[NFC][CodeGen] Tidy up TargetRegisterInfo stack realignment functions

Currently needsStackRealignment returns false if canRealignStack returns false.
This means that the behavior of needsStackRealignment does not correspond to
it's name and description; a function might need stack realignment, but if it
is not possible then this function returns false. Furthermore,
needsStackRealignment is not virtual and therefore some backends have made use
of canRealignStack to indicate whether a function needs stack realignment.

This patch attempts to clarify the situation by separating them and introducing
new names:

 - shouldRealignStack - true if there is any reason the stack should be
   realigned

 - canRealignStack - true if we are still able to realign the stack (e.g. we
   can still reserve/have reserved a frame pointer)

 - hasStackRealignment = shouldRealignStack && canRealignStack (not target
   customisable)

Targets can now override shouldRealignStack to indicate that stack realignment
is required.

This change will make it easier in a future change to handle the case where we
need to realign the stack but can't do so (for example when the register
allocator creates an aligned spill after the frame pointer has been
eliminated).

Differential Revision: https://reviews.llvm.org/D98716

Change-Id: Ib9a4d21728bf9d08a545b4365418d3ffe1af4d87

Author: tmatheson-arm

llvm/include/llvm/CodeGen/TargetRegisterInfo.h

@@ -914,9 +914,12 @@ class TargetRegisterInfo : public MCRegisterInfo {
 
   /// True if storage within the function requires the stack pointer to be
   /// aligned more than the normal calling convention calls for.
-  /// This cannot be overriden by the target, but canRealignStack can be
-  /// overridden.
-  bool needsStackRealignment(const MachineFunction &MF) const;
+  virtual bool shouldRealignStack(const MachineFunction &MF) const;
+
+  /// True if stack realignment is required and still possible.
+  bool hasStackRealignment(const MachineFunction &MF) const {
+    return shouldRealignStack(MF) && canRealignStack(MF);
+  }
 
   /// Get the offset from the referenced frame index in the instruction,
   /// if there is one.

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

@@ -1377,7 +1377,7 @@ void CodeViewDebug::beginFunctionImpl(const MachineFunction *MF) {
   CurFn->CSRSize = MFI.getCVBytesOfCalleeSavedRegisters();
   CurFn->FrameSize = MFI.getStackSize();
   CurFn->OffsetAdjustment = MFI.getOffsetAdjustment();
-  CurFn->HasStackRealignment = TRI->needsStackRealignment(*MF);
+  CurFn->HasStackRealignment = TRI->hasStackRealignment(*MF);
 
   // For this function S_FRAMEPROC record, figure out which codeview register
   // will be the frame pointer.

llvm/lib/CodeGen/GCRootLowering.cpp

@@ -317,8 +317,8 @@ bool GCMachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
   // size, we use UINT64_MAX to represent this.
   const MachineFrameInfo &MFI = MF.getFrameInfo();
   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
-  const bool DynamicFrameSize = MFI.hasVarSizedObjects() ||
-    RegInfo->needsStackRealignment(MF);
+  const bool DynamicFrameSize =
+      MFI.hasVarSizedObjects() || RegInfo->hasStackRealignment(MF);
   FI->setFrameSize(DynamicFrameSize ? UINT64_MAX : MFI.getStackSize());
 
   // Find all safe points.

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -1370,7 +1370,7 @@ bool CombinerHelper::optimizeMemcpy(MachineInstr &MI, Register Dst,
     // Don't promote to an alignment that would require dynamic stack
     // realignment.
     const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
-    if (!TRI->needsStackRealignment(MF))
+    if (!TRI->hasStackRealignment(MF))
       while (NewAlign > Alignment && DL.exceedsNaturalStackAlignment(NewAlign))
         NewAlign = NewAlign / 2;
 
@@ -1475,7 +1475,7 @@ bool CombinerHelper::optimizeMemmove(MachineInstr &MI, Register Dst,
     // Don't promote to an alignment that would require dynamic stack
     // realignment.
     const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
-    if (!TRI->needsStackRealignment(MF))
+    if (!TRI->hasStackRealignment(MF))
       while (NewAlign > Alignment && DL.exceedsNaturalStackAlignment(NewAlign))
         NewAlign = NewAlign / 2;
 

llvm/lib/CodeGen/MachineFrameInfo.cpp

@@ -173,7 +173,7 @@ uint64_t MachineFrameInfo::estimateStackSize(const MachineFunction &MF) const {
   // value.
   Align StackAlign;
   if (adjustsStack() || hasVarSizedObjects() ||
-      (RegInfo->needsStackRealignment(MF) && getObjectIndexEnd() != 0))
+      (RegInfo->hasStackRealignment(MF) && getObjectIndexEnd() != 0))
     StackAlign = TFI->getStackAlign();
   else
     StackAlign = TFI->getTransientStackAlign();

llvm/lib/CodeGen/PrologEpilogInserter.cpp

@@ -877,10 +877,9 @@ void PEI::calculateFrameObjectOffsets(MachineFunction &MF) {
   // incoming stack pointer if a frame pointer is required and is closer
   // to the incoming rather than the final stack pointer.
   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
-  bool EarlyScavengingSlots = (TFI.hasFP(MF) &&
-                               TFI.isFPCloseToIncomingSP() &&
+  bool EarlyScavengingSlots = (TFI.hasFP(MF) && TFI.isFPCloseToIncomingSP() &&
                                RegInfo->useFPForScavengingIndex(MF) &&
-                               !RegInfo->needsStackRealignment(MF));
+                               !RegInfo->hasStackRealignment(MF));
   if (RS && EarlyScavengingSlots) {
     SmallVector<int, 2> SFIs;
     RS->getScavengingFrameIndices(SFIs);
@@ -1063,7 +1062,7 @@ void PEI::calculateFrameObjectOffsets(MachineFunction &MF) {
     // value.
     Align StackAlign;
     if (MFI.adjustsStack() || MFI.hasVarSizedObjects() ||
-        (RegInfo->needsStackRealignment(MF) && MFI.getObjectIndexEnd() != 0))
+        (RegInfo->hasStackRealignment(MF) && MFI.getObjectIndexEnd() != 0))
       StackAlign = TFI.getStackAlign();
     else
       StackAlign = TFI.getTransientStackAlign();

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -6243,7 +6243,7 @@ static SDValue getMemcpyLoadsAndStores(SelectionDAG &DAG, const SDLoc &dl,
     // Don't promote to an alignment that would require dynamic stack
     // realignment.
     const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
-    if (!TRI->needsStackRealignment(MF))
+    if (!TRI->hasStackRealignment(MF))
       while (NewAlign > Alignment && DL.exceedsNaturalStackAlignment(NewAlign))
         NewAlign = NewAlign / 2;
 

llvm/lib/CodeGen/StackMaps.cpp

@@ -511,7 +511,7 @@ void StackMaps::recordStackMapOpers(const MCSymbol &MILabel,
   const MachineFrameInfo &MFI = AP.MF->getFrameInfo();
   const TargetRegisterInfo *RegInfo = AP.MF->getSubtarget().getRegisterInfo();
   bool HasDynamicFrameSize =
-      MFI.hasVarSizedObjects() || RegInfo->needsStackRealignment(*(AP.MF));
+      MFI.hasVarSizedObjects() || RegInfo->hasStackRealignment(*(AP.MF));
   uint64_t FrameSize = HasDynamicFrameSize ? UINT64_MAX : MFI.getStackSize();
 
   auto CurrentIt = FnInfos.find(AP.CurrentFnSym);

llvm/lib/CodeGen/TargetRegisterInfo.cpp

@@ -461,21 +461,13 @@ bool TargetRegisterInfo::canRealignStack(const MachineFunction &MF) const {
   return !MF.getFunction().hasFnAttribute("no-realign-stack");
 }
 
-bool TargetRegisterInfo::needsStackRealignment(
-    const MachineFunction &MF) const {
+bool TargetRegisterInfo::shouldRealignStack(const MachineFunction &MF) const {
   const MachineFrameInfo &MFI = MF.getFrameInfo();
   const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
   const Function &F = MF.getFunction();
-  Align StackAlign = TFI->getStackAlign();
-  bool requiresRealignment = ((MFI.getMaxAlign() > StackAlign) ||
-                              F.hasFnAttribute(Attribute::StackAlignment));
-  if (F.hasFnAttribute("stackrealign") || requiresRealignment) {
-    if (canRealignStack(MF))
-      return true;
-    LLVM_DEBUG(dbgs() << "Can't realign function's stack: " << F.getName()
-                      << "\n");
-  }
-  return false;
+  return F.hasFnAttribute("stackrealign") ||
+         (MFI.getMaxAlign() > TFI->getStackAlign()) ||
+         F.hasFnAttribute(Attribute::StackAlignment);
 }
 
 bool TargetRegisterInfo::regmaskSubsetEqual(const uint32_t *mask0,

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

@@ -252,7 +252,7 @@ bool AArch64FrameLowering::homogeneousPrologEpilog(
   // Bail on stack adjustment needed on return for simplicity.
   const MachineFrameInfo &MFI = MF.getFrameInfo();
   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
-  if (MFI.hasVarSizedObjects() || RegInfo->needsStackRealignment(MF))
+  if (MFI.hasVarSizedObjects() || RegInfo->hasStackRealignment(MF))
     return false;
   if (Exit && getArgumentPopSize(MF, *Exit))
     return false;
@@ -363,7 +363,7 @@ bool AArch64FrameLowering::hasFP(const MachineFunction &MF) const {
     return true;
   if (MFI.hasVarSizedObjects() || MFI.isFrameAddressTaken() ||
       MFI.hasStackMap() || MFI.hasPatchPoint() ||
-      RegInfo->needsStackRealignment(MF))
+      RegInfo->hasStackRealignment(MF))
     return true;
   // With large callframes around we may need to use FP to access the scavenging
   // emergency spillslot.
@@ -616,7 +616,7 @@ bool AArch64FrameLowering::canUseAsPrologue(
   const AArch64RegisterInfo *RegInfo = Subtarget.getRegisterInfo();
 
   // Don't need a scratch register if we're not going to re-align the stack.
-  if (!RegInfo->needsStackRealignment(*MF))
+  if (!RegInfo->hasStackRealignment(*MF))
     return true;
   // Otherwise, we can use any block as long as it has a scratch register
   // available.
@@ -678,7 +678,7 @@ bool AArch64FrameLowering::shouldCombineCSRLocalStackBump(
   if (MFI.hasVarSizedObjects())
     return false;
 
-  if (RegInfo->needsStackRealignment(MF))
+  if (RegInfo->hasStackRealignment(MF))
     return false;
 
   // This isn't strictly necessary, but it simplifies things a bit since the
@@ -1375,7 +1375,7 @@ void AArch64FrameLowering::emitPrologue(MachineFunction &MF,
   if (NumBytes) {
     // Alignment is required for the parent frame, not the funclet
     const bool NeedsRealignment =
-        !IsFunclet && RegInfo->needsStackRealignment(MF);
+        !IsFunclet && RegInfo->hasStackRealignment(MF);
     unsigned scratchSPReg = AArch64::SP;
 
     if (NeedsRealignment) {
@@ -1981,13 +1981,13 @@ StackOffset AArch64FrameLowering::resolveFrameOffsetReference(
     // Argument access should always use the FP.
     if (isFixed) {
       UseFP = hasFP(MF);
-    } else if (isCSR && RegInfo->needsStackRealignment(MF)) {
+    } else if (isCSR && RegInfo->hasStackRealignment(MF)) {
       // References to the CSR area must use FP if we're re-aligning the stack
       // since the dynamically-sized alignment padding is between the SP/BP and
       // the CSR area.
       assert(hasFP(MF) && "Re-aligned stack must have frame pointer");
       UseFP = true;
-    } else if (hasFP(MF) && !RegInfo->needsStackRealignment(MF)) {
+    } else if (hasFP(MF) && !RegInfo->hasStackRealignment(MF)) {
       // If the FPOffset is negative and we're producing a signed immediate, we
       // have to keep in mind that the available offset range for negative
       // offsets is smaller than for positive ones. If an offset is available
@@ -2029,9 +2029,10 @@ StackOffset AArch64FrameLowering::resolveFrameOffsetReference(
     }
   }
 
-  assert(((isFixed || isCSR) || !RegInfo->needsStackRealignment(MF) || !UseFP) &&
-         "In the presence of dynamic stack pointer realignment, "
-         "non-argument/CSR objects cannot be accessed through the frame pointer");
+  assert(
+      ((isFixed || isCSR) || !RegInfo->hasStackRealignment(MF) || !UseFP) &&
+      "In the presence of dynamic stack pointer realignment, "
+      "non-argument/CSR objects cannot be accessed through the frame pointer");
 
   if (isSVE) {
     StackOffset FPOffset =
@@ -2041,10 +2042,9 @@ StackOffset AArch64FrameLowering::resolveFrameOffsetReference(
         StackOffset::get(MFI.getStackSize() - AFI->getCalleeSavedStackSize(),
                          ObjectOffset);
     // Always use the FP for SVE spills if available and beneficial.
-    if (hasFP(MF) &&
-        (SPOffset.getFixed() ||
-         FPOffset.getScalable() < SPOffset.getScalable() ||
-         RegInfo->needsStackRealignment(MF))) {
+    if (hasFP(MF) && (SPOffset.getFixed() ||
+                      FPOffset.getScalable() < SPOffset.getScalable() ||
+                      RegInfo->hasStackRealignment(MF))) {
       FrameReg = RegInfo->getFrameRegister(MF);
       return FPOffset;
     }