[ARM] R11 not pushed adjacent to link register with PAC-M and AAPCS frame chain fix #82801

jwestwood921 · 2024-02-23T17:37:31Z

When code for M class architecture was compiled with AAPCS and PAC enabled, the frame pointer, r11, was not pushed to the stack adjacent to the link register. Due to PAC being enabled, r12 was placed between r11 and lr. This patch fixes this by adding an extra case to the already existing code that splits the GPR push in two when R11 is the frame pointer and certain paremeters are met. The differential revision for this previous change can be found here: https://reviews.llvm.org/D125649. This now ensures that r11 and lr are pushed in a separate push instruction to the other GPRs when PAC and AAPCS are enabled, meaning the frame pointer and link register are now pushed onto the stack adjacent to each other.

llvmbot · 2024-02-23T17:38:03Z

@llvm/pr-subscribers-backend-arm

Author: James Westwood (jwestwood921)

Changes

When code for M class architecture was compiled with AAPCS and PAC enabled, the frame pointer, r11, was not pushed to the stack adjacent to the link register. Due to PAC being enabled, r12 was placed between r11 and lr. This patch fixes this by adding an extra case to the already existing code that splits the GPR push in two when R11 is the frame pointer and certain paremeters are met. The differential revision for this previous change can be found here: https://reviews.llvm.org/D125649. This now ensures that r11 and lr are pushed in a separate push instruction to the other GPRs when PAC and AAPCS are enabled, meaning the frame pointer and link register are now pushed onto the stack adjacent to each other.

Patch is 21.29 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/82801.diff

6 Files Affected:

(modified) llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp (+2-2)
(modified) llvm/lib/Target/ARM/ARMFrameLowering.cpp (+124-62)
(modified) llvm/lib/Target/ARM/ARMSubtarget.cpp (+37-1)
(modified) llvm/lib/Target/ARM/ARMSubtarget.h (+16-6)
(modified) llvm/lib/Target/ARM/Thumb1FrameLowering.cpp (+2-2)
(added) llvm/test/CodeGen/Thumb2/pacbti-m-frame-chain.ll (+82)

diff --git a/llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp b/llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp
index 9adf758b46c481..42e2b89260e16a 100644
--- a/llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp
@@ -62,14 +62,14 @@ ARMBaseRegisterInfo::ARMBaseRegisterInfo()
 const MCPhysReg*
 ARMBaseRegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
   const ARMSubtarget &STI = MF->getSubtarget<ARMSubtarget>();
-  bool UseSplitPush = STI.splitFramePushPop(*MF);
+  bool UseSplitPush = STI.splitFramePushPopR7(*MF);
   const Function &F = MF->getFunction();
 
   if (F.getCallingConv() == CallingConv::GHC) {
     // GHC set of callee saved regs is empty as all those regs are
     // used for passing STG regs around
     return CSR_NoRegs_SaveList;
-  } else if (STI.splitFramePointerPush(*MF)) {
+  } else if (STI.framePointerRequiredForSEHUnwind(*MF)) {
     return CSR_Win_SplitFP_SaveList;
   } else if (F.getCallingConv() == CallingConv::CFGuard_Check) {
     return CSR_Win_AAPCS_CFGuard_Check_SaveList;
diff --git a/llvm/lib/Target/ARM/ARMFrameLowering.cpp b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
index eeb7f64aa5810e..f288e8be7d3816 100644
--- a/llvm/lib/Target/ARM/ARMFrameLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
@@ -718,9 +718,13 @@ static int getMaxFPOffset(const ARMSubtarget &STI, const ARMFunctionInfo &AFI,
   // This is a conservative estimation: Assume the frame pointer being r7 and
   // pc("r15") up to r8 getting spilled before (= 8 registers).
   int MaxRegBytes = 8 * 4;
-  if (STI.splitFramePointerPush(MF)) {
+  if (STI.r11AndLRNotAdjacent(MF) &&
+      STI.getRegisterInfo()->getFrameRegister(MF) == ARM::R11)
     // Here, r11 can be stored below all of r4-r15 (3 registers more than
-    // above), plus d8-d15.
+    // above).
+    MaxRegBytes = 11 * 4;
+  if (STI.framePointerRequiredForSEHUnwind(MF)) {
+    // Here, r11 can be stored below all of r4-r15 plus d8-d15.
     MaxRegBytes = 11 * 4 + 8 * 8;
   }
   int FPCXTSaveSize =
@@ -788,7 +792,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
   }
 
   // Determine spill area sizes.
-  if (STI.splitFramePointerPush(MF)) {
+  if (STI.splitFramePushPopR11(MF)) {
     for (const CalleeSavedInfo &I : CSI) {
       Register Reg = I.getReg();
       int FI = I.getFrameIdx();
@@ -834,7 +838,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
       case ARM::R10:
       case ARM::R11:
       case ARM::R12:
-        if (STI.splitFramePushPop(MF)) {
+        if (STI.splitFramePushPopR7(MF)) {
           GPRCS2Size += 4;
           break;
         }
@@ -897,13 +901,13 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
   unsigned GPRCS2Offset = GPRCS1Offset - GPRCS2Size;
   Align DPRAlign = DPRCSSize ? std::min(Align(8), Alignment) : Align(4);
   unsigned DPRGapSize = GPRCS1Size + FPCXTSaveSize + ArgRegsSaveSize;
-  if (!STI.splitFramePointerPush(MF)) {
+  if (!STI.framePointerRequiredForSEHUnwind(MF)) {
     DPRGapSize += GPRCS2Size;
   }
   DPRGapSize %= DPRAlign.value();
 
   unsigned DPRCSOffset;
-  if (STI.splitFramePointerPush(MF)) {
+  if (STI.framePointerRequiredForSEHUnwind(MF)) {
     DPRCSOffset = GPRCS1Offset - DPRGapSize - DPRCSSize;
     GPRCS2Offset = DPRCSOffset - GPRCS2Size;
   } else {
@@ -922,8 +926,8 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
   AFI->setGPRCalleeSavedArea2Offset(GPRCS2Offset);
   AFI->setDPRCalleeSavedAreaOffset(DPRCSOffset);
 
-  // Move past area 2.
-  if (GPRCS2Size > 0 && !STI.splitFramePointerPush(MF)) {
+  // Move past area 2, unless following Win_AAPCS_CFGuard calling convention.
+  if (GPRCS2Size > 0 && !STI.framePointerRequiredForSEHUnwind(MF)) {
     GPRCS2Push = LastPush = MBBI++;
     DefCFAOffsetCandidates.addInst(LastPush, GPRCS2Size);
   }
@@ -963,13 +967,14 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
   } else
     NumBytes = DPRCSOffset;
 
-  if (GPRCS2Size > 0 && STI.splitFramePointerPush(MF)) {
+  // Move past area 2 if following Win_AAPCS_CFGuard calling convention.
+  if (GPRCS2Size > 0 && STI.framePointerRequiredForSEHUnwind(MF)) {
     GPRCS2Push = LastPush = MBBI++;
     DefCFAOffsetCandidates.addInst(LastPush, GPRCS2Size);
   }
 
   bool NeedsWinCFIStackAlloc = NeedsWinCFI;
-  if (STI.splitFramePointerPush(MF) && HasFP)
+  if (STI.framePointerRequiredForSEHUnwind(MF) && HasFP)
     NeedsWinCFIStackAlloc = false;
 
   if (STI.isTargetWindows() && WindowsRequiresStackProbe(MF, NumBytes)) {
@@ -1074,7 +1079,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
     AfterPush = std::next(GPRCS1Push);
     unsigned PushSize = sizeOfSPAdjustment(*GPRCS1Push);
     int FPOffset = PushSize + FramePtrOffsetInPush;
-    if (STI.splitFramePointerPush(MF)) {
+    if (STI.splitFramePushPopR11(MF)) {
       AfterPush = std::next(GPRCS2Push);
       emitRegPlusImmediate(!AFI->isThumbFunction(), MBB, AfterPush, dl, TII,
                            FramePtr, ARM::SP, 0, MachineInstr::FrameSetup);
@@ -1106,7 +1111,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
   // instructions below don't need to be replayed to unwind the stack.
   if (NeedsWinCFI && MBBI != MBB.begin()) {
     MachineBasicBlock::iterator End = MBBI;
-    if (HasFP && STI.splitFramePointerPush(MF))
+    if (HasFP && STI.framePointerRequiredForSEHUnwind(MF))
       End = AfterPush;
     insertSEHRange(MBB, {}, End, TII, MachineInstr::FrameSetup);
     BuildMI(MBB, End, dl, TII.get(ARM::SEH_PrologEnd))
@@ -1118,61 +1123,114 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
   // the necessary DWARF cf instructions to describe the situation. Start by
   // recording where each register ended up:
   if (GPRCS1Size > 0 && !NeedsWinCFI) {
-    MachineBasicBlock::iterator Pos = std::next(GPRCS1Push);
-    int CFIIndex;
-    for (const auto &Entry : CSI) {
-      Register Reg = Entry.getReg();
-      int FI = Entry.getFrameIdx();
-      switch (Reg) {
-      case ARM::R8:
-      case ARM::R9:
-      case ARM::R10:
-      case ARM::R11:
-      case ARM::R12:
-        if (STI.splitFramePushPop(MF))
+    if (STI.r11AndLRNotAdjacent(MF) &&
+        RegInfo->getFrameRegister(MF) == ARM::R11) {
+      MachineBasicBlock::iterator Pos = std::next(GPRCS1Push);
+      int CFIIndex;
+      for (const auto &Entry : CSI) {
+        Register Reg = Entry.getReg();
+        int FI = Entry.getFrameIdx();
+        switch (Reg) {
+        case ARM::R0:
+        case ARM::R1:
+        case ARM::R2:
+        case ARM::R3:
+        case ARM::R4:
+        case ARM::R5:
+        case ARM::R6:
+        case ARM::R7:
+        case ARM::R8:
+        case ARM::R9:
+        case ARM::R10:
+        case ARM::R12:
+          CFIIndex = MF.addFrameInst(MCCFIInstruction::createOffset(
+              nullptr, MRI->getDwarfRegNum(Reg, true),
+              MFI.getObjectOffset(FI)));
+          BuildMI(MBB, Pos, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
+              .addCFIIndex(CFIIndex)
+              .setMIFlags(MachineInstr::FrameSetup);
           break;
-        [[fallthrough]];
-      case ARM::R0:
-      case ARM::R1:
-      case ARM::R2:
-      case ARM::R3:
-      case ARM::R4:
-      case ARM::R5:
-      case ARM::R6:
-      case ARM::R7:
-      case ARM::LR:
-        CFIIndex = MF.addFrameInst(MCCFIInstruction::createOffset(
-            nullptr, MRI->getDwarfRegNum(Reg, true), MFI.getObjectOffset(FI)));
-        BuildMI(MBB, Pos, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
-            .addCFIIndex(CFIIndex)
-            .setMIFlags(MachineInstr::FrameSetup);
-        break;
+        }
+      }
+    } else {
+      MachineBasicBlock::iterator Pos = std::next(GPRCS1Push);
+      int CFIIndex;
+      for (const auto &Entry : CSI) {
+        Register Reg = Entry.getReg();
+        int FI = Entry.getFrameIdx();
+        switch (Reg) {
+        case ARM::R8:
+        case ARM::R9:
+        case ARM::R10:
+        case ARM::R11:
+        case ARM::R12:
+          if (STI.splitFramePushPopR7(MF))
+            break;
+          [[fallthrough]];
+        case ARM::R0:
+        case ARM::R1:
+        case ARM::R2:
+        case ARM::R3:
+        case ARM::R4:
+        case ARM::R5:
+        case ARM::R6:
+        case ARM::R7:
+        case ARM::LR:
+          CFIIndex = MF.addFrameInst(MCCFIInstruction::createOffset(
+              nullptr, MRI->getDwarfRegNum(Reg, true),
+              MFI.getObjectOffset(FI)));
+          BuildMI(MBB, Pos, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
+              .addCFIIndex(CFIIndex)
+              .setMIFlags(MachineInstr::FrameSetup);
+          break;
+        }
       }
     }
   }
 
   if (GPRCS2Size > 0 && !NeedsWinCFI) {
     MachineBasicBlock::iterator Pos = std::next(GPRCS2Push);
-    for (const auto &Entry : CSI) {
-      Register Reg = Entry.getReg();
-      int FI = Entry.getFrameIdx();
-      switch (Reg) {
-      case ARM::R8:
-      case ARM::R9:
-      case ARM::R10:
-      case ARM::R11:
-      case ARM::R12:
-        if (STI.splitFramePushPop(MF)) {
-          unsigned DwarfReg = MRI->getDwarfRegNum(
-              Reg == ARM::R12 ? ARM::RA_AUTH_CODE : Reg, true);
+    if (STI.r11AndLRNotAdjacent(MF) &&
+        RegInfo->getFrameRegister(MF) == ARM::R11) {
+      for (const auto &Entry : CSI) {
+        Register Reg = Entry.getReg();
+        int FI = Entry.getFrameIdx();
+        switch (Reg) {
+        case ARM::R11:
+        case ARM::LR:
+          unsigned DwarfReg = MRI->getDwarfRegNum(Reg, true);
           unsigned Offset = MFI.getObjectOffset(FI);
           unsigned CFIIndex = MF.addFrameInst(
               MCCFIInstruction::createOffset(nullptr, DwarfReg, Offset));
           BuildMI(MBB, Pos, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
               .addCFIIndex(CFIIndex)
               .setMIFlags(MachineInstr::FrameSetup);
+          break;
+        }
+      }
+    } else {
+      MachineBasicBlock::iterator Pos = std::next(GPRCS2Push);
+      for (const auto &Entry : CSI) {
+        Register Reg = Entry.getReg();
+        int FI = Entry.getFrameIdx();
+        switch (Reg) {
+        case ARM::R8:
+        case ARM::R9:
+        case ARM::R10:
+        case ARM::R11:
+        case ARM::R12:
+          if (STI.splitFramePushPopR7(MF)) {
+            unsigned DwarfReg = MRI->getDwarfRegNum(
+                Reg == ARM::R12 ? ARM::RA_AUTH_CODE : Reg, true);
+            unsigned Offset = MFI.getObjectOffset(FI);
+            unsigned CFIIndex = MF.addFrameInst(
+                MCCFIInstruction::createOffset(nullptr, DwarfReg, Offset));
+            BuildMI(MBB, Pos, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
+                .addCFIIndex(CFIIndex)
+                .setMIFlags(MachineInstr::FrameSetup);
+          }
+          break;
         }
-        break;
       }
     }
   }
@@ -1382,7 +1440,8 @@ void ARMFrameLowering::emitEpilogue(MachineFunction &MF,
                    MachineInstr::FrameDestroy);
 
     // Increment past our save areas.
-    if (AFI->getGPRCalleeSavedArea2Size() && STI.splitFramePointerPush(MF))
+    if (AFI->getGPRCalleeSavedArea2Size() &&
+        STI.framePointerRequiredForSEHUnwind(MF))
       MBBI++;
 
     if (MBBI != MBB.end() && AFI->getDPRCalleeSavedAreaSize()) {
@@ -1399,7 +1458,8 @@ void ARMFrameLowering::emitEpilogue(MachineFunction &MF,
                    MachineInstr::FrameDestroy);
     }
 
-    if (AFI->getGPRCalleeSavedArea2Size() && !STI.splitFramePointerPush(MF))
+    if (AFI->getGPRCalleeSavedArea2Size() &&
+        !STI.framePointerRequiredForSEHUnwind(MF))
       MBBI++;
     if (AFI->getGPRCalleeSavedArea1Size()) MBBI++;
 
@@ -1539,7 +1599,8 @@ void ARMFrameLowering::emitPushInst(MachineBasicBlock &MBB,
     unsigned LastReg = 0;
     for (; i != 0; --i) {
       Register Reg = CSI[i-1].getReg();
-      if (!(Func)(Reg, STI.splitFramePushPop(MF))) continue;
+      if (!(Func)(Reg, STI.splitFramePushPopR7(MF)))
+        continue;
 
       // D-registers in the aligned area DPRCS2 are NOT spilled here.
       if (Reg >= ARM::D8 && Reg < ARM::D8 + NumAlignedDPRCS2Regs)
@@ -1632,7 +1693,8 @@ void ARMFrameLowering::emitPopInst(MachineBasicBlock &MBB,
     for (; i != 0; --i) {
       CalleeSavedInfo &Info = CSI[i-1];
       Register Reg = Info.getReg();
-      if (!(Func)(Reg, STI.splitFramePushPop(MF))) continue;
+      if (!(Func)(Reg, STI.splitFramePushPopR7(MF)))
+        continue;
 
       // The aligned reloads from area DPRCS2 are not inserted here.
       if (Reg >= ARM::D8 && Reg < ARM::D8 + NumAlignedDPRCS2Regs)
@@ -1640,7 +1702,7 @@ void ARMFrameLowering::emitPopInst(MachineBasicBlock &MBB,
       if (Reg == ARM::LR && !isTailCall && !isVarArg && !isInterrupt &&
           !isCmseEntry && !isTrap && AFI->getArgumentStackToRestore() == 0 &&
           STI.hasV5TOps() && MBB.succ_empty() && !hasPAC &&
-          !STI.splitFramePointerPush(MF)) {
+          !STI.splitFramePushPopR11(MF)) {
         Reg = ARM::PC;
         // Fold the return instruction into the LDM.
         DeleteRet = true;
@@ -2001,7 +2063,7 @@ bool ARMFrameLowering::spillCalleeSavedRegisters(
         .addImm(-4)
         .add(predOps(ARMCC::AL));
   }
-  if (STI.splitFramePointerPush(MF)) {
+  if (STI.splitFramePushPopR11(MF)) {
     emitPushInst(MBB, MI, CSI, PushOpc, PushOneOpc, false,
                  &isSplitFPArea1Register, 0, MachineInstr::FrameSetup);
     emitPushInst(MBB, MI, CSI, FltOpc, 0, true, &isARMArea3Register,
@@ -2046,7 +2108,7 @@ bool ARMFrameLowering::restoreCalleeSavedRegisters(
   unsigned LdrOpc =
       AFI->isThumbFunction() ? ARM::t2LDR_POST : ARM::LDR_POST_IMM;
   unsigned FltOpc = ARM::VLDMDIA_UPD;
-  if (STI.splitFramePointerPush(MF)) {
+  if (STI.splitFramePushPopR11(MF)) {
     emitPopInst(MBB, MI, CSI, PopOpc, LdrOpc, isVarArg, false,
                 &isSplitFPArea2Register, 0);
     emitPopInst(MBB, MI, CSI, FltOpc, 0, isVarArg, true, &isARMArea3Register,
@@ -2362,7 +2424,7 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
     if (Spilled) {
       NumGPRSpills++;
 
-      if (!STI.splitFramePushPop(MF)) {
+      if (!STI.splitFramePushPopR7(MF)) {
         if (Reg == ARM::LR)
           LRSpilled = true;
         CS1Spilled = true;
@@ -2384,7 +2446,7 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
         break;
       }
     } else {
-      if (!STI.splitFramePushPop(MF)) {
+      if (!STI.splitFramePushPopR7(MF)) {
         UnspilledCS1GPRs.push_back(Reg);
         continue;
       }
diff --git a/llvm/lib/Target/ARM/ARMSubtarget.cpp b/llvm/lib/Target/ARM/ARMSubtarget.cpp
index 922fa93226f298..cc86afb835d0c4 100644
--- a/llvm/lib/Target/ARM/ARMSubtarget.cpp
+++ b/llvm/lib/Target/ARM/ARMSubtarget.cpp
@@ -494,11 +494,47 @@ bool ARMSubtarget::ignoreCSRForAllocationOrder(const MachineFunction &MF,
          ARM::GPRRegClass.contains(PhysReg);
 }
 
-bool ARMSubtarget::splitFramePointerPush(const MachineFunction &MF) const {
+bool ARMSubtarget::r11AndLRNotAdjacent(const MachineFunction &MF) const {
+  const std::vector<CalleeSavedInfo> CSI =
+      MF.getFrameInfo().getCalleeSavedInfo();
+
+  if (CSI.size() > 1 &&
+      MF.getInfo<ARMFunctionInfo>()->shouldSignReturnAddress()) {
+
+    bool r11InCSI = false;
+    bool lrInCSI = false;
+    unsigned long r11Idx = 0;
+    unsigned long lrIdx = 0;
+
+    for (unsigned long i = 0; i < CSI.size(); i++) {
+      if (CSI[i].getReg() == ARM::LR) {
+        lrIdx = i;
+        lrInCSI = true;
+      } else if (CSI[i].getReg() == ARM::R11) {
+        r11Idx = i;
+        r11InCSI = true;
+      }
+    }
+    if (lrIdx + 1 != r11Idx && r11InCSI && lrInCSI)
+      return true;
+  }
+  return false;
+}
+
+bool ARMSubtarget::framePointerRequiredForSEHUnwind(
+    const MachineFunction &MF) const {
   const Function &F = MF.getFunction();
+  const std::vector<CalleeSavedInfo> CSI =
+      MF.getFrameInfo().getCalleeSavedInfo();
+
   if (!MF.getTarget().getMCAsmInfo()->usesWindowsCFI() ||
       !F.needsUnwindTableEntry())
     return false;
   const MachineFrameInfo &MFI = MF.getFrameInfo();
   return MFI.hasVarSizedObjects() || getRegisterInfo()->hasStackRealignment(MF);
 }
+
+bool ARMSubtarget::splitFramePushPopR11(const MachineFunction &MF) const {
+  return (r11AndLRNotAdjacent(MF) && getFramePointerReg() == ARM::R11) ||
+         framePointerRequiredForSEHUnwind(MF);
+}
diff --git a/llvm/lib/Target/ARM/ARMSubtarget.h b/llvm/lib/Target/ARM/ARMSubtarget.h
index 91f3978b041a3a..210d8fdb1440ee 100644
--- a/llvm/lib/Target/ARM/ARMSubtarget.h
+++ b/llvm/lib/Target/ARM/ARMSubtarget.h
@@ -441,18 +441,28 @@ class ARMSubtarget : public ARMGenSubtargetInfo {
   }
 
   /// Returns true if the frame setup is split into two separate pushes (first
-  /// r0-r7,lr then r8-r11), principally so that the frame pointer is adjacent
-  /// to lr. This is always required on Thumb1-only targets, as the push and
-  /// pop instructions can't access the high registers.
-  bool splitFramePushPop(const MachineFunction &MF) const {
-    if (MF.getInfo<ARMFunctionInfo>()->shouldSignReturnAddress())
+  /// r0-r7,lr then r8-r11), principally so that the frame pointer r7 is
+  /// adjacent to lr. This is always required on Thumb1-only targets, as the
+  /// push and pop instructions can't access the high registers.
+  bool splitFramePushPopR7(const MachineFunction &MF) const {
+    if (MF.getInfo<ARMFunctionInfo>()->shouldSignReturnAddress() &&
+        !createAAPCSFrameChain())
       return true;
     return (getFramePointerReg() == ARM::R7 &&
             MF.getTarget().Options.DisableFramePointerElim(MF)) ||
            isThumb1Only();
   }
 
-  bool splitFramePointerPush(const MachineFunction &MF) const;
+  bool framePointerRequiredForSEHUnwind(const MachineFunction &MF) const;
+
+  // Returns true if R11 and lr are not adjacent to each other in the list of
+  // callee saved registers in a frame.
+  bool r11AndLRNotAdjacent(const MachineFunction &MF) const;
+
+  // Returns true if the frame setup is split into two separate pushes (first
+  // r0-r10,r12 then r11,lr), principally so that the frame pointer r11 is
+  // adjacent to lr.
+  bool splitFramePushPopR11(const MachineFunction &MF) const;
 
   bool useStride4VFPs() const;
 
diff --git a/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp b/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
index 0f4ece64bff532..0ad7b96453d1c5 100644
--- a/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
+++ b/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
@@ -222,7 +222,7 @@ void Thumb1FrameLowering::emitPrologue(MachineFunction &MF,
     case ARM::R8:
     case ARM::R9:
     case ARM::R10:
-      if (STI.splitFramePushPop(MF)) {
+      if (STI.splitFramePushPopR7(MF)) {
         GPRCS2Size += 4;
         break;
       }
@@ -366,7 +366,7 @@ void Thumb1FrameLowering::emitPrologue(MachineFunction &MF,
       case ARM::R10:
       case ARM::R11:
       case ARM::R12:
-        if (STI.splitFramePushPop(MF))
+        if (STI.splitFramePushPopR7(MF))
           break;
         [[fallthrough]];
       case ARM::R0:
diff --git a/llvm/test/CodeGen/Thumb2/pacbti-m-frame-chain.ll b/llvm/test/CodeGen/Thumb2/pacbti-m-frame-chain.ll
new file mode 100644
index 00000000000000..77759355e576a2
--- /dev/null
+++ b/llvm/test/CodeGen/Thumb2/pacbti-m-frame-chain.ll
@@ -0,0 +1,82 @@
+; RUN: llc -filetype asm -o - %s --frame-pointer=all -mattr=+aapcs-frame-chain -mattr=+aapcs-frame-chain-leaf -force-dwarf-frame-section | FileCheck %s
+target triple = "thumbv8m.main-none-none-eabi"
+
+; int f() {
+;     return 0;
+; }
+;
+; int x(int, char *);
+; int y(int n) {
+; char a[n];
+; return 1 + x(n, a);
+; }
+
+define hidden i32 @f() local_unnamed_addr {
+entry:
+    ret i32 0;
+}
+
+define hidden i32 @x(i32 noundef %n) local_unnamed_addr {
+entry:
+  %vla = alloca i8, i32 %n, align 1
+  %call = call i32 @y(i32 noundef %n, ptr noundef nonnull %vla)
+  %add = add nsw i32 %call, 1
+  ret i32 %add
+}
+
+declare dso_local i32 @y(i32 noundef, ptr noundef) local_unnamed_addr
+
+; CHECK-LABEL: f:
+; CHECK:       pac     r12, lr, sp
+; CHECK-NEXT:  .save   {ra_auth_code}
+; CHECK-NEXT:  str     r12, [sp, #-4]!
+; CHECK-NEXT:  .cfi_def_cfa_offset 4
+; CHECK-NEXT:  .cfi_offset r12, -8
+; CHECK-NEXT:  .save   {r11, lr}
+; CHECK-NEXT:  push.w  {r11, lr}
+; CHECK-NEXT:  .cfi_offset lr, -4
+; CHECK-NEXT:  .cfi_offset r11, -12
+; CHECK-NEXT:  .setfp  r11, sp
+; CHECK-NEXT:  mov     r11, sp
+; CHECK-NEXT:  .cfi_def_cfa r11, 12
+; CHECK-NEXT:  m...
[truncated]

efriedma-quic · 2024-02-23T19:34:04Z

llvm/lib/Target/ARM/ARMSubtarget.h

+  // Returns true if the frame setup is split into two separate pushes (first
+  // r0-r10,r12 then r11,lr), principally so that the frame pointer r11 is
+  // adjacent to lr.
+  bool splitFramePushPopR11(const MachineFunction &MF) const;


Would it make sense to introduce an enum of some sort here? Having a bunch of separate boolean methods representing different variations is confusing.

As far as I can tell, the following variations exist:

The normal push: push r4-r11+lr all at once

The Thumb1 split push: push r4-r7+lr, then r8-r11

The Thumb1 r11 frame chain split push: push lr, then r11, then r0-r7, then r8-r11.

The branch signing unsplit push: push r4-r12+lr all at once.

The branch signing split push: push r4-r7+lr, then r8-r12

The branch signing r11 frame chain split push: push r4-r10+r12, then r11+lr

The Windows SEH split push: push r4-r10, then r11+lr

I agree that using an enum is a better option. I think having a separate enum value for each push variation may be overkill, as many push variations are implemented in the same way by the frame lowering files. For example, the normal push and branch signing unsplit push are handled the same way, there is just an extra register in the latter variation. It might therefore be a better option to have an enum with values based around which register, if any, causes the register push to be split, instead of a value for each specific push variation.

That sounds fine.

efriedma-quic · 2024-02-23T21:07:34Z

llvm/lib/Target/ARM/ARMSubtarget.h

+
+  // Returns true if R11 and lr are not adjacent to each other in the list of
+  // callee saved registers in a frame.
+  bool r11AndLRNotAdjacent(const MachineFunction &MF) const;


This method doesn't seem to do what the description says... it unconditionally returns false when signing is disabled, but there are other reasons they might not be adjacent.

This method has been deleted, but functionality moved to the getPushPopSpiltVariation method. The description for this functionality has been updated to be more accurate.

efriedma-quic · 2024-02-23T21:18:34Z

llvm/lib/Target/ARM/ARMSubtarget.h

      return true;
    return (getFramePointerReg() == ARM::R7 &&
            MF.getTarget().Options.DisableFramePointerElim(MF)) ||
           isThumb1Only();
  }

-  bool splitFramePointerPush(const MachineFunction &MF) const;
+  bool framePointerRequiredForSEHUnwind(const MachineFunction &MF) const;


"framePointerRequiredForSEHUnwind" doesn't describe what the actual effect is here.

This method name has now been deleted.

…hes should be split with a single enum.

efriedma-quic · 2024-02-29T22:03:45Z

llvm/lib/Target/ARM/ARMSubtarget.h

+    /// r4-r11+lr (+r12 if necessary) can be pushed in a single instruction.
+    NoSplit,
+    /// The registers need to be split into a push of r4-r7+lr and another
+    /// containing r8-r11 (+r12 if necessary).


Maybe worth noting the interaction between R7Split and the Thumb1 AAPCS split push (HasFrameRecordArea).

efriedma-quic · 2024-02-29T22:08:28Z

llvm/lib/Target/ARM/ARMSubtarget.cpp

+  if ((MF.getInfo<ARMFunctionInfo>()->shouldSignReturnAddress() &&
+       !createAAPCSFrameChain()) ||
+      ((getFramePointerReg() == ARM::R7 &&
+        MF.getTarget().Options.DisableFramePointerElim(MF)) ||


Suggested change

MF.getTarget().Options.DisableFramePointerElim(MF)) ||

MF.getTarget().Options.DisableFramePointerElim(MF))) ||

efriedma-quic · 2024-02-29T22:09:47Z

llvm/lib/Target/ARM/Thumb1FrameLowering.cpp

@@ -222,7 +222,8 @@ void Thumb1FrameLowering::emitPrologue(MachineFunction &MF,
    case ARM::R8:
    case ARM::R9:
    case ARM::R10:
-      if (STI.splitFramePushPop(MF)) {
+      if (STI.getPushPopSplitVariation(MF) ==
+          ARMSubtarget::PushPopSplitVariation::R7Split) {


Given this is Thumb1FrameLowering... isn't this always true?

It looks like Thumb1FrameLowering is initialized only when isThumb1Only() is true, so the variation will always be R7Split. Therefore the if statement is unnecessary.

efriedma-quic · 2024-02-29T22:13:43Z

llvm/lib/Target/ARM/ARMFrameLowering.cpp

@@ -963,13 +976,18 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
  } else
    NumBytes = DPRCSOffset;

-  if (GPRCS2Size > 0 && STI.splitFramePointerPush(MF)) {
+  // Move past area 2 if following Win_AAPCS_CFGuard calling convention.


This comment doesn't look right? Functions defined with the CFGuard_Check should be extremely rare, if they show up at all; it's used to model the behavior of runtime routines.

Yes, that's a mistake on my end, thanks for catching it. The comment should refer to the CSR_Win_SplitFP calling convention.

efriedma-quic · 2024-02-29T22:26:12Z

llvm/lib/Target/ARM/ARMSubtarget.cpp

+  // Returns R11SplitAAPCSBranchSigning if R11 and lr are not adjacent to each
+  // other in the list of callee saved registers in a frame, and branch
+  // signing is enabled.
+  if (CSI.size() > 1 &&


Can we simplify this code? It seems sort of awkward to dig through the callee-saves here. And given the normal way this works, the only register that can show up between R11 and LR (R14), is R12.

I think it can probably be simplified. All that needs to be checked is if R11 is the frame pointer and R12 is in the callee saved register list. These are both already checked in the if statement, so I think the rest of the code is unnecessary. The rest of the code was there as if R11 and LR were next to each other, only one push instruction is needed. However, with the way the callee saved register list is constructed when R12 is required, I don't think this is possible.

efriedma-quic · 2024-02-29T22:31:38Z

llvm/lib/Target/ARM/ARMFrameLowering.cpp

+        case ARM::R10:
+        case ARM::R11:
+        case ARM::R12:
+          if (STI.getPushPopSplitVariation(MF) ==


Is it actually possible for this condition to be false? We know GPRCS2Size is non-zero, so it's not unsplit. We're not on Windows, so it's not the SEH split. And the new code explicitly checks for the branch signing split. So by process of elimination, this must be R7Split.

Yes, that reasoning is correct. I'll remove the if statement.

…ded comments.

efriedma-quic

LGTM

… AAPCS frame chain fix (llvm#82801)" This reverts commit 00e4a41. This patch was found to cause miscompilations and compilation failures.

#84019) … AAPCS frame chain fix (#82801)" This reverts commit 00e4a41. This patch was found to cause miscompilations and compilation failures.

llvmbot added the backend:ARM label Feb 23, 2024

efriedma-quic reviewed Feb 23, 2024

View reviewed changes

Replaced individual boolean methods to determine how the register pus…

b0438c9

…hes should be split with a single enum.

efriedma-quic reviewed Feb 29, 2024

View reviewed changes

Fixes for review comments: removed unnecessary if statements and amen…

a281e71

…ded comments.

efriedma-quic approved these changes Mar 1, 2024

View reviewed changes

Merge branch 'main' into r11-preserved-with-pac-aapcs

7f4a094

jwestwood921 merged commit 00e4a41 into llvm:main Mar 4, 2024
4 checks passed

jwestwood921 deleted the r11-preserved-with-pac-aapcs branch March 4, 2024 12:00

jwestwood921 mentioned this pull request Mar 5, 2024

Revert "[ARM] R11 not pushed adjacent to link register with PAC-M and… #84019

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	MF.getTarget().Options.DisableFramePointerElim(MF)) \|\|
	MF.getTarget().Options.DisableFramePointerElim(MF))) \|\|

[ARM] R11 not pushed adjacent to link register with PAC-M and AAPCS frame chain fix #82801

[ARM] R11 not pushed adjacent to link register with PAC-M and AAPCS frame chain fix #82801

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