaults to "*unused*".
+Setting it to "" disables matching.
+
 =item -V
 
 Shows the verbose version, including configuration information compiled
@@ -2705,12 +2712,30 @@ correctly.
 =item UNUSED
 
 Warns that the specified signal is never sinked.  Verilator is fairly
-liberal in the usage calculations; making a signal public, or accessing
+liberal in the usage calculations; making a signal public, a signal
-only a single array element marks the entire signal as used.
+matching --unused-regexp ("*unused*") or accessing only a single array
+element marks the entire signal as used.
 
 Disabled by default as this is a code style warning; it will simulate
 correctly.
 
+A recommended style for unused nets is to put at the bottom of a file code
+similar to the following:
+
+    wire _unused_ok = &{1'b0,
+    			sig_not_used_a,
+    			sig_not_used_yet_b,  // To be fixed
+    			1'b0};
+
+The reduction AND and constant zeros mean the net will always be zero, so
+won't use simulation time.  The redundant leading and trailing zeros avoid
+syntax errors if there are no signals between them.  The magic name
+"unused" (-unused-regexp) is recognized by Verilator and suppresses
+warnings; if using other lint tools, either teach to tool to ignore signals
+with "unused" in the name, or put the appropriate lint_off around the wire.
+Having unused signals in one place makes it easy to find what is unused,
+and reduces the number of lint_off pragmas, reducing bugs.
+
 =item VARHIDDEN
 
 Warns that a task, function, or begin/end block is declaring a variable by

src/V3Options.cpp

@@ -872,6 +872,9 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc, char
 	    else if ( !strcmp (sw, "-top-module") && (i+1)<argc ) {
 		shift; m_topModule = argv[i];
 	    }
+	    else if ( !strcmp (sw, "-unused-regexp") && (i+1)<argc ) {
+		shift; m_unusedRegexp = argv[i];
+	    }
 	    else if ( !strcmp (sw, "-x-assign") && (i+1)<argc) {
 		shift;
 		if (!strcmp (argv[i], "0")) { m_xAssign="0"; }
@@ -1083,6 +1086,7 @@ V3Options::V3Options() {
     m_makeDir = "obj_dir";
     m_bin = "";
     m_flags = "";
+    m_unusedRegexp = "*unused*";
     m_xAssign = "fast";
 
     m_language = V3LangCode::mostRecent();

src/V3Options.h

@@ -147,8 +147,9 @@ class V3Options {
     string	m_modPrefix;	// main switch: --mod-prefix
     string	m_pipeFilter;	// main switch: --pipe-filter
     string	m_prefix;	// main switch: --prefix
-    string	m_xAssign;	// main switch: --x-assign
     string	m_topModule;	// main switch: --top-module
+    string	m_unusedRegexp;	// main switch: --unused-regexp
+    string	m_xAssign;	// main switch: --x-assign
 
     // Consider moving m_language into FileLine, so can know language per-node
     V3LangCode	m_language;	// main switch: --language
@@ -266,6 +267,7 @@ class V3Options {
     string pipeFilter() const { return m_pipeFilter; }
     string prefix() const { return m_prefix; }
     string topModule() const { return m_topModule; }
+    string unusedRegexp() const { return m_unusedRegexp; }
     string xAssign() const { return m_xAssign; }
 
     const V3StringSet& cppFiles() const { return m_cppFiles; }

src/V3Undriven.cpp

@@ -133,6 +133,11 @@ public:
 	    }
 	}
     }
+    bool unusedMatch(AstVar* nodep) {
+	const char* regexpp = v3Global.opt.unusedRegexp().c_str();
+	if (!regexpp || !*regexpp) return false;
+	return V3Options::wildmatch(nodep->prettyName().c_str(), regexpp);
+    }
     void reportViolations() {
 	// Combine bits into overall state
 	AstVar* nodep = m_varp;
@@ -163,18 +168,22 @@ public:
 	    } else if (!anyD && !anyU) {
 		// UNDRIVEN is considered more serious - as is more likely a bug,
 		// thus undriven+unused bits get UNUSED warnings, as they're not as buggy.
-		nodep->v3warn(UNUSED, "Signal is not driven, nor used: "<<nodep->prettyName());
+		if (!unusedMatch(nodep)) {
+		    nodep->v3warn(UNUSED, "Signal is not driven, nor used: "<<nodep->prettyName());
+		}
 	    } else if (allD && !anyU) {
-		nodep->v3warn(UNUSED, "Signal is not used: "<<nodep->prettyName());
+		if (!unusedMatch(nodep)) {
+		    nodep->v3warn(UNUSED, "Signal is not used: "<<nodep->prettyName());
+		}
 	    } else if (!anyD && allU) {
 		nodep->v3warn(UNDRIVEN, "Signal is not driven: "<<nodep->prettyName());
 	    } else {
 		// Bits have different dispositions
-		if (anynotDU) {
+		if (anynotDU && !unusedMatch(nodep)) {
 		    nodep->v3warn(UNUSED, "Bits of signal are not driven, nor used: "<<nodep->prettyName()
 				  <<bitNames(BN_BOTH));
 		}
-		if (anyDnotU) {
+		if (anyDnotU && !unusedMatch(nodep)) {
 		    nodep->v3warn(UNUSED, "Bits of signal are not used: "<<nodep->prettyName()
 				  <<bitNames(BN_UNUSED));
 		}

test_regress/t/t_lint_unused_bad.v

@@ -22,6 +22,8 @@ module t (/*AUTOARG*/
    reg 	  sysused;
    initial $bboxed(sysused);
 
+   wire   _unused_ok;
+
 endmodule
 
 module sub;