Introduce DIExpression::foldConstantMath()

DIExpressions can get very long and have a lot of redundant operations.
This function uses simple pattern matching to fold constant math that
can be evaluated at compile time.

Author: rastogishubham

llvm/include/llvm/IR/DebugInfoMetadata.h

@@ -3121,6 +3121,11 @@ class DIExpression : public MDNode {
   /// expression and constant on failure.
   std::pair<DIExpression *, const ConstantInt *>
   constantFold(const ConstantInt *CI);
+
+  /// Try to shorten an expression with constant math operations that can be
+  /// evaluated at compile time. Returns a new expression on success, or the old
+  /// expression if there is nothing to be reduced.
+  DIExpression *foldConstantMath();
 };
 
 inline bool operator==(const DIExpression::FragmentInfo &A,

llvm/lib/IR/CMakeLists.txt

@@ -17,6 +17,7 @@ add_llvm_component_library(LLVMCore
   DataLayout.cpp
   DebugInfo.cpp
   DebugInfoMetadata.cpp
+  DIExpressionOptimizer.cpp
   DebugProgramInstruction.cpp
   DebugLoc.cpp
   DiagnosticHandler.cpp

llvm/lib/IR/DIExpressionOptimizer.cpp

@@ -0,0 +1,379 @@
+//===- DIExpressionOptimizer.cpp - Constant folding of DIExpressions ------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements functions to constant fold DIExpressions. Which were
+// declared in DIExpressionOptimizer.h
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/BinaryFormat/Dwarf.h"
+#include "llvm/IR/DebugInfoMetadata.h"
+
+using namespace llvm;
+
+/// Returns true if the Op is a DW_OP_constu.
+static std::optional<uint64_t> isConstantVal(DIExpression::ExprOperand Op) {
+  if (Op.getOp() == dwarf::DW_OP_constu)
+    return Op.getArg(0);
+  return std::nullopt;
+}
+
+/// 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 \p Const1 and \p Const2 by applying \p Operator and returning
+/// the result, if there is an overflow, return a std::nullopt.
+static std::optional<uint64_t>
+foldOperationIfPossible(uint64_t Const1, uint64_t Const2,
+                        dwarf::LocationAtom Operator) {
+
+  bool ResultOverflowed;
+  switch (Operator) {
+  case dwarf::DW_OP_plus: {
+    auto Result = SaturatingAdd(Const1, Const2, &ResultOverflowed);
+    if (ResultOverflowed)
+      return std::nullopt;
+    return Result;
+  }
+  case dwarf::DW_OP_minus: {
+    if (Const1 < Const2)
+      return std::nullopt;
+    return Const1 - Const2;
+  }
+  case dwarf::DW_OP_shl: {
+    if ((uint64_t)countl_zero(Const1) < Const2)
+      return std::nullopt;
+    return Const1 << Const2;
+  }
+  case dwarf::DW_OP_shr: {
+    if ((uint64_t)countr_zero(Const1) < Const2)
+      return std::nullopt;
+    return Const1 >> Const2;
+  }
+  case dwarf::DW_OP_mul: {
+    auto Result = SaturatingMultiply(Const1, Const2, &ResultOverflowed);
+    if (ResultOverflowed)
+      return std::nullopt;
+    return Result;
+  }
+  case dwarf::DW_OP_div: {
+    if (Const2)
+      return Const1 / Const2;
+    return std::nullopt;
+  }
+  default:
+    return std::nullopt;
+  }
+}
+
+/// Returns true if the two operations \p Operator1 and \p Operator2 are
+/// commutative and can be folded.
+static bool operationsAreFoldableAndCommutative(dwarf::LocationAtom Operator1,
+                                                dwarf::LocationAtom Operator2) {
+  return Operator1 == Operator2 &&
+         (Operator1 == dwarf::DW_OP_plus || Operator1 == dwarf::DW_OP_mul);
+}
+
+/// 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.
+void startFromBeginning(uint64_t &Loc, DIExpressionCursor &Cursor,
+                        ArrayRef<uint64_t> WorkingOps) {
+  Cursor.assignNewExpr(WorkingOps);
+  Loc = 0;
+}
+
+/// 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 SmallVector<uint64_t>
+canonicalizeDwarfOperations(ArrayRef<uint64_t> WorkingOps) {
+  DIExpressionCursor Cursor(WorkingOps);
+  uint64_t Loc = 0;
+  SmallVector<uint64_t> ResultOps;
+  while (Loc < WorkingOps.size()) {
+    auto Op = Cursor.peek();
+    /// Expression has no operations, break.
+    if (!Op)
+      break;
+    auto OpRaw = Op->getOp();
+
+    if (OpRaw >= dwarf::DW_OP_lit0 && OpRaw <= dwarf::DW_OP_lit31) {
+      ResultOps.push_back(dwarf::DW_OP_constu);
+      ResultOps.push_back(OpRaw - dwarf::DW_OP_lit0);
+      consumeOneOperator(Cursor, Loc, *Cursor.peek());
+      continue;
+    }
+    if (OpRaw == dwarf::DW_OP_plus_uconst) {
+      ResultOps.push_back(dwarf::DW_OP_constu);
+      ResultOps.push_back(Op->getArg(0));
+      ResultOps.push_back(dwarf::DW_OP_plus);
+      consumeOneOperator(Cursor, Loc, *Cursor.peek());
+      continue;
+    }
+    uint64_t PrevLoc = Loc;
+    consumeOneOperator(Cursor, Loc, *Cursor.peek());
+    ResultOps.append(WorkingOps.begin() + PrevLoc, WorkingOps.begin() + Loc);
+  }
+  return ResultOps;
+}
+
+/// This function will convert:
+/// 1. DW_OP_constu <const-val> DW_OP_plus to DW_OP_plus_uconst
+/// 2. DW_OP_constu, 0 to DW_OP_lit0
+static SmallVector<uint64_t>
+optimizeDwarfOperations(ArrayRef<uint64_t> WorkingOps) {
+  DIExpressionCursor Cursor(WorkingOps);
+  uint64_t Loc = 0;
+  SmallVector<uint64_t> ResultOps;
+  while (Loc < WorkingOps.size()) {
+    auto Op1 = Cursor.peek();
+    /// Expression has no operations, exit.
+    if (!Op1)
+      break;
+    auto Op1Raw = Op1->getOp();
+
+    if (Op1Raw == dwarf::DW_OP_constu && Op1->getArg(0) == 0) {
+      ResultOps.push_back(dwarf::DW_OP_lit0);
+      consumeOneOperator(Cursor, Loc, *Cursor.peek());
+      continue;
+    }
+
+    auto Op2 = Cursor.peekNext();
+    /// Expression has no more operations, copy into ResultOps and exit.
+    if (!Op2) {
+      uint64_t PrevLoc = Loc;
+      consumeOneOperator(Cursor, Loc, *Cursor.peek());
+      ResultOps.append(WorkingOps.begin() + PrevLoc, WorkingOps.begin() + Loc);
+      break;
+    }
+    auto Op2Raw = Op2->getOp();
+
+    if (Op1Raw == dwarf::DW_OP_constu && Op2Raw == dwarf::DW_OP_plus) {
+      ResultOps.push_back(dwarf::DW_OP_plus_uconst);
+      ResultOps.push_back(Op1->getArg(0));
+      consumeOneOperator(Cursor, Loc, *Cursor.peek());
+      consumeOneOperator(Cursor, Loc, *Cursor.peek());
+      continue;
+    }
+    uint64_t PrevLoc = Loc;
+    consumeOneOperator(Cursor, Loc, *Cursor.peek());
+    ResultOps.append(WorkingOps.begin() + PrevLoc, WorkingOps.begin() + Loc);
+  }
+  return ResultOps;
+}
+
+/// {DW_OP_constu, 0, DW_OP_[plus, minus, shl, shr]} -> {}
+/// {DW_OP_constu, 1, DW_OP_[mul, div]} -> {}
+static bool tryFoldNoOpMath(uint64_t Const1,
+                            ArrayRef<DIExpression::ExprOperand> Ops,
+                            uint64_t &Loc, DIExpressionCursor &Cursor,
+                            SmallVectorImpl<uint64_t> &WorkingOps) {
+
+  if (isNeutralElement(Ops[1].getOp(), Const1)) {
+    WorkingOps.erase(WorkingOps.begin() + Loc, WorkingOps.begin() + Loc + 3);
+    startFromBeginning(Loc, Cursor, WorkingOps);
+    return true;
+  }
+  return false;
+}
+
+/// {DW_OP_constu, Const1, DW_OP_constu, Const2, DW_OP_[plus,
+/// minus, mul, div, shl, shr] -> {DW_OP_constu, Const1 [+, -, *, /, <<, >>]
+/// Const2}
+static bool tryFoldConstants(uint64_t Const1,
+                             ArrayRef<DIExpression::ExprOperand> Ops,
+                             uint64_t &Loc, DIExpressionCursor &Cursor,
+                             SmallVectorImpl<uint64_t> &WorkingOps) {
+
+  auto Const2 = isConstantVal(Ops[1]);
+  if (!Const2)
+    return false;
+
+  auto Result = foldOperationIfPossible(
+      Const1, *Const2, static_cast<dwarf::LocationAtom>(Ops[2].getOp()));
+  if (!Result) {
+    consumeOneOperator(Cursor, Loc, Ops[0]);
+    return true;
+    }
+    WorkingOps.erase(WorkingOps.begin() + Loc + 2,
+                     WorkingOps.begin() + Loc + 5);
+    WorkingOps[Loc] = dwarf::DW_OP_constu;
+    WorkingOps[Loc + 1] = *Result;
+    startFromBeginning(Loc, Cursor, WorkingOps);
+    return true;
+}
+
+/// {DW_OP_constu, Const1, DW_OP_[plus, mul], DW_OP_constu, Const2,
+/// DW_OP_[plus, mul]} -> {DW_OP_constu, Const1 [+, *] Const2, DW_OP_[plus,
+/// mul]}
+static bool tryFoldCommutativeMath(uint64_t Const1,
+                                   ArrayRef<DIExpression::ExprOperand> Ops,
+                                   uint64_t &Loc, DIExpressionCursor &Cursor,
+                                   SmallVectorImpl<uint64_t> &WorkingOps) {
+
+  auto Const2 = isConstantVal(Ops[2]);
+  auto Operand1 = static_cast<dwarf::LocationAtom>(Ops[1].getOp());
+  auto Operand2 = static_cast<dwarf::LocationAtom>(Ops[3].getOp());
+
+  if (!Const2 || !operationsAreFoldableAndCommutative(Operand1, Operand2))
+    return false;
+
+  auto Result = foldOperationIfPossible(Const1, *Const2, Operand1);
+  if (!Result) {
+    consumeOneOperator(Cursor, Loc, Ops[0]);
+    return true;
+  }
+  WorkingOps.erase(WorkingOps.begin() + Loc + 3, WorkingOps.begin() + Loc + 6);
+  WorkingOps[Loc] = dwarf::DW_OP_constu;
+  WorkingOps[Loc + 1] = *Result;
+  startFromBeginning(Loc, Cursor, WorkingOps);
+  return true;
+}
+
+/// {DW_OP_constu, Const1, DW_OP_[plus, mul], DW_OP_LLVM_arg, Arg1,
+/// DW_OP_[plus, mul], DW_OP_constu, Const2, DW_OP_[plus, mul]} ->
+/// {DW_OP_constu, Const1 [+, *] Const2, DW_OP_[plus, mul], DW_OP_LLVM_arg,
+/// Arg1, DW_OP_[plus, mul]}
+static bool tryFoldCommutativeMathWithArgInBetween(
+    uint64_t Const1, ArrayRef<DIExpression::ExprOperand> Ops, uint64_t &Loc,
+    DIExpressionCursor &Cursor, SmallVectorImpl<uint64_t> &WorkingOps) {
+
+  auto Const2 = isConstantVal(Ops[4]);
+  auto Operand1 = static_cast<dwarf::LocationAtom>(Ops[1].getOp());
+  auto Operand2 = static_cast<dwarf::LocationAtom>(Ops[3].getOp());
+  auto Operand3 = static_cast<dwarf::LocationAtom>(Ops[5].getOp());
+
+  if (!Const2 || Ops[2].getOp() != dwarf::DW_OP_LLVM_arg ||
+      !operationsAreFoldableAndCommutative(Operand1, Operand2) ||
+      !operationsAreFoldableAndCommutative(Operand2, Operand3))
+    return false;
+
+  auto Result = foldOperationIfPossible(Const1, *Const2, Operand1);
+  if (!Result) {
+    consumeOneOperator(Cursor, Loc, Ops[0]);
+    return true;
+  }
+  WorkingOps.erase(WorkingOps.begin() + Loc + 6, WorkingOps.begin() + Loc + 9);
+  WorkingOps[Loc] = dwarf::DW_OP_constu;
+  WorkingOps[Loc + 1] = *Result;
+  startFromBeginning(Loc, Cursor, WorkingOps);
+  return true;
+}
+
+DIExpression *DIExpression::foldConstantMath() {
+
+  SmallVector<uint64_t, 8> WorkingOps(Elements.begin(), Elements.end());
+  uint64_t Loc = 0;
+  SmallVector<uint64_t> ResultOps = canonicalizeDwarfOperations(WorkingOps);
+  DIExpressionCursor Cursor(ResultOps);
+  SmallVector<DIExpression::ExprOperand, 8> Ops;
+
+  // Iterate over all Operations in a DIExpression to match the smallest pattern
+  // that can be folded.
+  while (Loc < ResultOps.size()) {
+    Ops.clear();
+
+    auto Op = Cursor.peek();
+    // Expression has no operations, exit.
+    if (!Op)
+      break;
+
+    auto Const1 = isConstantVal(*Op);
+
+    if (!Const1) {
+      // Early exit, all of the following patterns start with a constant value.
+      consumeOneOperator(Cursor, Loc, *Op);
+      continue;
+    }
+
+    Ops.push_back(*Op);
+
+    Op = Cursor.peekNext();
+    // All following patterns require at least 2 Operations, exit.
+    if (!Op)
+      break;
+
+    Ops.push_back(*Op);
+
+    // Try to fold a constant no-op, such as {+ 0}
+    if (tryFoldNoOpMath(*Const1, Ops, Loc, Cursor, ResultOps))
+      continue;
+
+    Op = Cursor.peekNextN(2);
+    // Op[1] could still match a pattern, skip iteration.
+    if (!Op) {
+      consumeOneOperator(Cursor, Loc, Ops[0]);
+      continue;
+    }
+
+    Ops.push_back(*Op);
+
+    // Try to fold a pattern of two constants such as {C1 + C2}.
+    if (tryFoldConstants(*Const1, Ops, Loc, Cursor, ResultOps))
+      continue;
+
+    Op = Cursor.peekNextN(3);
+    // Op[1] and Op[2] could still match a pattern, skip iteration.
+    if (!Op) {
+      consumeOneOperator(Cursor, Loc, Ops[0]);
+      continue;
+    }
+
+    Ops.push_back(*Op);
+
+    // Try to fold commutative constant math, such as {C1 + C2 +}.
+    if (tryFoldCommutativeMath(*Const1, Ops, Loc, Cursor, ResultOps))
+      continue;
+
+    Op = Cursor.peekNextN(4);
+    if (!Op) {
+      consumeOneOperator(Cursor, Loc, Ops[0]);
+      continue;
+    }
+
+    Ops.push_back(*Op);
+    Op = Cursor.peekNextN(5);
+    if (!Op) {
+      consumeOneOperator(Cursor, Loc, Ops[0]);
+      continue;
+    }
+
+    Ops.push_back(*Op);
+
+    // Try to fold commutative constant math with an LLVM_Arg in between, such
+    // as {C1 + Arg + C2 +}.
+    if (tryFoldCommutativeMathWithArgInBetween(*Const1, Ops, Loc, Cursor,
+                                               ResultOps))
+      continue;
+
+    consumeOneOperator(Cursor, Loc, Ops[0]);
+  }
+  ResultOps = optimizeDwarfOperations(ResultOps);
+  auto *Result = DIExpression::get(getContext(), ResultOps);
+  assert(Result->isValid() && "concatenated expression is not valid");
+  return Result;
+}