[BOLT] Introduce helpers to match MCInsts one at a time (NFC)

Introduce matchInst helper function to capture and/or match the operands
of MCInst. Unlike the existing `MCPlusBuilder::MCInstMatcher` machinery,
matchInst is intended for the use cases when precise control over the
instruction order is required. For example, when validating PtrAuth
hardening, all registers are usually considered unsafe after a function
call, even though callee-saved registers should preserve their old
values *under normal operation*.

Author: atrosinenko

bolt/include/bolt/Core/MCInstUtils.h

@@ -162,6 +162,134 @@ static inline raw_ostream &operator<<(raw_ostream &OS,
   return Ref.print(OS);
 }
 
+/// Instruction-matching helpers operating on a single instruction at a time.
+///
+/// Unlike MCPlusBuilder::MCInstMatcher, this matchInst() function focuses on
+/// the cases where a precise control over the instruction order is important:
+///
+///     // Bring the short names into the local scope:
+///     using namespace MCInstMatcher;
+///     // Declare the registers to capture:
+///     Reg Xn, Xm;
+///     // Capture the 0th and 1st operands, match the 2nd operand against the
+///     // just captured Xm register, match the 3rd operand against literal 0:
+///     if (!matchInst(MaybeAdd, AArch64::ADDXrs, Xm, Xn, Xm, Imm(0))
+///       return AArch64::NoRegister;
+///     // Match the 0th operand against Xm:
+///     if (!matchInst(MaybeBr, AArch64::BR, Xm))
+///       return AArch64::NoRegister;
+///     // Return the matched register:
+///     return Xm.get();
+namespace MCInstMatcher {
+
+// The base class to match an operand of type T.
+//
+// The subclasses of OpMatcher are intended to be allocated on the stack and
+// to only be used by passing them to matchInst() and by calling their get()
+// function, thus the peculiar `mutable` specifiers: to make the calling code
+// compact and readable, the templated matchInst() function has to accept both
+// long-lived Imm/Reg wrappers declared as local variables (intended to capture
+// the first operand's value and match the subsequent operands, whether inside
+// a single instruction or across multiple instructions), as well as temporary
+// wrappers around literal values to match, f.e. Imm(42) or Reg(AArch64::XZR).
+template <typename T> class OpMatcher {
+  mutable std::optional<T> Value;
+  mutable std::optional<T> SavedValue;
+
+  // Remember/restore the last Value - to be called by matchInst.
+  void remember() const { SavedValue = Value; }
+  void restore() const { Value = SavedValue; }
+
+  template <class... OpMatchers>
+  friend bool matchInst(const MCInst &, unsigned, const OpMatchers &...);
+
+protected:
+  OpMatcher(std::optional<T> ValueToMatch) : Value(ValueToMatch) {}
+
+  bool matchValue(T OpValue) const {
+    // Check that OpValue does not contradict the existing Value.
+    bool MatchResult = !Value || *Value == OpValue;
+    // If MatchResult is false, all matchers will be reset before returning from
+    // matchInst, including this one, thus no need to assign conditionally.
+    Value = OpValue;
+
+    return MatchResult;
+  }
+
+public:
+  /// Returns the captured value.
+  T get() const {
+    assert(Value.has_value());
+    return *Value;
+  }
+};
+
+class Reg : public OpMatcher<MCPhysReg> {
+  bool matches(const MCOperand &Op) const {
+    if (!Op.isReg())
+      return false;
+
+    return matchValue(Op.getReg());
+  }
+
+  template <class... OpMatchers>
+  friend bool matchInst(const MCInst &, unsigned, const OpMatchers &...);
+
+public:
+  Reg(std::optional<MCPhysReg> RegToMatch = std::nullopt)
+      : OpMatcher<MCPhysReg>(RegToMatch) {}
+};
+
+class Imm : public OpMatcher<int64_t> {
+  bool matches(const MCOperand &Op) const {
+    if (!Op.isImm())
+      return false;
+
+    return matchValue(Op.getImm());
+  }
+
+  template <class... OpMatchers>
+  friend bool matchInst(const MCInst &, unsigned, const OpMatchers &...);
+
+public:
+  Imm(std::optional<int64_t> ImmToMatch = std::nullopt)
+      : OpMatcher<int64_t>(ImmToMatch) {}
+};
+
+/// Tries to match Inst and updates Ops on success.
+///
+/// If Inst has the specified Opcode and its operand list prefix matches Ops,
+/// this function returns true and updates Ops, otherwise false is returned and
+/// values of Ops are kept as before matchInst was called.
+///
+/// Please note that while Ops are technically passed by a const reference to
+/// make invocations like `matchInst(MI, Opcode, Imm(42))` possible, all their
+/// fields are marked mutable.
+template <class... OpMatchers>
+bool matchInst(const MCInst &Inst, unsigned Opcode, const OpMatchers &...Ops) {
+  if (Inst.getOpcode() != Opcode)
+    return false;
+  assert(sizeof...(Ops) <= Inst.getNumOperands() &&
+         "Too many operands are matched for the Opcode");
+
+  // Ask each matcher to remember its current value in case of rollback.
+  (Ops.remember(), ...);
+
+  // Check if all matchers match the corresponding operands.
+  auto It = Inst.begin();
+  auto AllMatched = (Ops.matches(*(It++)) && ... && true);
+
+  // If match failed, restore the original captured values.
+  if (!AllMatched) {
+    (Ops.restore(), ...);
+    return false;
+  }
+
+  return true;
+}
+
+} // namespace MCInstMatcher
+
 } // namespace bolt
 } // namespace llvm
 

bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp

@@ -19,6 +19,7 @@
 #include "Utils/AArch64BaseInfo.h"
 #include "bolt/Core/BinaryBasicBlock.h"
 #include "bolt/Core/BinaryFunction.h"
+#include "bolt/Core/MCInstUtils.h"
 #include "bolt/Core/MCPlusBuilder.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/MC/MCContext.h"
@@ -390,81 +391,58 @@ class AArch64MCPlusBuilder : public MCPlusBuilder {
 
     // Iterate over the instructions of BB in reverse order, matching opcodes
     // and operands.
-    MCPhysReg TestedReg = 0;
-    MCPhysReg ScratchReg = 0;
+
     auto It = BB.end();
-    auto StepAndGetOpcode = [&It, &BB]() -> int {
-      if (It == BB.begin())
-        return -1;
-      --It;
-      return It->getOpcode();
+    auto StepBack = [&]() {
+      while (It != BB.begin()) {
+        --It;
+        if (!isCFI(*It))
+          return true;
+      }
+      return false;
     };
-
-    switch (StepAndGetOpcode()) {
-    default:
-      // Not matched the branch instruction.
+    // Step to the last non-CFI instruction.
+    if (!StepBack())
       return std::nullopt;
-    case AArch64::Bcc:
-      // Bcc EQ, .Lon_success
-      if (It->getOperand(0).getImm() != AArch64CC::EQ)
-        return std::nullopt;
-      // Not checking .Lon_success (see above).
 
-      // SUBSXrs XZR, TestedReg, ScratchReg, 0 (used by "CMP reg, reg" alias)
-      if (StepAndGetOpcode() != AArch64::SUBSXrs ||
-          It->getOperand(0).getReg() != AArch64::XZR ||
-          It->getOperand(3).getImm() != 0)
+    using namespace llvm::bolt::MCInstMatcher;
+    Reg TestedReg;
+    Reg ScratchReg;
+
+    if (matchInst(*It, AArch64::Bcc, Imm(AArch64CC::EQ) /*, .Lon_success*/)) {
+      if (!StepBack() || !matchInst(*It, AArch64::SUBSXrs, Reg(AArch64::XZR),
+                                    TestedReg, ScratchReg, Imm(0)))
         return std::nullopt;
-      TestedReg = It->getOperand(1).getReg();
-      ScratchReg = It->getOperand(2).getReg();
 
       // Either XPAC(I|D) ScratchReg, ScratchReg
       // or     XPACLRI
-      switch (StepAndGetOpcode()) {
-      default:
+      if (!StepBack())
         return std::nullopt;
-      case AArch64::XPACLRI:
+      if (matchInst(*It, AArch64::XPACLRI)) {
         // No operands to check, but using XPACLRI forces TestedReg to be X30.
-        if (TestedReg != AArch64::LR)
-          return std::nullopt;
-        break;
-      case AArch64::XPACI:
-      case AArch64::XPACD:
-        if (It->getOperand(0).getReg() != ScratchReg ||
-            It->getOperand(1).getReg() != ScratchReg)
+        if (TestedReg.get() != AArch64::LR)
           return std::nullopt;
-        break;
+      } else if (!matchInst(*It, AArch64::XPACI, ScratchReg, ScratchReg) &&
+                 !matchInst(*It, AArch64::XPACD, ScratchReg, ScratchReg)) {
+        return std::nullopt;
       }
 
-      // ORRXrs ScratchReg, XZR, TestedReg, 0 (used by "MOV reg, reg" alias)
-      if (StepAndGetOpcode() != AArch64::ORRXrs)
+      if (!StepBack() || !matchInst(*It, AArch64::ORRXrs, ScratchReg,
+                                    Reg(AArch64::XZR), TestedReg, Imm(0)))
         return std::nullopt;
-      if (It->getOperand(0).getReg() != ScratchReg ||
-          It->getOperand(1).getReg() != AArch64::XZR ||
-          It->getOperand(2).getReg() != TestedReg ||
-          It->getOperand(3).getImm() != 0)
-        return std::nullopt;
-
-      return std::make_pair(TestedReg, &*It);
 
-    case AArch64::TBZX:
-      // TBZX ScratchReg, 62, .Lon_success
-      ScratchReg = It->getOperand(0).getReg();
-      if (It->getOperand(1).getImm() != 62)
-        return std::nullopt;
-      // Not checking .Lon_success (see above).
+      return std::make_pair(TestedReg.get(), &*It);
+    }
 
-      // EORXrs ScratchReg, TestedReg, TestedReg, 1
-      if (StepAndGetOpcode() != AArch64::EORXrs)
-        return std::nullopt;
-      TestedReg = It->getOperand(1).getReg();
-      if (It->getOperand(0).getReg() != ScratchReg ||
-          It->getOperand(2).getReg() != TestedReg ||
-          It->getOperand(3).getImm() != 1)
+    if (matchInst(*It, AArch64::TBZX, ScratchReg, Imm(62) /*, .Lon_success*/)) {
+      if (!StepBack() || !matchInst(*It, AArch64::EORXrs, Reg(ScratchReg),
+                                    TestedReg, TestedReg, Imm(1)))
         return std::nullopt;
 
-      return std::make_pair(TestedReg, &*It);
+      return std::make_pair(TestedReg.get(), &*It);
     }
+
+    return std::nullopt;
   }
 
   std::optional<MCPhysReg> getAuthCheckedReg(const MCInst &Inst,