Fix make jobserver with submakes (#3758).

Changes

@@ -27,6 +27,7 @@ Verilator 5.003 devel
 * Fix return type of $countbits functions to int (#3725). [Ryszard Rozak, Antmicro Ltd]
 * Fix missing UNUSED warnings with --coverage (#3736). [alejandro-castro-ortegon]
 * Fix tracing parameters overridden with -G (#3723). [Iztok Jeras]
+* Fix make jobserver with submakes (#3758). [Gus Smith]
 * Fix wait 0.
 * Fix comparing ranged slices of unpacked arrays.
 * Fix empty string literals converting to string types (#3774). [miree]

docs/guide/environment.rst

@@ -20,6 +20,12 @@ associated programs.
    this variable to launch GNU make.  If this variable is not specified,
    "make" is used.
 
+.. option:: MAKEFLAGS
+
+   Flags created by :command:`make` to pass to submakes. Verilator searches
+   this variable to determine if a jobserver is used; see
+   :vlopt:`--build-jobs`.
+
 .. option:: OBJCACHE
 
    Optionally specifies a caching or distribution program to place in front

docs/guide/exe_verilator.rst

@@ -144,6 +144,10 @@ Summary:
    be a positive integer specifying the maximum number of parallel build
    jobs.
 
+   This forms the :command:`make` option ``-j`` value, unless the
+   :option:`MAKEFLAGS` environment variable contains ``-jobserver-auth``,
+   in which case Verilator assumes that make's jobserver is being used.
+
    See also :vlopt:`-j`.
 
 .. option:: --cc

src/V3Options.cpp

@@ -624,6 +624,10 @@ string V3Options::getenvMAKE() { return V3Os::getenvStr("MAKE", "gmake"); }
 string V3Options::getenvMAKE() { return V3Os::getenvStr("MAKE", "make"); }
 #endif
 
+string V3Options::getenvMAKEFLAGS() {  //
+    return V3Os::getenvStr("MAKEFLAGS", "");
+}
+
 string V3Options::getenvPERL() {  //
     return V3Os::getenvStr("PERL", "perl");
 }

src/V3Options.h

@@ -660,6 +660,7 @@ public:
     // Also add to V3Options::showVersion()
     static string getenvBuiltins(const string& var);
     static string getenvMAKE();
+    static string getenvMAKEFLAGS();
     static string getenvPERL();
     static string getenvSYSTEMC();
     static string getenvSYSTEMC_ARCH();

src/Verilator.cpp

@@ -686,7 +686,10 @@ static string buildMakeCmd(const string& makefile, const string& target) {
     cmd << v3Global.opt.getenvMAKE();
     cmd << " -C " << v3Global.opt.makeDir();
     cmd << " -f " << makefile;
-    if (jobs > 0) cmd << " -j " << jobs;
+    // Unless using make's jobserver, do a -j
+    if (v3Global.opt.getenvMAKEFLAGS().find("-jobserver-auth") == string::npos) {
+        if (jobs > 0) cmd << " -j " << jobs;
+    }
     for (const string& flag : makeFlags) cmd << ' ' << flag;
     if (!target.empty()) cmd << ' ' << target;