[ARM] add common parts for PACBTI-M support in the backend

[stuij]

Summary

This patch encapsulates decision logic about when and how to generate
PAC/BTI related code. It's a part shared by PAC-RET, BTI placement,
build attribute emission, etc, so it make sense committing it
separately in order to unblock the aforementioned parts, which can
proceed concurrently.

This patch adds a few member functions to ARMFunctionInfo, which are currently
unused, therefore there is no testing for them at the moment. This code is
tested in follow-up PAC/BTI code gen patches.

This patch is part of a series that adds support for the PACBTI-M extension of
the Armv8.1-M architecture, as detailed here:

https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension

The PACBTI-M specification can be found in the Armv8-M Architecture Reference
Manual:

https://developer.arm.com/documentation/ddi0553/latest

The following people contributed to this patch:

• Momchil Velikov
• Ties Stuij

[ostannard]

llvm/lib/Target/ARM/ARMMachineFunctionInfo.cpp

16

This could do with a comment explaining the meaning of the two return values.

77

It would be better to split the BTI stuff out into a separate function too, this early return will make adding anything else to this constructor harder.

[stuij]

addressed review comments: added a comment and split BTI code out of ARMFunctionInfo

[danielkiss]

LGTM

llvm/lib/Target/ARM/ARMMachineFunctionInfo.h

class ARMFunctionInfo : public MachineFunctionInfo {
 
   /// The amount the literal pool has been increasedby due to promoted globals.
   int PromotedGlobalsIncrease = 0;
 
   /// True if r0 will be preserved by a call to this function (e.g. C++
   /// con/destructors).
   bool PreservesR0 = false;
 
+  /// True if the function should sign its return address.
+  bool SignReturnAddress = false;
+
+  /// True if the fucntion should sign its return address, even if LR is not
+  /// saved.
+  bool SignReturnAddressAll = false;
+
+  /// True if BTI instructions should be placed at potential indirect jump
+  /// destinations.
+  bool BranchTargetEnforcement = false;
+
 public:
   ARMFunctionInfo() = default;
 
   explicit ARMFunctionInfo(MachineFunction &MF);
 
   bool isThumbFunction() const { return isThumb; }
   bool isThumb1OnlyFunction() const { return isThumb && !hasThumb2; }
   bool isThumb2Function() const { return isThumb && hasThumb2; }
void setPromotedConstpoolIncrease(int Sz) {
     PromotedGlobalsIncrease = Sz;
   }
 
   DenseMap<unsigned, unsigned> EHPrologueRemappedRegs;
   DenseMap<unsigned, unsigned> EHPrologueOffsetInRegs;
 
   void setPreservesR0() { PreservesR0 = true; }
   bool getPreservesR0() const { return PreservesR0; }
+
+  bool shouldSignReturnAddress() const {
+    return shouldSignReturnAddress(LRSpilled);
+  }
+
+  bool shouldSignReturnAddress(bool SpillsLR) const {
+    if (!SignReturnAddress)
+      return false;
+    if (SignReturnAddressAll)
+      return true;
+    return LRSpilled;
+  }
+
+  bool branchTargetEnforcement() const { return BranchTargetEnforcement; }
 };
 
 } // end namespace llvm
 
 #endif // LLVM_LIB_TARGET_ARM_ARMMACHINEFUNCTIONINFO_H

llvm/lib/Target/ARM/ARMMachineFunctionInfo.cpp

 //===-- ARMMachineFunctionInfo.cpp - ARM machine function info ------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #include "ARMMachineFunctionInfo.h"
 #include "ARMSubtarget.h"
 
 using namespace llvm;
 
 void ARMFunctionInfo::anchor() {}
 
+static bool GetBranchTargetEnforcement(MachineFunction &MF) {
+  const auto &Subtarget = MF.getSubtarget<ARMSubtarget>();
+  if (!Subtarget.isMClass() || !Subtarget.hasV7Ops())
+    return false;
+
+  const Function &F = MF.getFunction();
+  if (!F.hasFnAttribute("branch-target-enforcement")) {
+    if (const auto *BTE = mdconst::extract_or_null<ConstantInt>(
+            F.getParent()->getModuleFlag("branch-target-enforcement")))
+      return BTE->getZExtValue();
+    return false;
+  }
+
+  const StringRef BTIEnable =
+      F.getFnAttribute("branch-target-enforcement").getValueAsString();
+  assert(BTIEnable.equals_insensitive("true") ||
+         BTIEnable.equals_insensitive("false"));
+  return BTIEnable.equals_insensitive("true");
+}
+
+// The pair returns values for the ARMFunctionInfo members
+// SignReturnAddress and SignReturnAddressAll respectively.
+static std::pair<bool, bool> GetSignReturnAddress(const Function &F) {
+  if (!F.hasFnAttribute("sign-return-address")) {
+    const Module &M = *F.getParent();
+    if (const auto *Sign = mdconst::extract_or_null<ConstantInt>(
+            M.getModuleFlag("sign-return-address"))) {
+      if (Sign->getZExtValue()) {
+        if (const auto *All = mdconst::extract_or_null<ConstantInt>(
+                M.getModuleFlag("sign-return-address-all")))
+          return {true, All->getZExtValue()};
+        return {true, false};
+      }
+    }
+    return {false, false};
+  }
+
+  StringRef Scope = F.getFnAttribute("sign-return-address").getValueAsString();
+  if (Scope.equals("none"))
+    return {false, false};
+
+  if (Scope.equals("all"))
+    return {true, true};
+
+  assert(Scope.equals("non-leaf"));
+  return {true, false};
+}
+
 ARMFunctionInfo::ARMFunctionInfo(MachineFunction &MF)
     : isThumb(MF.getSubtarget<ARMSubtarget>().isThumb()),
       hasThumb2(MF.getSubtarget<ARMSubtarget>().hasThumb2()),
       IsCmseNSEntry(MF.getFunction().hasFnAttribute("cmse_nonsecure_entry")),
-      IsCmseNSCall(MF.getFunction().hasFnAttribute("cmse_nonsecure_call")) {}
+      IsCmseNSCall(MF.getFunction().hasFnAttribute("cmse_nonsecure_call")),
+      BranchTargetEnforcement(GetBranchTargetEnforcement(MF)) {
+
+  const auto &Subtarget = MF.getSubtarget<ARMSubtarget>();
+  if (Subtarget.isMClass() && Subtarget.hasV7Ops())
+    std::tie(SignReturnAddress, SignReturnAddressAll) =
+        GetSignReturnAddress(MF.getFunction());
+}