Reduce max RSS usage (#3483)

By constant folding nodes earlier in V3Expand, we can save some max RSS on large designs.

src/V3Const.cpp

@@ -3583,7 +3583,11 @@ public:
     }
     virtual ~ConstVisitor() override {
         if (m_doCpp) {
-            V3Stats::addStat("Optimizations, Const bit op reduction", m_statBitOpReduction);
+            if (m_globalPass) {
+                V3Stats::addStat("Optimizations, Const bit op reduction", m_statBitOpReduction);
+            } else {
+                V3Stats::addStatSum("Optimizations, Const bit op reduction", m_statBitOpReduction);
+            }
         }
     }
 
@@ -3676,6 +3680,12 @@ AstNode* V3Const::constifyEdit(AstNode* nodep) {
     return nodep;
 }
 
+AstNode* V3Const::constifyEditCpp(AstNode* nodep) {
+    ConstVisitor visitor{ConstVisitor::PROC_CPP, /* globalPass: */ false};
+    nodep = visitor.mainAcceptEdit(nodep);
+    return nodep;
+}
+
 void V3Const::constifyAllLive(AstNetlist* nodep) {
     // Only call from Verilator.cpp, as it uses user#'s
     // This only pushes constants up, doesn't make any other edits

src/V3Const.h

@@ -39,6 +39,9 @@ public:
     static void constifyCpp(AstNetlist* nodep);
     // Only the current node and lower
     // Return new node that may have replaced nodep
+    static AstNode* constifyEditCpp(AstNode* nodep);
+    // Only the current node and lower
+    // Return new node that may have replaced nodep
     static AstNode* constifyEdit(AstNode* nodep);
     // Only the current node and lower, with special SenTree optimization
     // Return new node that may have replaced nodep

src/V3Expand.cpp

@@ -32,6 +32,7 @@
 #include "V3Expand.h"
 #include "V3Stats.h"
 #include "V3Ast.h"
+#include "V3Const.h"
 
 #include <algorithm>
 
@@ -160,6 +161,7 @@ private:
                            new AstShiftL{fl, llowp,
                                          new AstConst{fl, static_cast<uint32_t>(loffset)},
                                          VL_EDATASIZE}}};
+            newp = V3Const::constifyEditCpp(newp);
         } else {
             newp = llowp;
         }
@@ -520,8 +522,9 @@ private:
                             cleanmask.setMask(VL_BITBIT_E(destp->widthMin()));
                             newp = new AstAnd{lfl, newp, new AstConst{lfl, cleanmask}};
                         }
-
-                        addWordAssign(nodep, w, destp, new AstOr{lfl, oldvalp, newp});
+                        AstNode* const orp
+                            = V3Const::constifyEditCpp(new AstOr{lfl, oldvalp, newp});
+                        addWordAssign(nodep, w, destp, orp);
                     }
                 }
                 VL_DO_DANGLING(rhsp->deleteTree(), rhsp);
@@ -541,7 +544,8 @@ private:
                 AstNode* const shifted = new AstShiftL{
                     lfl, rhsp, new AstConst{lfl, static_cast<uint32_t>(lsb)}, destp->width()};
                 AstNode* const cleaned = new AstAnd{lfl, shifted, new AstConst{lfl, cleanmask}};
-                AstNode* const newp = new AstAssign{nfl, destp, new AstOr{lfl, oldvalp, cleaned}};
+                AstNode* const orp = V3Const::constifyEditCpp(new AstOr{lfl, oldvalp, cleaned});
+                AstNode* newp = new AstAssign{nfl, destp, orp};
                 insertBefore(nodep, newp);
             }
             return true;