Fix slow compilation of generated sampled value code (#6652)

For handling $past and similar functions, we used to collect sampled
values of variables at the beginning of the main _eval function. If we
have many of these, this can grow _eval very large which can make C++
compilation very slow. Apply usual fix of emitting the necessary code in
a separate function and then splitting it based on size.

src/V3Clock.cpp

@@ -24,6 +24,7 @@
 #include "V3Clock.h"
 
 #include "V3Const.h"
+#include "V3Sched.h"
 
 VL_DEFINE_DEBUG_FUNCTIONS;
 
@@ -62,7 +63,7 @@ class ClockVisitor final : public VNVisitor {
     // NODE STATE
 
     // STATE
-    AstCFunc* const m_evalp = nullptr;  // The '_eval' function
+    AstCFunc* m_sampleCFuncp = nullptr;  // The CFunc to populate with sampled value assignments
 
     // VISITORS
     void visit(AstCoverToggle* nodep) override {
@@ -98,29 +99,36 @@ class ClockVisitor final : public VNVisitor {
     //========== Move sampled assignments
     void visit(AstVarScope* nodep) override {
         AstVar* const varp = nodep->varp();
-        if (varp->valuep() && varp->name().substr(0, strlen("__Vsampled")) == "__Vsampled") {
+        if (!varp->valuep()) return;
+        if (!VString::startsWith(varp->name(), "__Vsampled")) return;
+
+        // Create the containing function on first encounter
+        if (!m_sampleCFuncp) {
+            m_sampleCFuncp = V3Sched::util::makeSubFunction(v3Global.rootp(), "_sample", false);
+        }
+
         FileLine* const flp = nodep->fileline();
         AstNodeExpr* const rhsp = VN_AS(varp->valuep()->unlinkFrBack(), NodeExpr);
         AstVarRef* const lhsp = new AstVarRef{flp, nodep, VAccess::WRITE};
-            AstAssign* const assignp = new AstAssign{flp, lhsp, rhsp};
-            if (AstNode* const stmtsp = m_evalp->stmtsp()) {
-                stmtsp->addHereThisAsNext(assignp);
-            } else {
-                m_evalp->addStmtsp(assignp);
-            }
+        m_sampleCFuncp->addStmtsp(new AstAssign{flp, lhsp, rhsp});
         varp->direction(VDirection::NONE);  // Restore defaults
         varp->primaryIO(false);
     }
-    }
 
     //--------------------
     void visit(AstNode* nodep) override { iterateChildren(nodep); }
 
 public:
     // CONSTRUCTORS
-    explicit ClockVisitor(AstNetlist* netlistp)
-        : m_evalp{netlistp->evalp()} {
+    explicit ClockVisitor(AstNetlist* netlistp) {
         iterate(netlistp);
+        // If we need a sample function, call it at the begining of eval
+        if (m_sampleCFuncp) {
+            V3Sched::util::splitCheck(m_sampleCFuncp);
+            AstCCall* const callp = new AstCCall{m_sampleCFuncp->fileline(), m_sampleCFuncp};
+            callp->dtypeSetVoid();
+            netlistp->evalp()->stmtsp()->addHereThisAsNext(callp->makeStmt());
+        }
     }
     ~ClockVisitor() override = default;
 };

test_regress/t/t_flag_csplit.v

@@ -39,6 +39,7 @@ module t (/*AUTOARG*/
          $write("[%0t] cyc==%0d  sum=%x\n", $time, cyc, w[CNT]);
 `endif
          if (w[CNT] !== `EXPECTED_SUM) $stop;
+         $display("cyc: %0d $past(cyc): %0d", cyc, $past(cyc));
          $write("*-* All Finished *-*\n");
          $finish;
       end
@@ -55,7 +56,7 @@ module sub (input clk, input [31:0] i, output [31:0] z);
    assign z = z_tmp;
 
    always @(posedge z_tmp == 32'b11) begin
-     $display("%m z_tmp[0]: %0d", z_tmp);
+     $display("%m z_tmp: %0d, $past(z_tmp): $0d", z_tmp, $past(z_tmp));
    end
 
 endmodule

test_regress/t/t_flag_csplit_groups.py

@@ -125,7 +125,7 @@ test.file_grep_not(test.obj_dir + "/" + test.vm_prefix + "_classes.mk", "vm_clas
 test.file_grep_not(test.obj_dir + "/" + test.vm_prefix + "_classes.mk", "vm_classes_2")
 
 # Check combine count
-test.file_grep(test.stats, r'Node count, CFILE + (\d+)', (219 if test.vltmt else 205))
+test.file_grep(test.stats, r'Node count, CFILE + (\d+)', (239 if test.vltmt else 222))
 test.file_grep(test.stats, r'Makefile targets, VM_CLASSES_FAST + (\d+)', 2)
 test.file_grep(test.stats, r'Makefile targets, VM_CLASSES_SLOW + (\d+)', 2)