[llvm] Introduce DIExpression::foldConstantMath() (PR #71718)

[Adrian Prantl via llvm-commits]

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@@ -2011,6 +2010,347 @@ DIExpression::constantFold(const ConstantInt *CI) {
           ConstantInt::get(getContext(), NewInt)};
 }
 
+// Returns true if the Op is a DW_OP_constu.
+static bool isConstantVal(uint64_t Op) { return Op == dwarf::DW_OP_constu; }
+
+// Returns true if an operation and operand result in a No Op.
+static bool isNeutralElement(uint64_t Op, uint64_t Val) {
+  switch (Op) {
+  case dwarf::DW_OP_plus:
+  case dwarf::DW_OP_minus:
+  case dwarf::DW_OP_shl:
+  case dwarf::DW_OP_shr:
+    return Val == 0;
+  case dwarf::DW_OP_mul:
+  case dwarf::DW_OP_div:
+    return Val == 1;
+  default:
+    return false;
+  }
+}
+
+// Try to fold constant math operations and return the result if possible.
+static std::optional<uint64_t>
+foldOperationIfPossible(uint64_t Op, uint64_t Operand1, uint64_t Operand2) {
+  bool ResultOverflowed;
+  switch (Op) {
+  case dwarf::DW_OP_plus: {
+    auto Result = SaturatingAdd(Operand1, Operand2, &ResultOverflowed);
+    if (ResultOverflowed)
+      return std::nullopt;
+    return Result;
+  }
+  case dwarf::DW_OP_minus: {
+    if (Operand1 < Operand2)
+      return std::nullopt;
+    return Operand1 - Operand2;
+  }
+  case dwarf::DW_OP_shl: {
+    if (Operand2 > 64)
+      return std::nullopt;
+    return Operand1 << Operand2;
+  }
+  case dwarf::DW_OP_shr: {
+    if (Operand2 > 64)
+      return std::nullopt;
+    return Operand1 >> Operand2;
+  }
+  case dwarf::DW_OP_mul: {
+    auto Result = SaturatingMultiply(Operand1, Operand2, &ResultOverflowed);
+    if (ResultOverflowed)
+      return std::nullopt;
+    return Result;
+  }
+  case dwarf::DW_OP_div: {
+    if (Operand2)
+      return Operand1 / Operand2;
+    return std::nullopt;
+  }
+  default:
+    return std::nullopt;
+  }
+}
+
+// Returns true if the two operations are commutative and can be folded.
+static bool operationsAreFoldableAndCommutative(uint64_t Op1, uint64_t Op2) {
+  if (Op1 != Op2)
+    return false;
+  switch (Op1) {
+  case dwarf::DW_OP_plus:
+  case dwarf::DW_OP_mul:
+    return true;
+  default:
+    return false;
+  }
+}
+
+// Consume one operator and its operand(s).
+static void consumeOneOperator(DIExpressionCursor &Cursor, uint64_t &Loc,
+                               const DIExpression::ExprOperand &Op) {
+  Cursor.consume(1);
+  Loc = Loc + Op.getSize();
+}
+
+// Reset the Cursor to the beginning of the WorkingOps.
+static void startFromBeginning(uint64_t &Loc, DIExpressionCursor &Cursor,
+                               ArrayRef<uint64_t> WorkingOps) {
+  Cursor.assignNewExpr(WorkingOps);
+  Loc = 0;
+}
+
+// Move the Cursor to the position of an operation by starting from the
+// beginning and consuming the number of operations that came before.
+static void moveCursorToCurrentOp(uint64_t &Loc, DIExpressionCursor &Cursor,
+                                  ArrayRef<uint64_t> WorkingOps,
+                                  uint32_t NumOfOpsConsumed) {
+  Cursor.assignNewExpr(WorkingOps);
+  Cursor.consume(NumOfOpsConsumed);
+  assert(Cursor.peek()->getOp() == WorkingOps[Loc] &&
+         "Cursor position does not match position of iterator 'Loc' to "
+         "WorkingOps");
+}
+
+// This function will canonicalize:
+// 1. DW_OP_plus_uconst to DW_OP_constu <const-val> DW_OP_plus
+// 2. DW_OP_lit<n> to DW_OP_constu <n>
+static void canonicalizeDwarfOperations(SmallVectorImpl<uint64_t> &WorkingOps) {
+  DIExpressionCursor Cursor(WorkingOps);
+  uint64_t Loc = 0;
+  uint32_t NumOfOpsConsumed = 0;
+  while (Loc < WorkingOps.size()) {
+    auto Op = Cursor.peek();
+    // Expression has no operations, break.
+    if (!Op)
+      break;
+    auto OpRaw = Op->getOp();
+    auto OpArg = Op->getArg(0);
+
+    if (OpRaw >= dwarf::DW_OP_lit0 && OpRaw <= dwarf::DW_OP_lit31) {
+      WorkingOps[Loc] = dwarf::DW_OP_constu;
+      WorkingOps.insert(WorkingOps.begin() + Loc + 1,
----------------
adrian-prantl wrote:

Instead of doing these expensive insert operations, why not just build up a result on the fly return a new array?

https://github.com/llvm/llvm-project/pull/71718