ARM: fold prologue/epilogue sp updates into push/pop for code size

ARM prologues usually look like:
    push {r7, lr}
    sub sp, sp, brson#4

If code size is extremely important, this can be optimised to the single
instruction:
    push {r6, r7, lr}

where we don't actually care about the contents of r6, but pushing it subtracts
4 from sp as a side effect.

This should implement such a conversion, predicated on the "minsize" function
attribute (-Oz) since I've yet to find any code it actually makes faster.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194264 91177308-0d34-0410-b5e6-96231b3b80d8

Author: TNorthover

lib/Target/ARM/ARMBaseInstrInfo.cpp

@@ -1857,6 +1857,103 @@ void llvm::emitARMRegPlusImmediate(MachineBasicBlock &MBB,
   }
 }
 
+bool llvm::tryFoldSPUpdateIntoPushPop(MachineFunction &MF,
+                                      MachineInstr *MI,
+                                      unsigned NumBytes) {
+  // This optimisation potentially adds lots of load and store
+  // micro-operations, it's only really a great benefit to code-size.
+  if (!MF.getFunction()->hasFnAttribute(Attribute::MinSize))
+    return false;
+
+  // If only one register is pushed/popped, LLVM can use an LDR/STR
+  // instead. We can't modify those so make sure we're dealing with an
+  // instruction we understand.
+  bool IsPop = isPopOpcode(MI->getOpcode());
+  bool IsPush = isPushOpcode(MI->getOpcode());
+  if (!IsPush && !IsPop)
+    return false;
+
+  bool IsVFPPushPop = MI->getOpcode() == ARM::VSTMDDB_UPD ||
+                      MI->getOpcode() == ARM::VLDMDIA_UPD;
+  bool IsT1PushPop = MI->getOpcode() == ARM::tPUSH ||
+                     MI->getOpcode() == ARM::tPOP ||
+                     MI->getOpcode() == ARM::tPOP_RET;
+
+  assert((IsT1PushPop || (MI->getOperand(0).getReg() == ARM::SP &&
+                          MI->getOperand(1).getReg() == ARM::SP)) &&
+         "trying to fold sp update into non-sp-updating push/pop");
+
+  // The VFP push & pop act on D-registers, so we can only fold an adjustment
+  // by a multiple of 8 bytes in correctly. Similarly rN is 4-bytes. Don't try
+  // if this is violated.
+  if (NumBytes % (IsVFPPushPop ? 8 : 4) != 0)
+    return false;
+
+  // ARM and Thumb2 push/pop insts have explicit "sp, sp" operands (+
+  // pred) so the list starts at 4. Thumb1 starts after the predicate.
+  int RegListIdx = IsT1PushPop ? 2 : 4;
+
+  // Calculate the space we'll need in terms of registers.
+  unsigned FirstReg = MI->getOperand(RegListIdx).getReg();
+  unsigned RD0Reg, RegsNeeded;
+  if (IsVFPPushPop) {
+    RD0Reg = ARM::D0;
+    RegsNeeded = NumBytes / 8;
+  } else {
+    RD0Reg = ARM::R0;
+    RegsNeeded = NumBytes / 4;
+  }
+
+  // We're going to have to strip all list operands off before
+  // re-adding them since the order matters, so save the existing ones
+  // for later.
+  SmallVector<MachineOperand, 4> RegList;
+  for (int i = MI->getNumOperands() - 1; i >= RegListIdx; --i)
+    RegList.push_back(MI->getOperand(i));
+
+  MachineBasicBlock *MBB = MI->getParent();
+  const TargetRegisterInfo *TRI = MF.getRegInfo().getTargetRegisterInfo();
+
+  // Now try to find enough space in the reglist to allocate NumBytes.
+  for (unsigned CurReg = FirstReg - 1; CurReg >= RD0Reg && RegsNeeded;
+       --CurReg, --RegsNeeded) {
+    if (!IsPop) {
+      // Pushing any register is completely harmless, mark the
+      // register involved as undef since we don't care about it in
+      // the slightest.
+      RegList.push_back(MachineOperand::CreateReg(CurReg, false, false,
+                                                  false, false, true));
+      continue;
+    }
+
+    // However, we can only pop an extra register if it's not live. Otherwise we
+    // might clobber a return value register. We assume that once we find a live
+    // return register all lower ones will be too so there's no use proceeding.
+    if (MBB->computeRegisterLiveness(TRI, CurReg, MI) !=
+        MachineBasicBlock::LQR_Dead)
+      return false;
+
+    // Mark the unimportant registers as <def,dead> in the POP.
+    RegList.push_back(MachineOperand::CreateReg(CurReg, true, false, true));
+  }
+
+  if (RegsNeeded > 0)
+    return false;
+
+  // Finally we know we can profitably perform the optimisation so go
+  // ahead: strip all existing registers off and add them back again
+  // in the right order.
+  for (int i = MI->getNumOperands() - 1; i >= RegListIdx; --i)
+    MI->RemoveOperand(i);
+
+  // Add the complete list back in.
+  MachineInstrBuilder MIB(MF, &*MI);
+  for (int i = RegList.size() - 1; i >= 0; --i)
+    MIB.addOperand(RegList[i]);
+
+  return true;
+}
+
 bool llvm::rewriteARMFrameIndex(MachineInstr &MI, unsigned FrameRegIdx,
                                 unsigned FrameReg, int &Offset,
                                 const ARMBaseInstrInfo &TII) {

lib/Target/ARM/ARMBaseInstrInfo.h

@@ -362,6 +362,17 @@ bool isIndirectBranchOpcode(int Opc) {
   return Opc == ARM::BX || Opc == ARM::MOVPCRX || Opc == ARM::tBRIND;
 }
 
+static inline bool isPopOpcode(int Opc) {
+  return Opc == ARM::tPOP_RET || Opc == ARM::LDMIA_RET ||
+         Opc == ARM::t2LDMIA_RET || Opc == ARM::tPOP || Opc == ARM::LDMIA_UPD ||
+         Opc == ARM::t2LDMIA_UPD || Opc == ARM::VLDMDIA_UPD;
+}
+
+static inline bool isPushOpcode(int Opc) {
+  return Opc == ARM::tPUSH || Opc == ARM::t2STMDB_UPD ||
+         Opc == ARM::STMDB_UPD || Opc == ARM::VSTMDDB_UPD;
+}
+
 /// getInstrPredicate - If instruction is predicated, returns its predicate
 /// condition, otherwise returns AL. It also returns the condition code
 /// register by reference.
@@ -401,6 +412,13 @@ void emitThumbRegPlusImmediate(MachineBasicBlock &MBB,
                                const ARMBaseRegisterInfo& MRI,
                                unsigned MIFlags = 0);
 
+/// Tries to add registers to the reglist of a given base-updating
+/// push/pop instruction to adjust the stack by an additional
+/// NumBytes. This can save a few bytes per function in code-size, but
+/// obviously generates more memory traffic. As such, it only takes
+/// effect in functions being optimised for size.
+bool tryFoldSPUpdateIntoPushPop(MachineFunction &MF, MachineInstr *MI,
+                                unsigned NumBytes);
 
 /// rewriteARMFrameIndex / rewriteT2FrameIndex -
 /// Rewrite MI to access 'Offset' bytes from the FP. Return false if the

lib/Target/ARM/ARMFrameLowering.cpp

@@ -93,11 +93,7 @@ static bool isCSRestore(MachineInstr *MI,
                         const ARMBaseInstrInfo &TII,
                         const uint16_t *CSRegs) {
   // Integer spill area is handled with "pop".
-  if (MI->getOpcode() == ARM::LDMIA_RET ||
-      MI->getOpcode() == ARM::t2LDMIA_RET ||
-      MI->getOpcode() == ARM::LDMIA_UPD ||
-      MI->getOpcode() == ARM::t2LDMIA_UPD ||
-      MI->getOpcode() == ARM::VLDMDIA_UPD) {
+  if (isPopOpcode(MI->getOpcode())) {
     // The first two operands are predicates. The last two are
     // imp-def and imp-use of SP. Check everything in between.
     for (int i = 5, e = MI->getNumOperands(); i != e; ++i)
@@ -221,42 +217,37 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
   }
 
   // Move past area 1.
-  if (GPRCS1Size > 0) MBBI++;
+  MachineBasicBlock::iterator LastPush = MBB.end(), FramePtrPush;
+  if (GPRCS1Size > 0)
+    FramePtrPush = LastPush = MBBI++;
 
   // Determine starting offsets of spill areas.
   bool HasFP = hasFP(MF);
   unsigned DPRCSOffset  = NumBytes - (GPRCS1Size + GPRCS2Size + DPRCSSize);
   unsigned GPRCS2Offset = DPRCSOffset + DPRCSSize;
   unsigned GPRCS1Offset = GPRCS2Offset + GPRCS2Size;
-  if (HasFP)
+  int FramePtrOffsetInPush = 0;
+  if (HasFP) {
+    FramePtrOffsetInPush = MFI->getObjectOffset(FramePtrSpillFI) + GPRCS1Size;
     AFI->setFramePtrSpillOffset(MFI->getObjectOffset(FramePtrSpillFI) +
                                 NumBytes);
+  }
   AFI->setGPRCalleeSavedArea1Offset(GPRCS1Offset);
   AFI->setGPRCalleeSavedArea2Offset(GPRCS2Offset);
   AFI->setDPRCalleeSavedAreaOffset(DPRCSOffset);
 
-  // Set FP to point to the stack slot that contains the previous FP.
-  // For iOS, FP is R7, which has now been stored in spill area 1.
-  // Otherwise, if this is not iOS, all the callee-saved registers go
-  // into spill area 1, including the FP in R11.  In either case, it is
-  // now safe to emit this assignment.
-  if (HasFP) {
-    int FramePtrOffset = MFI->getObjectOffset(FramePtrSpillFI) + GPRCS1Size;
-    emitRegPlusImmediate(!AFI->isThumbFunction(), MBB, MBBI, dl, TII,
-                         FramePtr, ARM::SP, FramePtrOffset,
-                         MachineInstr::FrameSetup);
-  }
-
   // Move past area 2.
-  if (GPRCS2Size > 0) MBBI++;
+  if (GPRCS2Size > 0) {
+    LastPush = MBBI++;
+  }
 
   // Move past area 3.
   if (DPRCSSize > 0) {
-    MBBI++;
+    LastPush = MBBI++;
     // Since vpush register list cannot have gaps, there may be multiple vpush
     // instructions in the prologue.
     while (MBBI->getOpcode() == ARM::VSTMDDB_UPD)
-      MBBI++;
+      LastPush = MBBI++;
   }
 
   // Move past the aligned DPRCS2 area.
@@ -272,8 +263,12 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
 
   if (NumBytes) {
     // Adjust SP after all the callee-save spills.
-    emitSPUpdate(isARM, MBB, MBBI, dl, TII, -NumBytes,
-                 MachineInstr::FrameSetup);
+    if (tryFoldSPUpdateIntoPushPop(MF, LastPush, NumBytes))
+      FramePtrOffsetInPush += NumBytes;
+    else
+      emitSPUpdate(isARM, MBB, MBBI, dl, TII, -NumBytes,
+                   MachineInstr::FrameSetup);
+
     if (HasFP && isARM)
       // Restore from fp only in ARM mode: e.g. sub sp, r7, #24
       // Note it's not safe to do this in Thumb2 mode because it would have
@@ -286,6 +281,18 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
       AFI->setShouldRestoreSPFromFP(true);
   }
 
+  // Set FP to point to the stack slot that contains the previous FP.
+  // For iOS, FP is R7, which has now been stored in spill area 1.
+  // Otherwise, if this is not iOS, all the callee-saved registers go
+  // into spill area 1, including the FP in R11.  In either case, it
+  // is in area one and the adjustment needs to take place just after
+  // that push.
+  if (HasFP)
+    emitRegPlusImmediate(!AFI->isThumbFunction(), MBB, ++FramePtrPush, dl, TII,
+                         FramePtr, ARM::SP, FramePtrOffsetInPush,
+                         MachineInstr::FrameSetup);
+
+
   if (STI.isTargetELF() && hasFP(MF))
     MFI->setOffsetAdjustment(MFI->getOffsetAdjustment() -
                              AFI->getFramePtrSpillOffset());
@@ -380,12 +387,17 @@ void ARMFrameLowering::emitEpilogue(MachineFunction &MF,
     if (NumBytes != 0)
       emitSPUpdate(isARM, MBB, MBBI, dl, TII, NumBytes);
   } else {
+    MachineBasicBlock::iterator FirstPop = MBBI;
+
     // Unwind MBBI to point to first LDR / VLDRD.
     const uint16_t *CSRegs = RegInfo->getCalleeSavedRegs(&MF);
     if (MBBI != MBB.begin()) {
-      do
+      do {
+        if (isPopOpcode(MBBI->getOpcode()))
+          FirstPop = MBBI;
+
         --MBBI;
-      while (MBBI != MBB.begin() && isCSRestore(MBBI, TII, CSRegs));
+      } while (MBBI != MBB.begin() && isCSRestore(MBBI, TII, CSRegs));
       if (!isCSRestore(MBBI, TII, CSRegs))
         ++MBBI;
     }
@@ -429,8 +441,8 @@ void ARMFrameLowering::emitEpilogue(MachineFunction &MF,
                                  ARM::SP)
             .addReg(FramePtr));
       }
-    } else if (NumBytes)
-      emitSPUpdate(isARM, MBB, MBBI, dl, TII, NumBytes);
+    } else if (NumBytes && !tryFoldSPUpdateIntoPushPop(MF, FirstPop, NumBytes))
+        emitSPUpdate(isARM, MBB, MBBI, dl, TII, NumBytes);
 
     // Increment past our save areas.
     if (AFI->getDPRCalleeSavedAreaSize()) {

lib/Target/ARM/Thumb1FrameLowering.cpp

@@ -164,11 +164,17 @@ void Thumb1FrameLowering::emitPrologue(MachineFunction &MF) const {
   AFI->setDPRCalleeSavedAreaOffset(DPRCSOffset);
   NumBytes = DPRCSOffset;
 
+  int FramePtrOffsetInBlock = 0;
+  if (tryFoldSPUpdateIntoPushPop(MF, prior(MBBI), NumBytes)) {
+    FramePtrOffsetInBlock = NumBytes;
+    NumBytes = 0;
+  }
+
   // Adjust FP so it point to the stack slot that contains the previous FP.
   if (HasFP) {
-    int FramePtrOffset = MFI->getObjectOffset(FramePtrSpillFI) + GPRCS1Size;
+    FramePtrOffsetInBlock += MFI->getObjectOffset(FramePtrSpillFI) + GPRCS1Size;
     AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tADDrSPi), FramePtr)
-      .addReg(ARM::SP).addImm(FramePtrOffset / 4)
+      .addReg(ARM::SP).addImm(FramePtrOffsetInBlock / 4)
       .setMIFlags(MachineInstr::FrameSetup));
     if (NumBytes > 508)
       // If offset is > 508 then sp cannot be adjusted in a single instruction,
@@ -292,8 +298,9 @@ void Thumb1FrameLowering::emitEpilogue(MachineFunction &MF,
           &MBB.front() != MBBI &&
           prior(MBBI)->getOpcode() == ARM::tPOP) {
         MachineBasicBlock::iterator PMBBI = prior(MBBI);
-        emitSPUpdate(MBB, PMBBI, TII, dl, *RegInfo, NumBytes);
-      } else
+        if (!tryFoldSPUpdateIntoPushPop(MF, PMBBI, NumBytes))
+          emitSPUpdate(MBB, PMBBI, TII, dl, *RegInfo, NumBytes);
+      } else if (!tryFoldSPUpdateIntoPushPop(MF, MBBI, NumBytes))
         emitSPUpdate(MBB, MBBI, TII, dl, *RegInfo, NumBytes);
     }
   }