[TableGen] Avoid evaluating RHS of a BinOp until short-circuit is complete (llvm#144021)

This patch adds an even more aggressive short-circuit on `!and` and
`!or` that completely avoids the evaluation of RHS operand until short
circuiting decisions are made.

Author: mshockwave

llvm/lib/TableGen/Record.cpp

@@ -1557,8 +1557,7 @@ const Init *BinOpInit::Fold(const Record *CurRec) const {
 }
 
 const Init *BinOpInit::resolveReferences(Resolver &R) const {
-  const Init *lhs = LHS->resolveReferences(R);
-  const Init *rhs = RHS->resolveReferences(R);
+  const Init *NewLHS = LHS->resolveReferences(R);
 
   unsigned Opc = getOpcode();
   if (Opc == AND || Opc == OR) {
@@ -1570,15 +1569,17 @@ const Init *BinOpInit::resolveReferences(Resolver &R) const {
     // limited version of short-circuit against all ones (`true` is casted
     // to 1 rather than all ones before we evaluate `!or`).
     if (const auto *LHSi = dyn_cast_or_null<IntInit>(
-            lhs->convertInitializerTo(IntRecTy::get(getRecordKeeper())))) {
+            NewLHS->convertInitializerTo(IntRecTy::get(getRecordKeeper())))) {
       if ((Opc == AND && !LHSi->getValue()) ||
           (Opc == OR && LHSi->getValue() == -1))
         return LHSi;
     }
   }
 
-  if (LHS != lhs || RHS != rhs)
-    return (BinOpInit::get(getOpcode(), lhs, rhs, getType()))
+  const Init *NewRHS = RHS->resolveReferences(R);
+
+  if (LHS != NewLHS || RHS != NewRHS)
+    return (BinOpInit::get(getOpcode(), NewLHS, NewRHS, getType()))
         ->Fold(R.getCurrentRecord());
   return this;
 }

llvm/test/TableGen/true-false.td

@@ -67,13 +67,18 @@ def rec7 {
   bits<3> flags = { true, false, true };
 }
 
-// `!and` and `!or` should be short-circuit such that `!tail` on empty list will never
-// be evaluated.
+// `!and` and `!or` should be short-circuited such that any of the `!head` or
+// `!tail` on empty list below will never be evaluated.
 // CHECK: def rec8
+// CHECK:   bit v = 0;
+// CHECK:   int v2 = -1;
 // CHECK:   list<int> newSeq = [];
 // CHECK:   list<int> newSeq2 = [];
 
 class Foo <list<int> seq = []> {
+  bit v = !and(false, !head(seq));
+  int v2 = !or(-1, !head(seq));
+
   bit unresolved = !ne(!find(NAME, "BAR"), -1);
   list<int> newSeq  = !if(!and(false, unresolved), !tail(seq), seq);
   list<int> newSeq2 = !if(!or(-1, unresolved), seq, !tail(seq));