[AVR] Respect the 'interrupt' function attribute

In the past, AVR functions were only lowered with interrupt-specific
machine code if the function was defined with the "avr-interrupt" or
"avr-signal" calling conventions.

This patch modifies the backend so that if the function does not have a
special calling convention, but does have an "interrupt" attribute,
that function is interpreted as a function with interrupts.

This also extracts the "is this function an interrupt" logic from
several disparate places in the backend into one AVRMachineFunctionInfo
attribute.

Bug found by Wilhelm Meier.

llvm/lib/Target/AVR/AVRFrameLowering.cpp

@@ -56,10 +56,11 @@ void AVRFrameLowering::emitPrologue(MachineFunction &MF,
   DebugLoc DL = (MBBI != MBB.end()) ? MBBI->getDebugLoc() : DebugLoc();
   const AVRSubtarget &STI = MF.getSubtarget<AVRSubtarget>();
   const AVRInstrInfo &TII = *STI.getInstrInfo();
+  const AVRMachineFunctionInfo *AFI = MF.getInfo<AVRMachineFunctionInfo>();
   bool HasFP = hasFP(MF);
 
   // Interrupt handlers re-enable interrupts in function entry.
-  if (CallConv == CallingConv::AVR_INTR) {
+  if (AFI->isInterruptHandler() && CallConv != CallingConv::AVR_SIGNAL) {
     BuildMI(MBB, MBBI, DL, TII.get(AVR::BSETs))
         .addImm(0x07)
         .setMIFlag(MachineInstr::FrameSetup);
@@ -74,8 +75,7 @@ void AVRFrameLowering::emitPrologue(MachineFunction &MF,
 
   // Emit special prologue code to save R1, R0 and SREG in interrupt/signal
   // handlers before saving any other registers.
-  if (CallConv == CallingConv::AVR_INTR ||
+  if (AFI->isInterruptHandler()) {
-      CallConv == CallingConv::AVR_SIGNAL) {
     BuildMI(MBB, MBBI, DL, TII.get(AVR::PUSHWRr))
         .addReg(AVR::R1R0, RegState::Kill)
         .setMIFlag(MachineInstr::FrameSetup);
@@ -99,7 +99,6 @@ void AVRFrameLowering::emitPrologue(MachineFunction &MF,
   }
 
   const MachineFrameInfo &MFI = MF.getFrameInfo();
-  const AVRMachineFunctionInfo *AFI = MF.getInfo<AVRMachineFunctionInfo>();
   unsigned FrameSize = MFI.getStackSize() - AFI->getCalleeSavedFrameSize();
 
   // Skip the callee-saved push instructions.
@@ -142,13 +141,11 @@ void AVRFrameLowering::emitPrologue(MachineFunction &MF,
 
 void AVRFrameLowering::emitEpilogue(MachineFunction &MF,
                                     MachineBasicBlock &MBB) const {
-  CallingConv::ID CallConv = MF.getFunction().getCallingConv();
+  const AVRMachineFunctionInfo *AFI = MF.getInfo<AVRMachineFunctionInfo>();
-  bool isHandler = (CallConv == CallingConv::AVR_INTR ||
-                    CallConv == CallingConv::AVR_SIGNAL);
 
   // Early exit if the frame pointer is not needed in this function except for
   // signal/interrupt handlers where special code generation is required.
-  if (!hasFP(MF) && !isHandler) {
+  if (!hasFP(MF) && !AFI->isInterruptHandler()) {
     return;
   }
 
@@ -158,14 +155,13 @@ void AVRFrameLowering::emitEpilogue(MachineFunction &MF,
 
   DebugLoc DL = MBBI->getDebugLoc();
   const MachineFrameInfo &MFI = MF.getFrameInfo();
-  const AVRMachineFunctionInfo *AFI = MF.getInfo<AVRMachineFunctionInfo>();
   unsigned FrameSize = MFI.getStackSize() - AFI->getCalleeSavedFrameSize();
   const AVRSubtarget &STI = MF.getSubtarget<AVRSubtarget>();
   const AVRInstrInfo &TII = *STI.getInstrInfo();
 
   // Emit special epilogue code to restore R1, R0 and SREG in interrupt/signal
   // handlers at the very end of the function, just before reti.
-  if (isHandler) {
+  if (AFI->isInterruptHandler()) {
     BuildMI(MBB, MBBI, DL, TII.get(AVR::POPRd), AVR::R0);
     BuildMI(MBB, MBBI, DL, TII.get(AVR::OUTARr))
         .addImm(0x3f)

llvm/lib/Target/AVR/AVRISelLowering.cpp

@@ -1429,10 +1429,12 @@ AVRTargetLowering::LowerReturn(SDValue Chain, CallingConv::ID CallConv,
     return Chain;
   }
 
+  const AVRMachineFunctionInfo *AFI = MF.getInfo<AVRMachineFunctionInfo>();
+
   unsigned RetOpc =
-      (CallConv == CallingConv::AVR_INTR || CallConv == CallingConv::AVR_SIGNAL)
+    AFI->isInterruptHandler()
-          ? AVRISD::RETI_FLAG
+        ? AVRISD::RETI_FLAG
-          : AVRISD::RET_FLAG;
+        : AVRISD::RET_FLAG;
 
   RetOps[0] = Chain; // Update chain.
 

llvm/lib/Target/AVR/AVRMachineFunctionInfo.h

@@ -31,6 +31,9 @@ class AVRMachineFunctionInfo : public MachineFunctionInfo {
   /// used inside the function.
   bool HasStackArgs;
 
+  /// Whether or not the function is an interrupt handler.
+  bool IsInterruptHandler;
+
   /// Size of the callee-saved register portion of the
   /// stack frame in bytes.
   unsigned CalleeSavedFrameSize;
@@ -41,11 +44,19 @@ class AVRMachineFunctionInfo : public MachineFunctionInfo {
 public:
   AVRMachineFunctionInfo()
       : HasSpills(false), HasAllocas(false), HasStackArgs(false),
-        CalleeSavedFrameSize(0), VarArgsFrameIndex(0) {}
+        IsInterruptHandler(false), CalleeSavedFrameSize(0),
+        VarArgsFrameIndex(0) {}
 
   explicit AVRMachineFunctionInfo(MachineFunction &MF)
       : HasSpills(false), HasAllocas(false), HasStackArgs(false),
-        CalleeSavedFrameSize(0), VarArgsFrameIndex(0) {}
+        CalleeSavedFrameSize(0), VarArgsFrameIndex(0) {
+    unsigned CallConv = MF.getFunction().getCallingConv();
+
+    this->IsInterruptHandler =
+      CallConv == CallingConv::AVR_INTR ||
+      CallConv == CallingConv::AVR_SIGNAL ||
+      MF.getFunction().hasFnAttribute("interrupt");
+  }
 
   bool getHasSpills() const { return HasSpills; }
   void setHasSpills(bool B) { HasSpills = B; }
@@ -56,6 +67,8 @@ public:
   bool getHasStackArgs() const { return HasStackArgs; }
   void setHasStackArgs(bool B) { HasStackArgs = B; }
 
+  bool isInterruptHandler() const { return IsInterruptHandler; }
+
   unsigned getCalleeSavedFrameSize() const { return CalleeSavedFrameSize; }
   void setCalleeSavedFrameSize(unsigned Bytes) { CalleeSavedFrameSize = Bytes; }
 

llvm/lib/Target/AVR/AVRRegisterInfo.cpp

@@ -22,6 +22,7 @@
 
 #include "AVR.h"
 #include "AVRInstrInfo.h"
+#include "AVRMachineFunctionInfo.h"
 #include "AVRTargetMachine.h"
 #include "MCTargetDesc/AVRMCTargetDesc.h"
 
@@ -34,19 +35,21 @@ AVRRegisterInfo::AVRRegisterInfo() : AVRGenRegisterInfo(0) {}
 
 const uint16_t *
 AVRRegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
-  CallingConv::ID CC = MF->getFunction().getCallingConv();
+  const AVRMachineFunctionInfo *AFI = MF->getInfo<AVRMachineFunctionInfo>();
 
-  return ((CC == CallingConv::AVR_INTR || CC == CallingConv::AVR_SIGNAL)
+  return AFI->isInterruptHandler()
               ? CSR_Interrupts_SaveList
-              : CSR_Normal_SaveList);
+              : CSR_Normal_SaveList;
 }
 
 const uint32_t *
 AVRRegisterInfo::getCallPreservedMask(const MachineFunction &MF,
                                       CallingConv::ID CC) const {
-  return ((CC == CallingConv::AVR_INTR || CC == CallingConv::AVR_SIGNAL)
+  const AVRMachineFunctionInfo *AFI = MF.getInfo<AVRMachineFunctionInfo>();
+
+  return AFI->isInterruptHandler()
               ? CSR_Interrupts_RegMask
-              : CSR_Normal_RegMask);
+              : CSR_Normal_RegMask;
 }
 
 BitVector AVRRegisterInfo::getReservedRegs(const MachineFunction &MF) const {

llvm/test/CodeGen/AVR/interrupts.ll

@@ -16,6 +16,22 @@ define avr_intrcc void @interrupt_handler() {
   ret void
 }
 
+define void @interrupt_handler_via_ir_attribute() #0 {
+; CHECK-LABEL: interrupt_handler_via_ir_attribute:
+; CHECK: sei
+; CHECK-NEXT: push r0
+; CHECK-NEXT: push r1
+; CHECK-NEXT: in r0, 63
+; CHECK-NEXT: push r0
+; CHECK: clr r0
+; CHECK: pop r0
+; CHECK-NEXT: out 63, r0
+; CHECK-NEXT: pop r1
+; CHECK-NEXT: pop r0
+; CHECK-NEXT: reti
+  ret void
+}
+
 define avr_signalcc void @signal_handler() {
 ; CHECK-LABEL: signal_handler:
 ; CHECK-NOT: sei
@@ -31,3 +47,6 @@ define avr_signalcc void @signal_handler() {
 ; CHECK-NEXT: reti
   ret void
 }
+
+attributes #0 = { "interrupt" }
+attributes #1 = { "signal" }