[X86] Make hasFP constant time

We need a frame pointer if there is a push/pop sequence after the
prologue in order to unwind the stack.  Scanning the instructions to
figure out if this happened made hasFP not constant-time which is a
violation of expectations.  Let's compute this up-front and reuse that
computation when we need it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256730 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetLowering.h

@@ -2269,6 +2269,12 @@ public:
     return false;
   }
 
+  /// Return true if the MachineFunction contains a COPY which would imply
+  /// HasOpaqueSPAdjustment.
+  virtual bool hasCopyImplyingStackAdjustment(MachineFunction *MF) const {
+    return false;
+  }
+
   /// Perform necessary initialization to handle a subset of CSRs explicitly
   /// via copies. This function is called at the beginning of instruction
   /// selection.

lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

@@ -633,6 +633,9 @@ bool SelectionDAGISel::runOnMachineFunction(MachineFunction &mf) {
     MRI.replaceRegWith(From, To);
   }
 
+  if (TLI->hasCopyImplyingStackAdjustment(MF))
+    MFI->setHasOpaqueSPAdjustment(true);
+
   // Freeze the set of reserved registers now that MachineFrameInfo has been
   // set up. All the information required by getReservedRegs() should be
   // available now.

lib/Target/X86/X86FrameLowering.cpp

@@ -78,23 +78,6 @@ X86FrameLowering::needsFrameIndexResolution(const MachineFunction &MF) const {
          MF.getInfo<X86MachineFunctionInfo>()->getHasPushSequences();
 }
 
-/// usesTheStack - This function checks if any of the users of EFLAGS
-/// copies the EFLAGS. We know that the code that lowers COPY of EFLAGS has
-/// to use the stack, and if we don't adjust the stack we clobber the first
-/// frame index.
-/// See X86InstrInfo::copyPhysReg.
-static bool usesTheStack(const MachineFunction &MF) {
-  const MachineRegisterInfo &MRI = MF.getRegInfo();
-
-  return any_of(MRI.reg_instructions(X86::EFLAGS),
-                [](const MachineInstr &RI) { return RI.isCopy(); });
-}
-
-static bool doesStackUseImplyFP(const MachineFunction &MF) {
-  bool IsWin64Prologue = MF.getTarget().getMCAsmInfo()->usesWindowsCFI();
-  return IsWin64Prologue && usesTheStack(MF);
-}
-
 /// hasFP - Return true if the specified function should have a dedicated frame
 /// pointer register.  This is true if the function has variable sized allocas
 /// or if frame pointer elimination is disabled.
@@ -108,8 +91,7 @@ bool X86FrameLowering::hasFP(const MachineFunction &MF) const {
           MFI->isFrameAddressTaken() || MFI->hasOpaqueSPAdjustment() ||
           MF.getInfo<X86MachineFunctionInfo>()->getForceFramePointer() ||
           MMI.callsUnwindInit() || MMI.hasEHFunclets() || MMI.callsEHReturn() ||
-          MFI->hasStackMap() || MFI->hasPatchPoint() ||
-          doesStackUseImplyFP(MF));
+          MFI->hasStackMap() || MFI->hasPatchPoint());
 }
 
 static unsigned getSUBriOpcode(unsigned IsLP64, int64_t Imm) {
@@ -961,11 +943,11 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF,
   // push and pop from the stack.
   if (Is64Bit && !Fn->hasFnAttribute(Attribute::NoRedZone) &&
       !TRI->needsStackRealignment(MF) &&
-      !MFI->hasVarSizedObjects() && // No dynamic alloca.
-      !MFI->adjustsStack() &&       // No calls.
-      !IsWin64CC &&                 // Win64 has no Red Zone
-      !usesTheStack(MF) &&          // Don't push and pop.
-      !MF.shouldSplitStack()) {     // Regular stack
+      !MFI->hasVarSizedObjects() &&    // No dynamic alloca.
+      !MFI->adjustsStack() &&          // No calls.
+      !IsWin64CC &&                    // Win64 has no Red Zone
+      !MFI->hasOpaqueSPAdjustment() && // Don't push and pop.
+      !MF.shouldSplitStack()) {        // Regular stack
     uint64_t MinSize = X86FI->getCalleeSavedFrameSize();
     if (HasFP) MinSize += SlotSize;
     StackSize = std::max(MinSize, StackSize > 128 ? StackSize - 128 : 0);

lib/Target/X86/X86ISelLowering.cpp

@@ -27982,6 +27982,18 @@ bool X86TargetLowering::isTypeDesirableForOp(unsigned Opc, EVT VT) const {
   }
 }
 
+/// This function checks if any of the users of EFLAGS copies the EFLAGS. We
+/// know that the code that lowers COPY of EFLAGS has to use the stack, and if
+/// we don't adjust the stack we clobber the first frame index.
+/// See X86InstrInfo::copyPhysReg.
+bool X86TargetLowering::hasCopyImplyingStackAdjustment(
+    MachineFunction *MF) const {
+  const MachineRegisterInfo &MRI = MF->getRegInfo();
+
+  return any_of(MRI.reg_instructions(X86::EFLAGS),
+                [](const MachineInstr &RI) { return RI.isCopy(); });
+}
+
 /// IsDesirableToPromoteOp - This method query the target whether it is
 /// beneficial for dag combiner to promote the specified node. If true, it
 /// should return the desired promotion type by reference.

lib/Target/X86/X86ISelLowering.h

@@ -697,6 +697,10 @@ namespace llvm {
     /// and some i16 instructions are slow.
     bool IsDesirableToPromoteOp(SDValue Op, EVT &PVT) const override;
 
+    /// Return true if the MachineFunction contains a COPY which would imply
+    /// HasOpaqueSPAdjustment.
+    bool hasCopyImplyingStackAdjustment(MachineFunction *MF) const override;
+
     MachineBasicBlock *
       EmitInstrWithCustomInserter(MachineInstr *MI,
                                   MachineBasicBlock *MBB) const override;

lib/Target/X86/X86InstrInfo.cpp

@@ -4453,7 +4453,8 @@ void X86InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
     // such as TF/IF/DF, which LLVM doesn't model.
     //
     // Notice that we have to adjust the stack if we don't want to clobber the
-    // first frame index. See X86FrameLowering.cpp - usesTheStack.
+    // first frame index.
+    // See X86ISelLowering.cpp - X86::hasCopyImplyingStackAdjustment.
 
 
     bool AXDead = (Reg == AX) ||